What are the possible ways to create objects in JavaScript

There are many ways to create objects in JS as below:

1. Object constructor

The simplest way to create an empty object is using the Object constructor. Currently this approach is not recommended.

var object = new Object();

2. Object's create method

The create method of Object creates a new object by passing the prototype object as a parameter.

var object = Object.create(null);

3. Object literal syntax

The object literal syntax is equivalent to create method when it passes null as parameter.

var object = {};

4. Function constructor

Create any function and apply the new operator to create object instances.

function Person(name) {
    var object = {};
    object.name = name;
    object.age = 21;
    return object;
}

var object = new Person("Sudheer");

5. Function constructor with prototype

This is similar to function constructor but it uses prototype for their properties and methods.

function Person(){}
Person.prototype.name = "Sudheer";
var object = new Person();

This is equivalent to an instance created with an object create method with a function prototype and then call that function with an instance and parameters as arguments.

function func {};

new func(x, y, z);

6. ES6 Class syntax

ES6 introduces class feature to create the objects.

class Person {
    constructor(name) {
        this.name = name;
    }
}

var object = new Person("Sudheer");

7. Singleton pattern

A Singleton is an object which can only be instantiated one time. Repeated calls to its constructor return the same instance and this way one can ensure that they don't accidentally create multiple instances.

var object = new function() {
    this.name = "Sudheer";
}

What is a prototype chain

Prototype chaining is used to build new types of objects based on existing ones. It is similar to inheritance in a class based language.

The prototype on object instance is available through Object.getPrototypeOf(object) or proto property whereas prototype on constructors function is available through Object.prototype.

What is the difference between Call, Apply and Bind

The difference between Call, Apply and Bind can be explained with below examples.

Call: The call() method invokes a function with a given this value and arguments provided one by one.

var employee1 = {firstName: 'John', lastName: 'Rodson'};
var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};

function invite(greeting1, greeting2) {
    console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
}

invite.call(employee1, 'Hello', 'How are you?'); // Hello John Rodson, How are you?
invite.call(employee2, 'Hello', 'How are you?'); // Hello Jimmy Baily, How are you?

Apply: Invokes the function with a given this value and allows you to pass in arguments as an array.

var employee1 = {firstName: 'John', lastName: 'Rodson'};
var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};

function invite(greeting1, greeting2) {
    console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
}

invite.apply(employee1, ['Hello', 'How are you?']); // Hello John Rodson, How are you?
invite.apply(employee2, ['Hello', 'How are you?']); // Hello Jimmy Baily, How are you?

Bind: returns a new function, allowing you to pass any number of arguments.

var employee1 = {firstName: 'John', lastName: 'Rodson'};
var employee2 = {firstName: 'Jimmy', lastName: 'Baily'};

function invite(greeting1, greeting2) {
    console.log(greeting1 + ' ' + this.firstName + ' ' + this.lastName+ ', '+ greeting2);
}

var inviteEmployee1 = invite.bind(employee1);
var inviteEmployee2 = invite.bind(employee2);
inviteEmployee1('Hello', 'How are you?'); // Hello John Rodson, How are you?
inviteEmployee2('Hello', 'How are you?'); // Hello Jimmy Baily, How are you?

Call and apply are pretty interchangeable. Both execute the current function immediately. You need to decide whether it’s easier to send in an array or a comma separated list of arguments. You can remember by treating Call is for comma (separated list) and Apply is for Array.

Whereas Bind creates a new function that will have this set to the first parameter passed to bind().

What is JSON and its common operations

JSON is a text-based data format following JavaScript object syntax, which was popularized by Douglas Crockford. It is useful when you want to transmit data across a network and it is basically just a text file with an extension of .json, and a MIME type of application/json.

Parsing: Converting a string to a native object.

JSON.parse(text)

Stringification: converting a native object to a string so it can be transmitted across the network.

JSON.stringify(object)

What is the purpose of the array slice method

The slice() method returns the selected elements in an array as a new array object. It selects the elements starting at the given start argument, and ends at the given optional end argument without including the last element. If you omit the second argument then it selects till the end.

Some of the examples of this method are:

let arrayIntegers = [1, 2, 3, 4, 5];
let arrayIntegers1 = arrayIntegers.slice(0,2); // returns [1,2]
let arrayIntegers2 = arrayIntegers.slice(2,3); // returns [3]
let arrayIntegers3 = arrayIntegers.slice(4); //returns [5]

Note: Slice method won't mutate the original array but it returns the subset as a new array.

What is the purpose of the array splice method

The splice() method is used either adds/removes items to/from an array, and then returns the removed item. The first argument specifies the array position for insertion or deletion whereas the optional second argument indicates the number of elements to be deleted. Each additional argument is added to the array.

Some of the examples of this method are:

let arrayIntegersOriginal1 = [1, 2, 3, 4, 5];
let arrayIntegersOriginal2 = [1, 2, 3, 4, 5];
let arrayIntegersOriginal3 = [1, 2, 3, 4, 5];

let arrayIntegers1 = arrayIntegersOriginal1.splice(0,2); // returns [1, 2]; original array: [3, 4, 5]
let arrayIntegers2 = arrayIntegersOriginal2.splice(3); // returns [4, 5]; original array: [1, 2, 3]
let arrayIntegers3 = arrayIntegersOriginal3.splice(3, 1, "a", "b", "c"); //returns [4]; original array: [1, 2, 3, "a", "b", "c", 5]

Note: Splice method modifies the original array and returns the deleted array.

What is the difference between slice and splice

Slice:

  • Doesn't modify the original array (immutable);
  • Returns the subset of original array;
  • Used to pick the elements from array;

Splice:

  • Modifies the original array (mutable);
  • Returns the deleted elements as array;
  • Used to insert or delete elements to/from array;

How do you compare Object and Map

Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Due to this reason, Objects have been used as Maps historically. But there are important differences that make using a Map preferable in certain cases.

  • The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
  • The keys in Map are ordered while keys added to Object are not. Thus, when iterating over it, a Map object returns keys in order of insertion.
  • You can get the size of a Map easily with the size property, while the number of properties in an Object must be determined manually.
  • A Map is an iterable and can thus be directly iterated, whereas iterating over an Object requires obtaining its keys in some fashion and iterating over them.
  • An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. As of ES5 this can be bypassed by using map = Object.create(null), but this is seldom done.
  • A Map may perform better in scenarios involving frequent addition and removal of key pairs.

What is the difference between == and === operators

JavaScript provides both strict (===, !==) and type-converting (==, !=) equality comparison. The strict operators take type of variable in consideration, while non-strict operators make type correction/conversion based upon values of variables. The strict operators follow the below conditions for different types.

1. Two strings are strictly equal when they have the same sequence of characters, same length, and same characters in corresponding positions.

2. Two numbers are strictly equal when they are numerically equal. i.e, Having the same number value. There are two special cases in this,

a. NaN is not equal to anything, including NaN.

b. Positive and negative zeros are equal to one another.

3. Two Boolean operands are strictly equal if both are true or both are false.

4. Two objects are strictly equal if they refer to the same Object.

5. Null and Undefined types are not equal with ===, but equal with ==. i.e, null === undefined --> false but null == undefined --> true

Some of the example which covers the above cases:

0 == false   // true
0 === false  // false
1 == "1"     // true
1 === "1"    // false
null == undefined // true
null === undefined // false
'0' == false // true
'0' === false // false
[]==[] or []===[] //false, refer different objects in memory
{}=={} or {}==={} //false, refer different objects in memory

What are lambda or arrow functions

An arrow function is a shorter syntax for a function expression and does not have its own this, arguments, super, or new.target. These functions are best suited for non-method functions, and they cannot be used as constructors.

What is a first class function

In Javascript, functions are first class objects. First-class functions means when functions in that language are treated like any other variable.

For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable. For example, in the below example, handler functions assigned to a listener:

const handler = () => console.log ('This is a click handler function');
document.addEventListener ('click', handler);

What is a first order function

First-order function is a function that doesn’t accept another function as an argument and doesn’t return a function as its return value.

const firstOrder = () => console.log ('I am a first order function!');

What is a higher order function

Higher-order function is a function that accepts another function as an argument or returns a function as a return value or both.

const firstOrderFunc = () => console.log ('Hello, I am a First order function');
const higherOrder = ReturnFirstOrderFunc => ReturnFirstOrderFunc();
higherOrder(firstOrderFunc);

What is a unary function

Unary function (i.e. monadic) is a function that accepts exactly one argument. It stands for a single argument accepted by a function.

Let us take an example of unary function:

const unaryFunction = a => console.log (a + 10); // Add 10 to the given argument and display the value

What is the currying function

Currying is the process of taking a function with multiple arguments and turning it into a sequence of functions each with only a single argument. Currying is named after a mathematician Haskell Curry. By applying currying, a n-ary function turns it into a unary function.

Let's take an example of n-ary function and how it turns into a currying function:

const multiArgFunction = (a, b, c) => a + b + c;
console.log(multiArgFunction(1,2,3));// 6

const curryUnaryFunction = a => b => c => a + b + c;
curryUnaryFunction (1); // returns a function: b => c =>  1 + b + c
curryUnaryFunction (1) (2); // returns a function: c => 3 + c
curryUnaryFunction (1) (2) (3); // returns the number 6

Curried functions are great to improve code reusability and functional composition.

What is a pure function

A Pure function is a function where the return value is only determined by its arguments without any side effects. i.e, If you call a function with the same arguments 'n' number of times and 'n' number of places in the application then it will always return the same value.

Let's take an example to see the difference between pure and impure functions:

//Impure
let numberArray = [];
const impureAddNumber = number => numberArray.push(number);
//Pure
const pureAddNumber = number => argNumberArray =>
  argNumberArray.concat([number]);

//Display the results
console.log (impureAddNumber(6)); // returns 1
console.log (numberArray); // returns [6]
console.log (pureAddNumber(7) (numberArray)); // returns [6, 7]
console.log (numberArray); // returns [6]

As per above code snippets, Push function is impure itself by altering the array and returning an push number index which is independent of parameter value. Whereas Concat on the other hand takes the array and concatenates it with the other array producing a whole new array without side effects. Also, the return value is a concatenation of the previous array.

Remember that Pure functions are important as they simplify unit testing without any side effects and no need for dependency injection. They also avoid tight coupling and make it harder to break your application by not having any side effects. These principles are coming together with Immutability concept of ES6 by giving preference to const over let usage.

What is the purpose of the let keyword

The let statement declares a block scope local variable. Hence the variables defined with let keyword are limited in scope to the block, statement, or expression on which it is used. Whereas variables declared with the var keyword used to define a variable globally, or locally to an entire function regardless of block scope.

Let's take an example to demonstrate the usage:

let counter = 30;

if (counter === 30) {
    let counter = 31;
    console.log(counter); // 31
}

console.log(counter); // 30 (because the variable in if block won't exist here)

What is the difference between let and var

var:

  • It is been available from the beginning of JavaScript;
  • It has function scope;
  • Variables will be hoisted;

let:

  • Introduced as part of ES6;
  • It has block scope;
  • Hoisted but not initialized;

Let's take an example to see the difference:

function userDetails(username) {
    if(username) {
        console.log(salary); // undefined due to hoisting
        console.log(age); // ReferenceError: Cannot access 'age' before initialization
        let age = 30;
        var salary = 10000;
    }

    console.log(salary); //10000 (accessible to due function scope)
    console.log(age); //error: age is not defined(due to block scope)
}

userDetails('John');

What is the reason to choose the name let as a keyword

let is a mathematical statement that was adopted by early programming languages like Scheme and Basic. It has been borrowed from dozens of other languages that use let already as a traditional keyword as close to var as possible.

How do you redeclare variables in switch block without an error

If you try to redeclare variables in a switch block then it will cause errors because there is only one block. For example, the below code block throws a syntax error as below:

let counter = 1;

switch(x) {
    case 0:
        let name;
        break;

    case 1:
        let name; // SyntaxError for redeclaration.
        break;
}

To avoid this error, you can create a nested block inside a case clause and create a new block scoped lexical environment.

let counter = 1;

switch(x) {
    case 0: {
        let name;
        break;
    }
    case 1: {
        let name; // No SyntaxError for redeclaration.
        break;
    }
}

What is the Temporal Dead Zone

The Temporal Dead Zone is a behavior in JavaScript that occurs when declaring a variable with the let and const keywords, but not with var. In ECMAScript 6, accessing a let or const variable before its declaration (within its scope) causes a ReferenceError. The time span when that happens, between the creation of a variable’s binding and its declaration, is called the temporal dead zone.

Let's see this behavior with an example:

function somemethod() {
    console.log(counter1); // undefined
    console.log(counter2); // ReferenceError

    var counter1 = 1;
    let counter2 = 2;
}

What is IIFE (Immediately Invoked Function Expression)

IIFE (Immediately Invoked Function Expression) is a JavaScript function that runs as soon as it is defined. The signature of it would be as below:

(function ()
    {
        // logic here
    }
)();

The primary reason to use an IIFE is to obtain data privacy because any variables declared within the IIFE cannot be accessed by the outside world. i.e, If you try to access variables with IIFE then it throws an error as below.

(function ()
    {
        var message = "IIFE";
        console.log(message);
    }
)();

console.log(message); // Error: message is not defined

What is the benefit of using modules

There are a lot of benefits to using modules in favour of a sprawling. Some of the benefits are:

  • Maintainability;
  • Reusability;
  • Namespacing;

What is memoization

Memoization is a programming technique which attempts to increase a function’s performance by caching its previously computed results. Each time a memoized function is called, its parameters are used to index the cache. If the data is present, then it can be returned, without executing the entire function. Otherwise the function is executed and then the result is added to the cache. Let's take an example of adding function with memoization.

const memoizAddition = () => {
    let cache = {};
	
    return (value) => {
        if (value in cache) {
            console.log('Fetching from cache');
            return cache[value]; // Here, cache.value cannot be used as property name starts with the number which is not a valid JavaScript  identifier. Hence, can only be accessed using the square bracket notation.
        } else {
            console.log('Calculating result');
            let result = value + 20;
            cache[value] = result;
            return result;
        }
    }
}

// returned function from memoizAddition
const addition = memoizAddition();
console.log(addition(20)); //output: 40 calculated
console.log(addition(20)); //output: 40 cached

What is Hoisting

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. Remember that JavaScript only hoists declarations, not initialisation. Let's take a simple example of variable hoisting:

console.log(message); //output : undefined
var message = 'The variable Has been hoisted';

The above code looks like as below to the interpreter:

var message;
console.log(message);
message = 'The variable Has been hoisted';

What are classes in ES6

In ES6, Javascript classes are primarily syntactic sugar over JavaScript’s existing prototype-based inheritance. For example, the prototype based inheritance written in function expression as below:

function Bike(model, color) {
    this.model = model;
    this.color = color;
}

Bike.prototype.getDetails = function() {
    return this.model + ' bike has' + this.color + ' color';
};

Whereas ES6 classes can be defined as an alternative:

class Bike {
    constructor(color, model) {
        this.color = color;
        this.model = model;
    }

    getDetails() {
        return this.model + ' bike has ' + this.color + ' color';
    }
}

What are closures

A closure is the combination of a function and the lexical environment within which that function was declared. i.e, It is an inner function that has access to the outer or enclosing function’s variables. The closure has three scope chains:

  1. Own scope where variables defined between its curly brackets;
  2. Outer function’s variables;
  3. Global variables;

Let's take an example of closure concept:

function Welcome(name) {
    var greetingInfo = function(message) {
        console.log(message + name);
    }
	
    return greetingInfo;
}

var myFunction = Welcome('John');
myFunction('Welcome '); // output: Welcome John
myFunction('Hello Mr.'); // output: Hello Mr.John

As per the above code, the inner function (i.e, greetingInfo) has access to the variables in the outer function scope (i.e, Welcome) even after the outer function has returned.

What are modules

Modules refer to small units of independent, reusable code and also act as the foundation of many JavaScript design patterns. Most of the JavaScript modules export an object literal, a function, or a constructor.

Why do you need modules

Below are the list of benefits using modules in JS ecosystem:

  • Maintainability;
  • Reusability;
  • Namespacing;

What is scope in JS

Scope is the accessibility of variables, functions, and objects in some particular part of your code during runtime. In other words, scope determines the visibility of variables and other resources in areas of your code.

What is a Service Worker

A Service Worker is basically a script (JavaScript file) that runs in the background, separate from a web page and provides features that don't need a web page or user interaction. Some of the major features of service workers are Rich offline experiences (offline first web application development), periodic background syncs, push notifications, intercept and handle network requests and programmatically managing a cache of responses.

How do you manipulate DOM using a Service Worker

Service Worker can't access the DOM directly. But it can communicate with the pages it controls by responding to messages sent via the postMessage interface, and those pages can manipulate the DOM.

How do you reuse information across Service Worker restarts

The problem with Service Worker is that it gets terminated when not in use, and restarted when it's next needed, so you cannot rely on global state within a Service Worker's onfetch and onmessage handlers. In this case, Service Workers will have access to IndexedDB API in order to persist and reuse across restarts.

What is IndexedDB

IndexedDB is a low-level API for client-side storage of larger amounts of structured data, including files/blobs. This API uses indexes to enable high-performance searches of this data.

What is Web Storage

Web Storage is an API that provides a mechanism by which browsers can store key/value pairs locally within the user's browser, in a much more intuitive fashion than using cookies. The Web Storage provides two mechanisms for storing data on the client.

  • Local storage: It stores data for current origin with no expiration date.
  • Session storage: It stores data for one session and the data is lost when the browser tab is closed.

What is a post message

Post message is a method that enables cross-origin communication between Window objects. (i.e, between a page and a pop-up that it spawned, or between a page and an iframe embedded within it). Generally, scripts on different pages are allowed to access each other if and only if the pages follow same-origin policy (i.e, pages share the same protocol, port number, and host).

What is a cookie

A cookie is a piece of data that is stored on your computer to be accessed by your browser. Cookies are saved as key/value pairs. For example, you can create a cookie named username as below:

document.cookie = "username=John";

Why do you need a cookie

Cookies are used to remember information about the user profile (such as username). It basically involves two steps:

  • When a user visits a web page, the user profile can be stored in a cookie.
  • Next time the user visits the page, the cookie remembers the user profile.

What are the options in a cookie

There are few below options available for a cookie:

1. By default, the cookie is deleted when the browser is closed but you can change this behavior by setting expiry date (in UTC time):

document.cookie = "username=John; expires=Sat, 8 Jun 2019 12:00:00 UTC";

2. By default, the cookie belongs to a current page. But you can tell the browser what path the cookie belongs to using a path parameter:

document.cookie = "username=John; path=/services";

How do you delete a cookie

You can delete a cookie by setting the expiry date as a passed date. You don't need to specify a cookie value in this case. For example, you can delete a username cookie in the current page as below:

document.cookie = "username=; expires=Fri, 07 Jun 2019 00:00:00 UTC; path=/;";

Note: You should define the cookie path option to ensure that you delete the right cookie. Some browsers doesn't allow to delete a cookie unless you specify a path parameter.

What are the differences between cookie, local storage and session storage

Below are some of the differences between cookie, local storage and session storage:

What is the main difference between localStorage and sessionStorage

LocalStorage is the same as SessionStorage but it persists the data even when the browser is closed and reopened (i.e it has no expiration time) whereas in sessionStorage data gets cleared when the page session ends.

How do you access web storage

The Window object implements the WindowLocalStorage and WindowSessionStorage objects which has localStorage (window.localStorage) and sessionStorage (window.sessionStorage) properties respectively. These properties create an instance of the Storage object, through which data items can be set, retrieved and removed for a specific domain and storage type (session or local). For example, you can read and write on local storage objects as below:

localStorage.setItem('logo', document.getElementById('logo').value);
localStorage.getItem('logo');

What are the methods available on session storage

The session storage provided methods for reading, writing and clearing the session data:

// Save data to sessionStorage
sessionStorage.setItem('key', 'value');

// Get saved data from sessionStorage
let data = sessionStorage.getItem('key');

// Remove saved data from sessionStorage
sessionStorage.removeItem('key');

// Remove all saved data from sessionStorage
sessionStorage.clear();

What is a storage event and its event handler

The StorageEvent is an event that fires when a storage area has been changed in the context of another document. Whereas onstorage property is an EventHandler for processing storage events. The syntax would be as below:

window.onstorage = functionRef;

Let's take the example usage of onstorage event handler which logs the storage key and it's values:

window.onstorage = function(e) {
    console.log('The ' + e.key +
        ' key has been changed from ' + e.oldValue +
        ' to ' + e.newValue + '.');
};

Why do you need web storage

Web storage is more secure, and large amounts of data can be stored locally, without affecting website performance. Also, the information is never transferred to the server. Hence this is a more recommended approach than Cookies.

How do you check web storage browser support

You need to check browser support for localStorage and sessionStorage before using web storage:

if (typeof(Storage) !== "undefined") {
    // Code for localStorage/sessionStorage.
} else {
    // Sorry! No Web Storage support..
}

How do you check web workers browser support

You need to check browser support for web workers before using it:

if (typeof(Worker) !== "undefined") {
    // code for Web worker support.
} else {
    // Sorry! No Web Worker support..
}

Give an example of a web worker

You need to follow below steps to start using web workers for counting example.

1. Create a Web Worker File: You need to write a script to increment the count value. Let's name it as counter.js:

let i = 0;

function timedCount() {
    i = i + 1;
    postMessage(i);
    setTimeout("timedCount()", 500);
}

timedCount();

Here postMessage() method is used to post a message back to the HTML page.

2. Create a Web Worker Object: You can create a web worker object by checking for browser support. Let's name this file as web_worker_example.js:

if (typeof(w) == "undefined") {
    w = new Worker("counter.js");
}

and we can receive messages from web worker:

w.onmessage = function(event){
    document.getElementById("message").innerHTML = event.data;
};

3. Terminate a Web Worker: Web workers will continue to listen for messages (even after the external script is finished) until it is terminated. You can use the terminate() method to terminate listening to the messages:

w.terminate();

4. Reuse the Web Worker: If you set the worker variable to undefined you can reuse the code:

w = undefined;

What are the restrictions of web workers on DOM

WebWorkers don't have access to below JS objects since they are defined in an external files:

  • Window object;
  • Document object;
  • Parent object;

What is a promise

A promise is an object that may produce a single value some time in the future with either a resolved value or a reason that it’s not resolved (for example, network error). It will be in one of the 3 possible states: fulfilled, rejected, or pending.

The syntax of Promise creation looks like below:

const promise = new Promise(function(resolve, reject) {
    // promise description
})

The usage of a promise would be as below:

const promise = new Promise(resolve => {
    setTimeout(() => {
        resolve("I'm a Promise!");
    }, 5000);
}, reject => {

});

promise.then(value => console.log(value));

Why do you need a promise

Promises are used to handle asynchronous operations. They provide an alternative approach for callbacks by reducing the callback hell and writing the cleaner code.

What are the three states of promise

Promises have three states:

  • Pending: This is an initial state of the Promise before an operation begins;
  • Fulfilled: This state indicates that the specified operation was completed;
  • Rejected: This state indicates that the operation did not complete. In this case an error value will be thrown;

What is a callback function

A callback function is a function passed into another function as an argument. This function is invoked inside the outer function to complete an action. Let's take a simple example of how to use callback function:

function callbackFunction(name) {
    console.log('Hello ' + name);
}

function outerFunction(callback) {
    let name = prompt('Please enter your name.');
    callback(name);
}

outerFunction(callbackFunction);

Why do we need callbacks

The callbacks are needed because JS is an event driven language. That means instead of waiting for a response JS will keep executing while listening for other events. Let's take an example with the first function invoking an API call (simulated by setTimeout) and the next function which logs the message.

function firstFunction() {
    // Simulate a code delay
    setTimeout(function() {
        console.log('First function called');
    }, 1000 );
}

function secondFunction() {
    console.log('Second function called');
}

firstFunction();
secondFunction();

// Output
// Second function called
// First function called

As observed from the output, JS didn't wait for the response of the first function and the remaining code block got executed. So callbacks are used in a way to make sure that certain code doesn’t execute until the other code finishes execution.

What is a callback hell

Callback Hell is an anti-pattern with multiple nested callbacks which makes code hard to read and debug when dealing with asynchronous logic. The callback hell looks like below:

async1(function() {
    async2(function() {
        async3(function() {
            async4(function() {
                ....
            });
        });
    });
});

What are server-sent events

Server-sent events (SSE) is a server push technology enabling a browser to receive automatic updates from a server via HTTP connection without resorting to polling. These are a one way communications channel - events flow from server to client only. This has been used in Facebook/Twitter updates, stock price updates, news feeds, etc.

How do you receive server-sent event notifications

The EventSource object is used to receive server-sent event notifications. For example, you can receive messages from server as below:

if(typeof(EventSource) !== "undefined") {
    var source = new EventSource("sse_generator.js");
    
    source.onmessage = function(event) {
        document.getElementById("output").innerHTML += event.data + "<br>";
    };
}

How do you check browser support for server-sent events

You can perform browser support for server-sent events before using it as below:

if (typeof(EventSource) !== "undefined") {
    // Server-sent events supported. Let's have some code here!
} else {
    // No server-sent events supported
}

What are the events available for server sent events

Below are the list of events available for server sent events:

  • onopen - It is used when a connection to the server is opened;
  • onmessage - This event is used when a message is received;
  • onerror - It happens when an error occurs;

What are the main rules of promise

A promise must follow a specific set of rules:

  • A promise is an object that supplies a standard-compliant .then() method;
  • A pending promise may transition into either fulfilled or rejected state;
  • A fulfilled or rejected promise is settled and it must not transition into any other state;
  • Once a promise is settled, the value must not change;

What is callback in callback

You can nest one callback inside in another callback to execute the actions sequentially one by one. This is known as callbacks in callbacks.

loadScript('/script1.js', function(script) {
    console.log('first script is loaded');

    loadScript('/script2.js', function(script) {
        console.log('second script is loaded');

        loadScript('/script3.js', function(script) {
            console.log('third script is loaded');
            // after all scripts are loaded
        });
    });
});

What is promise chaining

The process of executing a sequence of asynchronous tasks one after another using promises is known as Promise chaining. Let's take an example of promise chaining for calculating the final result:

new Promise(function(resolve, reject) {
    setTimeout(() => resolve(1), 1000);
}).then(function(result) {
    console.log(result); // 1
    return result * 2;
}).then(function(result) {
    console.log(result); // 2
    return result * 3;
}).then(function(result) {
    console.log(result); // 6
    return result * 4;
});

In the above handlers, the result is passed to the chain of .then() handlers with the below work flow:

  1. The initial promise resolves in 1 second;
  2. After that .then handler is called by logging the result(1) and then return a promise with the value of result * 2;
  3. After that the value passed to the next .then handler by logging the result(2) and return a promise with result * 3;
  4. Finally the value passed to the last .then handler by logging the result(6) and return a promise with result * 4;

What is a strict mode in JS

 

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This way it prevents certain actions from being taken and throws more exceptions. The literal expression "use strict"; instructs the browser to use the JS code in the Strict mode.

Why do you need strict mode

Strict mode is useful to write "secure" JavaScript by notifying "bad syntax" into real errors. For example, it eliminates accidentally creating a global variable by throwing an error and also throws an error for assignment to a non-writable property, a getter-only property, a non-existing property, a non-existing variable, or a non-existing object.

What is the purpose of double exclamation

The double exclamation or negation (!!) ensures the resulting type is a boolean. If it was falsey (e.g. 0, null, undefined, etc.), it will be false, otherwise, true. For example, you can test IE version using this expression as below:

let isIE8 = false;
isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);
console.log(isIE8); // returns true or false

If you don't use this expression then it returns the original value.

console.log(navigator.userAgent.match(/MSIE 8.0/));  // returns either an Array or null

Note: The expression !! is not an operator, but it is just twice of ! operator.

What is the purpose of the delete operator

The delete keyword is used to delete the property as well as its value.

var user= {name: "John", age: 20};
delete user.age;

console.log(user); // {name: "John"}

What is the typeof operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable. It returns the type of a variable or an expression.

typeof "John Abraham";  // Returns "string"
typeof (1 + 2);         // Returns "number"

What is undefined property

The undefined property indicates that a variable has not been assigned a value, or not declared at all. The type of undefined value is undefined too.

var user;                   // Value is undefined, type is undefined
console.log(typeof(user));  //undefined

Any variable can be emptied by setting the value to undefined:

user = undefined;

What is null value

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values. The type of null value is object. You can empty the variable by setting the value to null.

var user = null;
console.log(typeof(user));  // object

What is the difference between null and undefined

Null:

  • It is an assignment value which indicates that variable points to no object.
  • Type of null is object.
  • The null value is a primitive value that represents the null, empty, or non-existent reference.
  • Indicates the absence of a value for a variable.
  • Converted to zero (0) while performing primitive operations.

Undefined:

  • It is not an assignment value where a variable has been declared but has not yet been assigned a value.
  • Type of undefined is undefined.
  • The undefined value is a primitive value used when a variable has not been assigned a value.
  • Indicates absence of variable itself.
  • Converted to NaN while performing primitive operations.

What is eval

The eval() function evaluates JavaScript code represented as a string. The string can be a JavaScript expression, variable, statement, or sequence of statements.

console.log(eval('1 + 2'));  // 3

What is the difference between window and document

Window:

  • It is the root level element in any web page;
  • By default window object is available implicitly in the page;
  • It has methods like alert(), confirm() and properties like document, location;

Document:

  • It is the direct child of the window object. This is also known as Document Object Model (DOM);
  • You can access it via window.document or document;
  • It provides methods like getElementById, getElementByTagName, createElement, etc;

How do you access history in JS

The window.history object contains the browser's history. You can load previous and next URLs in the history using back() and next() methods.

function goBack() {
    window.history.back()
}

function goForward() {
    window.history.forward()
}

Note: You can also access history without window prefix.

How do you detect caps lock key turned on or not

The mouseEvent getModifierState() is used to return a boolean value that indicates whether the specified modifier key is activated or not. The modifiers such as CapsLock, ScrollLock and NumLock are activated when they are clicked, and deactivated when they are clicked again.

Let's take an input element to detect the CapsLock on/off behavior with an example:

<input type="password" onmousedown="enterInput(event)">

<p id="feedback"></p>

<script>
    function enterInput(e) {
        var flag = e.getModifierState("CapsLock");
  
        if (flag) {
            document.getElementById("feedback").innerHTML = "CapsLock activated";
        } else {
            document.getElementById("feedback").innerHTML = "CapsLock not activated";
        }
    }
</script>

What is isNaN

The isNaN() function is used to determine whether a value is an illegal number (Not-a-Number) or not. i.e, This function returns true if the value equates to NaN. Otherwise it returns false.

isNaN('Hello'); // true
isNaN('100');   // false

What are the differences between undeclared and undefined variables

undeclared:

  • These variables do not exist in a program and are not declared;
  • If you try to read the value of an undeclared variable, then a runtime error is encountered;

undefined:

  • These variables declared in the program but have not assigned any value;
  • If you try to read the value of an undefined variable, an undefined value is returned;

What are global variables

Global variables are those that are available throughout the length of the code without any scope. The var keyword is used to declare a local variable but if you omit it then it will become global variable:

msg = "Hello";  // var is missing, it becomes global variable

What are the problems with global variables

The problem with global variables is the conflict of variable names of local and global scope. It is also difficult to debug and test the code that relies on global variables.

What is NaN property

The NaN property is a global property that represents "Not-a-Number" value. i.e, It indicates that a value is not a legal number. It is very rare to use NaN in a program but it can be used as return value for few cases.

Math.sqrt(-1);
parseInt("Hello");

What is the purpose of isFinite function

The isFinite() function is used to determine whether a number is a finite, legal number. It returns false if the value is +infinity, -infinity, or NaN (Not-a-Number), otherwise it returns true.

isFinite(Infinity);  // false
isFinite(NaN);       // false
isFinite(-Infinity); // false

isFinite(100);       // true

What is an event flow

Event flow is the order in which event is received on the web page. When you click an element that is nested in various other elements, before your click actually reaches its destination, or target element, it must trigger the click event for each of its parent elements first, starting at the top with the global window object. There are two ways of event flow:

  • Top to Bottom (Event Capturing)
  • Bottom to Top (Event Bubbling)

What is event bubbling

Event bubbling is a type of event propagation where the event first triggers on the innermost target element, and then successively triggers on the ancestors (parents) of the target element in the same nesting hierarchy till it reaches the outermost DOM element.

What is event capturing

Event capturing is a type of event propagation where the event is first captured by the outermost element, and then successively triggers on the descendants (children) of the target element in the same nesting hierarchy till it reaches the innermost DOM element.

How do you submit a form using JavaScript

You can submit a form using document.forms[0].submit(). All the form input's information is submitted using onsubmit event handler.

function submit() {
    document.forms[0].submit();
}

How do you find operating system details

The window.navigator object contains information about the visitor's browser OS details. Some of the OS properties are available under platform property:

console.log(navigator.platform);  // Win32

What is the difference between document load and DOMContentLoaded events

The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for assets (stylesheets, images, and subframes) to finish loading. Whereas The load event is fired when the whole page has loaded, including all dependent resources (stylesheets, images).

What is the difference between native, host and user objects

Native objects are objects that are part of the JavaScript language defined by the ECMAScript specification. For example, String, Math, RegExp, Object, Function etc core objects defined in the ECMAScript spec. Host objects are objects provided by the browser or runtime environment (Node). For example, window, XmlHttpRequest, DOM nodes etc are considered as host objects. User objects are objects defined in the javascript code. For example, User objects created for profile information.

What are the tools or techniques used for debugging JavaScript code

You can use below tools or techniques for debugging JS:

  • Chrome Devtools
  • debugger statement
  • Good old console.log statement

What are the pros and cons of promises over callbacks

Pros:

  • It avoids callback hell which is unreadable
  • Easy to write sequential asynchronous code with .then()
  • Easy to write parallel asynchronous code with Promise.all()
  • Solves some of the common problems of callbacks (call the callback too late, too early, many times and swallow errors/exceptions)

Cons:

  • It makes little complex code
  • You need to load a polyfill if ES6 is not supported

What is the difference between an attribute and a property

Attributes are defined on the HTML markup whereas properties are defined on the DOM. For example, the below HTML element has 2 attributes type and value:

<input type="text" value="Name:">

You can retrieve the attribute value as below:

const input = document.querySelector('input');
console.log(input.getAttribute('value')); // Good morning
console.log(input.value); // Good morning

And after you change the value of the text field to "Good evening", it becomes like:

console.log(input.getAttribute('value')); // Good morning
console.log(input.value); // Good evening

What is same-origin policy

The same-origin policy is a policy that prevents JavaScript from making requests across domain boundaries. An origin is defined as a combination of URI scheme, hostname, and port number. If you enable this policy then it prevents a malicious script on one page from obtaining access to sensitive data on another web page using Document Object Model (DOM).

What is the purpose of void 0

Void(0) is used to prevent the page from refreshing. This will be helpful to eliminate the unwanted side-effect, because it will return the undefined primitive value. It is commonly used for HTML documents that use href="JavaScript:Void(0);" within an <a> element. i.e, when you click a link, the browser loads a new page or refreshes the same page. But this behavior will be prevented using this expression. For example, the below link notify the message without reloading the page:

<a href="JavaScript:void(0);" onclick="alert('Well done!')">Click Me!</a>

Is JavaScript a compiled or interpreted language

JavaScript is an interpreted language, not a compiled language. An interpreter in the browser reads over the JavaScript code, interprets each line, and runs it. Nowadays modern browsers use a technology known as Just-In-Time (JIT) compilation, which compiles JavaScript to executable bytecode just as it is about to run.

Is JavaScript a case-sensitive language

Yes, JavaScript is a case sensitive language. The language keywords, variables, function & object names, and any other identifiers must always be typed with a consistent capitalization of letters.

What are events

Events are "things" that happen to HTML elements. When JavaScript is used in HTML pages, JavaScript can react on these events. Some of the examples of HTML events are:

  • Web page has finished loading
  • Input field was changed
  • Button was clicked

Let's describe the behavior of click event for button element:

<!doctype html>
<html>
    <head>
        <script>
            function greeting() {
            alert('Hello! Good morning');
        }
        </script>
    </head>
    <body>
        <button type="button" onclick="greeting()">Click me</button>
    </body>
</html>

What is the use of preventDefault method

The preventDefault() method cancels the event if it is cancelable, meaning that the default action or behaviour that belongs to the event will not occur. For example, prevent form submission when clicking on submit button and prevent opening the page URL when clicking on hyperlink are some common use cases.

document.getElementById("link").addEventListener("click", function(event) {
    event.preventDefault();
});

Note: Remember that not all events are cancelable.

What is the use of stopPropagation method

The stopPropagation method is used to stop the event from bubbling up the event chain. For example, the below nested divs with stopPropagation method prevents default event propagation when clicking on nested div(Div1).

<p>Click DIV1 Element</p>
<div onclick="secondFunc()">DIV 2
    <div onclick="firstFunc(event)">DIV 1</div>
</div>

<script>
    function firstFunc(event) {
        alert("DIV 1");
        event.stopPropagation();
    }

    function secondFunc() {
        alert("DIV 2");
    }
</script>

What is BOM

The Browser Object Model (BOM) allows JavaScript to "talk to" the browser. It consists of the objects navigator, history, screen, location and document which are children of the window. The Browser Object Model is not standardized and can change based on different browsers.

What is the use of setTimeout

The setTimeout() method is used to call a function or evaluate an expression after a specified number of milliseconds. For example, let's log a message after 2 seconds using setTimeout method:

setTimeout(function() { console.log("Good morning"); }, 2000);

What is the use of setInterval

The setInterval() method is used to call a function or evaluate an expression at specified intervals (in milliseconds). For example, let's log a message after 2 seconds using setInterval method:

setInterval(function() { console.log("Good morning"); }, 2000);

Why is JavaScript treated as Single threaded

JavaScript is a single-threaded language. Because the language specification does not allow the programmer to write code so that the interpreter can run parts of it in parallel in multiple threads or processes. Whereas languages like Java, Go, C++ can make multi-threaded and multi-process programs.

What is an event delegation

Event delegation is a technique for listening to events where you delegate a parent element as the listener for all of the events that happen inside it.

For example, if you wanted to detect field changes in inside a specific form, you can use event delegation technique:

var form = document.querySelector('#registration-form');

// Listen for changes to fields inside the form
form.addEventListener('input', function (event) {
    // Log the field that was changed
    console.log(event.target);
}, false);

What is the purpose of clearTimeout method

The clearTimeout() function is used in javascript to clear the timeout which has been set by setTimeout()function before that. i.e, The return value of setTimeout() function is stored in a variable and it’s passed into the clearTimeout() function to clear the timer.

For example, the below setTimeout method is used to display the message after 3 seconds. This timeout can be cleared by the clearTimeout() method.

var msg;

function greeting() {
    alert('Good morning');
}

function start() {
    msg = setTimeout(greeting, 3000);
}

function stop() {
    clearTimeout(msg);
}

What is the purpose of clearInterval method

The clearInterval() function is used in javascript to clear the interval which has been set by setInterval() function. i.e, The return value returned by setInterval() function is stored in a variable and it’s passed into the clearInterval() function to clear the interval.

For example, the below setInterval method is used to display the message for every 3 seconds. This interval can be cleared by the clearInterval() method.

var msg;

function greeting() {
    alert('Good morning');
}

function start() {
    msg = setInterval(greeting, 3000);
}

function stop() {
    clearInterval(msg);
}

How do you redirect new page in JS

In vanilla JS, you can redirect to a new page using the location property of window object. The syntax would be as follows:

window.location.href = 'newPage.html';

How do you validate an email in JS

You can validate an email in JS using regular expressions. It is recommended to do validations on the server side instead of the client side. Because the JS can be disabled on the client side.

function validateEmail(email) {
    var re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    return re.test(String(email).toLowerCase());
}

How do you get the current url with JS

You can use window.location.href expression to get the current url path and you can use the same expression for updating the URL too. You can also use document.URL for read-only purposes but this solution has issues in FF.

console.log('location.href', window.location.href); // Returns full URL

What are the various url properties of location object

The below Location object properties can be used to access URL components of the page:

  • href - The entire URL
  • protocol - The protocol of the URL
  • host - The hostname and port of the URL
  • hostname - The hostname of the URL
  • port - The port number in the URL
  • pathname - The path name of the URL
  • search - The query portion of the URL
  • hash - The anchor portion of the URL

How do get query string values in JS

You can use URLSearchParams to get query string values in JS. Let's see an example to get the client code value from URL query string:

const urlParams = new URLSearchParams(window.location.search);
const clientCode = urlParams.get('clientCode');

How do you check if a key exists in an object

You can check whether a key exists in an object or not using three approaches:

1. Using in operator: You can use the in operator whether a key exists in an object or not:

"key" in obj

and if you want to check if a key doesn't exist, remember to use parenthesis:

!("key" in obj)

2. Using hasOwnProperty method: You can use hasOwnProperty to particularly test for properties of the object instance (and not inherited properties):

obj.hasOwnProperty("key")  // true

3. Using undefined comparison: If you access a non-existing property from an object, the result is undefined. Let’s compare the properties against undefined to determine the existence of the property:

const user = {
    name: 'John'
};

console.log(user.name !== undefined);     // true
console.log(user.nickName !== undefined); // false

How do you loop through or enumerate JS object

You can use the for-in loop to loop through JS object. You can also make sure that the key you get is an actual property of an object, and doesn't come from the prototype using hasOwnProperty method.

var object = {
    "k1": "value1",
    "k2": "value2",
    "k3": "value3"
};

for (var key in object) {
    if (object.hasOwnProperty(key)) {
        console.log(key + " -> " + object[key]); // k1 -> value1 ...
    }
}

What is an arguments object

The arguments object is an Array-like object accessible inside functions that contains the values of the arguments passed to that function. For example, let's see how to use arguments object inside sum function:

function sum() {
    var total = 0;

    for (var i = 0, len = arguments.length; i < len; ++i) {
        total += arguments[i];
    }

    return total;
}

sum(1, 2, 3);  // returns 6

How do you make first letter of the string in an uppercase

You can create a function which uses a chain of string methods such as charAt, toUpperCase and slice methods to generate a string with the first letter in uppercase.

function capitalizeFirstLetter(string) {
    return string.charAt(0).toUpperCase() + string.slice(1);
}

What are the pros and cons of for loop

The for-loop is a commonly used iteration syntax in JS.

Pros:

  • Works on every environment;
  • You can use break and continue flow control statements;

Cons:

  • Too verbose;
  • Imperative;
  • You might face one-by-off errors;

How do you display the current date in JS

You can use new Date() to generate a new Date object containing the current date and time. For example, let's display the current date in mm/dd/yyyy:

var today = new Date();
var dd = String(today.getDate()).padStart(2, '0');
var mm = String(today.getMonth() + 1).padStart(2, '0'); //January is 0!
var yyyy = today.getFullYear();

today = mm + '/' + dd + '/' + yyyy;
console.log(today);

How do you compare two date objects

You need to use date.getTime() method to compare date values instead of comparison operators (==, !=, ===, and !== operators):

var d1 = new Date();
var d2 = new Date(d1);
console.log(d1.getTime() === d2.getTime());  // True
console.log(d1 === d2);  // False

How do you check if a string starts with another string

You can use ECMAScript 6's String.prototype.startsWith() method to check if a string starts with another string or not. But it is not yet supported in all browsers. Let's see an example to see this usage:

"Good morning".startsWith("Good");  // true
"Good morning".startsWith("morning");  // false

How do you add a key value pair in JS

There are two possible solutions to add new properties to an object. Let's take a simple object to explain these solutions.

var object = {
    key1: value1,
    key2: value2
};

1. Using dot notation: This solution is useful when you know the name of the property:

object.key3 = "value3";

2. Using square bracket notation: This solution is useful when the name of the property is dynamically determined:

obj["key3"] = "value3";

Is the !-- notation represents a special operator

No, that's not a special operator. But it is a combination of 2 standard operators one after the other:

  • A logical not (!)
  • A prefix decrement (--)

At first, the value decremented by one and then tested to see if it is equal to zero or not for determining the truthy/falsy value.

How do you assign default values to variables

You can use the logical or operator || in an assignment expression to provide a default value. The syntax looks like as below:

var a = b || c;

As per the above expression, variable 'a' will get the value of 'c' only if 'b' is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise 'a' will get the value of 'b'.

Can we define properties for functions

Yes, we can define properties for functions because functions are also objects:

fn = function(x) {
    // Function code goes here
}

fn.name = "John";

fn.profile = function(y) {
    // Profile code goes here
}

What is the way to find the number of parameters expected by a function

You can use function.length syntax to find the number of parameters expected by a function. Let's take an example of sum function to calculate the sum of numbers:

function sum(num1, num2, num3, num4) {
    return num1 + num2 + num3 + num4;
}

sum.length;  // 4 is the number of parameters expected

What is a polyfill

A polyfill is a piece of JS code used to provide modern functionality on older browsers that do not natively support it. For example, Silverlight plugin polyfill can be used to mimic the functionality of an HTML Canvas element on Microsoft Internet Explorer 7.

What are break and continue statements

The break statement is used to "jump out" of a loop. i.e, It breaks the loop and continues executing the code after the loop.

for (i = 0; i < 10; i++) {
    if (i === 5) { break; }
    text += "Number: " + i + "<br>";
}

The continue statement is used to "jump over" one iteration in the loop. i.e, It breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

for (i = 0; i < 10; i++) {
    if (i === 5) { continue; }
    text += "Number: " + i + "<br>";
}

What are JS labels

The label statement allows us to name loops and blocks in JavaScript. We can then use these labels to refer back to the code later. For example, the below code with labels avoids printing the numbers when they are same:

var i, j;

loop1:
for (i = 0; i < 3; i++) {
    loop2:
    for (j = 0; j < 3; j++) {
        if (i === j) {
            continue loop1;
        }
        console.log('i = ' + i + ', j = ' + j);
    }
}

// Output is:
//   "i = 1, j = 0"
//   "i = 2, j = 0"
//   "i = 2, j = 1"

How do you generate random integers

You can use Math.random() with Math.floor() to return random integers. For example, if you want generate random integers between 1 to 10, the multiplication factor should be 10:

Math.floor(Math.random() * 10) + 1;  // returns a random integer from 1 to 10
Math.floor(Math.random() * 100) + 1;  // returns a random integer from 1 to 100

Note: Math.random() returns a random number between 0 (inclusive), and 1 (exclusive).

Can you write a random integers function to print integers with in a range

Yes, you can create a proper random function to return a random number between min and max (both included):

function randomInteger(min, max) {
    return Math.floor( Math.random() * (max - min + 1) ) + min;
}

randomInteger(1, 100); // returns a random integer from 1 to 100
randomInteger(1, 1000); // returns a random integer from 1 to 1000

What are the string methods available in Regular expression

Regular Expressions has two string methods: search() and replace().

The search() method uses an expression to search for a match, and returns the position of the match.

var msg = "Hello John";
var n = msg.search(/John/i);  // 6

The replace() method is used to return a modified string where the pattern is replaced.

var msg = "Hello John";
var n = msg.replace(/John/i, "Buttler");  // Hello Buttler

What are modifiers in regular expression

  • i - Perform case-insensitive matching;
  • g - Perform a global match rather than stops at first match;
  • m - Perform multiline matching;

Let's take an example of global modifier:

var text = "Learn JS one by one";
var pattern = /one/g;
var result = text.match(pattern);  // one,one

What are regular expression patterns

Regular Expressions provide a group of patterns in order to match characters. Basically they are categorized into 3 types:

1. Brackets. These are used to find a range of characters. For example, below are some use cases:

  • [abc]: Used to find any of the characters between the brackets (a,b,c)
  • [0-9]: Used to find any of the digits between the brackets
  • (a|b): Used to find any of the alternatives separated with |

2. Metacharacters. These are characters with a special meaning. For example, below are some use cases:

  • \d: Used to find a digit
  • \s: Used to find a whitespace character
  • \b: Used to find a match at the beginning or ending of a word

3. Quantifiers. These are useful to define quantities. For example, below are some use cases:

  • n+: Used to find matches for any string that contains at least one n
  • n*: Used to find matches for any string that contains zero or more occurrences of n
  • n?: Used to find matches for any string that contains zero or one occurrences of n

How do you search a string for a pattern

You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result:

var pattern = /you/;
console.log(pattern.test("How are you?"));  // true

What is the purpose of exec method

The purpose of exec method is similar to test method but it executes a search for a match in a specified string and returns a result array, or null instead of returning true/false:

var pattern = /you/;
console.log(pattern.exec("How are you?"));  // ["you", index: 8, input: "How are you?", groups: undefined]

How do you change the style of a HTML element

You can change inline style or classname of a HTML element using JS.

1. Using style property. You can modify inline style using style property.

document.getElementById("title").style.fontSize = "30px";

2. Using ClassName property. It is easy to modify element class using className property.

document.getElementById("title").className = "custom-title";

What would be the result of 1+2+'3'

The output is going to be 33. Since 1 and 2 are numeric values, the result of the first two digits is going to be a numeric value 3. The next digit is a string type value because of that the addition of numeric value 3 and string type value 3 is just going to be a concatenation value 33.

What is a debugger statement

The debugger statement invokes any available debugging functionality, such as setting a breakpoint. If no debugging functionality is available, this statement has no effect. For example, in the below function a debugger statement has been inserted. So execution is paused at the debugger statement just like a breakpoint in the script source.

function getProfile() {
// code goes here
debugger;
// code goes here
}

What is the purpose of breakpoints in debugging

You can set breakpoints in the JS code once the debugger statement is executed and the debugger window pops up. At each breakpoint, JS will stop executing, and let you examine the JavaScript values. After examining values, you can resume the execution of code using the play button.

Can I use reserved words as identifiers

No, you cannot use the reserved words as variables, labels, object or function names. Let's see one simple example:

var else = "hello";  // Uncaught SyntaxError: Unexpected token else

How do you detect a mobile browser

You can use regex which returns a true or false value depending on whether or not the user is browsing with a mobile.

window.mobilecheck = function() {
    var mobileCheck = false;

    (function(a){if(/(android|bb\d+|meego).+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|ip(hone|od)|iris|kindle|lge |maemo|midp|mmp|mobile.+firefox|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce|xda|xiino/i.test(a)||/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|yas\-|your|zeto|zte\-/i.test(a.substr(0,4))) mobileCheck = true;})(navigator.userAgent||navigator.vendor||window.opera);

    return mobileCheck;
};

How do you detect a mobile browser without regexp

You can detect mobile browsers by simply running through a list of devices and checking if the useragent matches anything. This is an alternative solution for RegExp usage:

function detectmob() {
    if( navigator.userAgent.match(/Android/i)
    || navigator.userAgent.match(/webOS/i)
    || navigator.userAgent.match(/iPhone/i)
    || navigator.userAgent.match(/iPad/i)
    || navigator.userAgent.match(/iPod/i)
    || navigator.userAgent.match(/BlackBerry/i)
    || navigator.userAgent.match(/Windows Phone/i)
    ) {
        return true;
    } else {
        return false;
    }
}

Can you apply chaining on conditional operator

Yes, you can apply chaining on conditional operators similar to if ... else if ... else if ... else chain. The syntax is going to be as below:

function traceValue(someParam) {
    return condition1 ? value1
        : condition2 ? value2
        : condition3 ? value3
        : value4;
}

// The above conditional operator is equivalent to:

function traceValue(someParam) {
    if (condition1) { return value1; }
    else if (condition2) { return value2; }
    else if (condition3) { return value3; }
    else { return value4; }
}

What are the ways to execute JS after page load

You can execute JS after page load in many different ways.

1. window.onload

window.onload = function ...

2. document.onload

document.onload = function ...

3. body onload

<body onload="script();">

What is the difference between proto and prototype

The __proto__ object is the actual object that is used in the lookup chain to resolve methods, etc. Whereas prototype is the object that is used to build __proto__ when you create an object with new.

( new Employee ).__proto__ === Employee.prototype;
( new Employee ).prototype === undefined;

Give an example where do you really need semicolon

It is recommended to use semicolons after every statement in JavaScript. For example, in the below case it throws an error "... is not a function" at runtime due to missing semicolon.

// define a function
var fn = function () {
    // ...
} // semicolon missing at this line

// then execute some code inside a closure
(function () {
    // ...
})();

and it will be interpreted as:

var fn = function () {
    // ...
}(function () {
    // ...
})();

In this case, we are passing the second function as an argument to the first function and then trying to call the result of the first function call as a function. Hence, the second function will fail with a "... is not a function" error at runtime.

What is a freeze method

The freeze() method is used to freeze an object. Freezing an object does not allow adding new properties to an object, prevents from removing and prevents changing the enumerability, configurability, or writability of existing properties. i.e, It returns the passed object and does not create a frozen copy.

const obj = {
    prop: 100
};

Object.freeze(obj);
obj.prop = 200;  // Throws an error in strict mode

console.log(obj.prop);  //100

Note: It causes a TypeError if the argument passed is not an object.

What is the purpose of freeze method

Below are the main benefits of using freeze method:

  • It is used for freezing objects and arrays.
  • It is used to make an object immutable.

How do you detect JS disabled in the page

You can use the <noscript> tag to detect JS disabled or not. The code block inside <noscript> gets executed when JavaScript is disabled, and is typically used to display alternative content when the page generated in JavaScript.

<script type="javascript">
    // JS related code goes here
</script>
<noscript>
    <a href="next_page.html?noJS=true">JavaScript is disabled in the page. Please click Next Page</a>
</noscript>

What are various operators supported by JS

An operator is capable of manipulating (mathematical and logical computations) a certain value or operand. There are various operators supported by JavaScript as below:

  • Arithmetic Operators: Includes + (Addition), – (Subtraction), * (Multiplication), / (Division), % (Modulus), + + (Increment) and – – (Decrement)
  • Comparison Operators: Includes = = (Equal), != (Not Equal), === (Equal with type), > (Greater than), >= (Greater than or Equal to), < (Less than), <= (Less than or Equal to)
  • Logical Operators: Includes && (Logical AND), || (Logical OR), ! (Logical NOT)
  • Assignment Operators: Includes = (Assignment Operator), += (Add and Assignment Operator), –= (Subtract and Assignment Operator), *= (Multiply and Assignment), /= (Divide and Assignment), %= (Modules and Assignment)
  • Ternary Operators: It includes conditional (: ?) Operator
  • typeof Operator: It uses to find type of variable. The syntax looks like: typeof variable

What is a rest parameter

Rest parameter is an improved way to handle function parameters which allows us to represent an indefinite number of arguments as an array. The syntax would be as below:

function f(a, b, ...theArgs) {
    // ...
}

For example, let's take a sum example to calculate on dynamic number of parameters:

function total(...args) {
    let sum = 0;

    for(let i of args) {
        sum += i;
    }

    return sum;
}

console.log(total(1,2));  // 3
console.log(total(1,2,3));  // 6
console.log(total(1,2,3,4));  // 10
console.log(total(1,2,3,4,5));  // 15

Note: Rest parameter is added in ES2015 or ES6.

What happens if you do not use rest parameter as a last argument

The rest parameter should be the last argument, as its job is to collect all the remaining arguments into an array. For example, if you define a function like below it doesn’t make any sense and will throw an error.

function someFunc(a, ...b, c) {
    // You code goes here
    return;
}

What are the bitwise operators available in JS

Below are the list of bitwise logical operators used in JavaScript:

  • Bitwise AND ( & )
  • Bitwise OR ( | )
  • Bitwise XOR ( ^ )
  • Bitwise NOT ( ~ )
  • Left Shift ( << )
  • Sign Propagating Right Shift ( >> )
  • Zero fill Right Shift ( >>> )

What is a spread operator

Spread operator allows iterables ( arrays / objects / strings ) to be expanded into single arguments/elements. Let's take an example to see this behavior:

function calculateSum(x, y, z) {
    return x + y + z;
}

const numbers = [1, 2, 3];

console.log(calculateSum(...numbers));  // 6

How do you determine two values same or not using object

The Object.is() method determines whether two values are the same value. For example, the usage with different types of values would be:

Object.is('hello', 'hello');  // true
Object.is(window, window);  // true
Object.is([], []);  // false

Two values are the same if one of the following holds:

  • both undefined
  • both null
  • both true or both false
  • both strings of the same length with the same characters in the same order
  • both the same object (means both object have same reference)
  • both numbers and both +0 both -0 both NaN both non-zero and both not NaN and both have the same value

What is a Proxy object

The Proxy object is used to define custom behavior for fundamental operations such as property lookup, assignment, enumeration, function invocation, etc. The syntax would be as follows:

var p = new Proxy(target, handler);

Let's take an example of Proxy object:

var handler = {
    get: function(obj, prop) {
        return prop in obj ?
            obj[prop] :
            100;
    }
};

var p = new Proxy({}, handler);
p.a = 10;
p.b = null;

console.log(p.a, p.b);  // 10, null
console.log('c' in p, p.c);  // false, 100

In the above code, it uses get handler which define the behavior of the proxy when an operation is performed on it.

What is the purpose of seal method

The Object.seal() method is used to seal an object, by preventing new properties from being added to it and marking all existing properties as non-configurable. But values of present properties can still be changed as long as they are writable. Let's see the below example to understand more about seal() method:

const object = {
    property: 'Welcome JS world'
};
Object.seal(object);

object.property = 'Welcome to object world';
console.log(Object.isSealed(object));  // true

delete object.property;  // You cannot delete when sealed
console.log(object.property);  // Welcome to object world

What are the differences between freeze and seal methods

If an object is frozen using the Object.freeze() method then its properties become immutable and no changes can be made in them whereas if an object is sealed using the Object.seal() method then the changes can be made in the existing properties of the object.

How do you get enumerable key and value pairs

The Object.entries() method is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs, in the same order as that provided by a for...in loop. Let's see the functionality of object.entries() method in an example:

const object = {
    a: 'Good morning',
    b: 100
};

for (let [key, value] of Object.entries(object)) {
    console.log(`${key}: ${value}`);  // a: 'Good morning' b: 100
}

Note: The order is not guaranteed as object defined.

What is the main difference between Object.values and Object.entries method

The Object.values() method's behavior is similar to Object.entries() method but it returns an array of values instead [key, value] pairs.

const object = {
    a: 'Good morning',
    b: 100
};

for (let value of Object.values(object)) {
    console.log(`${value}`);  // 'Good morning' 100
}

How can you get the list of keys of any object

You can use the Object.keys() method which is used to return an array of a given object's own property names, in the same order as we get with a normal loop. For example, you can get the keys of a user object:

const user = {
    name: 'John',
    gender: 'male',
    age: 40
};

console.log(Object.keys(user));  // ['name', 'gender', 'age']

How do you create an object with prototype

The Object.create() method is used to create a new object with the specified prototype object and properties. i.e, It uses an existing object as the prototype of the newly created object. It returns a new object with the specified prototype object and properties.

const user = {
    name: 'John',
    printInfo: function () {
        console.log(`My name is ${this.name}.`);
    }
};

const admin = Object.create(user);

admin.name = "Nick";  // Remember that "name" is a property set on "admin" but not on "user" object

admin.printInfo();  // My name is Nick

What is a WeakSet

WeakSet is used to store a collection of weakly (weak references) held objects. The syntax would be as follows:

new WeakSet([iterable]);

Let's see the below example to explain it's behavior:

var ws = new WeakSet();
var user = {};

ws.add(user);
ws.has(user);    // true
ws.delete(user); // removes user from the set
ws.has(user);    // false, user has been removed

What are the differences between WeakSet and Set

The main difference is that references to objects in Set are strong while references to objects in WeakSet are weak. i.e, An object in WeakSet can be garbage collected if there is no other reference to it. Other differences are:

  • Sets can store any value whereas WeakSets can store only collections of objects;
  • WeakSet does not have size property unlike Set;
  • WeakSet does not have methods such as clear, keys, values, entries, forEach;
  • WeakSet is not iterable;

What is a WeakMap

The WeakMap object is a collection of key/value pairs in which the keys are weakly referenced. In this case, keys must be objects and the values can be arbitrary values. The syntax is looking like as below:

new WeakMap([iterable])

Let's see the below example to explain it's behavior:

var ws = new WeakMap();
var user = {};

ws.set(user);
ws.has(user);  // true
ws.delete(user);  // removes user from the map
ws.has(user);  // false, user has been removed

What are the differences between WeakMap and Map

The main difference is that references to key objects in Map are strong while references to key objects in WeakMap are weak. i.e, A key object in WeakMap can be garbage collected if there is no other reference to it. Other differences are:

  • Maps can store any key type whereas WeakMaps can store only collections of key objects;
  • WeakMap does not have size property unlike Map;
  • WeakMap does not have methods such as clear, keys, values, entries, forEach;
  • WeakMap is not iterable;

How do you encode an URL

The encodeURI() function is used to encode complete URI which has special characters except (, / ? : @ & = + $ #) characters.

var uri = 'https://mozilla.org/?x=шеллы';
var encoded = encodeURI(uri);
console.log(encoded);  // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B

How do you decode an URL

The decodeURI() function is used to decode a Uniform Resource Identifier (URI) previously created by encodeURI().

var uri = 'https://mozilla.org/?x=шеллы';
var encoded = encodeURI(uri);
console.log(encoded); // https://mozilla.org/?x=%D1%88%D0%B5%D0%BB%D0%BB%D1%8B

try {
    console.log(decodeURI(encoded));  // "https://mozilla.org/?x=шеллы"
} catch(e) {  // catches a malformed URI
    console.error(e);
}

How do you print the contents of web page

The window object provided a print() method which is used to print the contents of the current window. It opens a Print dialog box which lets you choose between various printing options. Let's see the usage of print method in an example:

<input type="button" value="Print" onclick="window.print()" />

Note: In most browsers, it will block while the print dialog is open.

What is an anonymous function

An anonymous function is a function without a name! Anonymous functions are commonly assigned to a variable name or used as a callback function. The syntax would be as below:

function (optionalParameters) {
    // do something
}

const myFunction = function() {  // Anonymous function assigned to a variable
    // do something
};

[1, 2, 3].map(function(element) {  // Anonymous function used as a callback function
    // do something
});

Let's see the above anonymous function in an example:

var x = function (a, b) { return a * b };
var z = x(5, 10);
console.log(z);  // 50

What is the precedence order between local and global variables

A local variable takes precedence over a global variable with the same name. Let's see this behavior in an example:

var msg = "Good morning";

function greeting() {
    msg = "Good Evening";
    console.log(msg);
}

greeting();  // Good Evening

What are JS accessors

ECMAScript 5 introduced JS object accessors or computed properties through getters and setters. Getters uses the get keyword whereas setters uses the set keyword.

var user = {
    firstName: "John",
    lastName : "Abraham",
    language : "en",
    get lang() {
        return this.language;
    }
    set lang(lang) {
        this.language = lang;
    }
};

console.log(user.lang);  // getter access lang as en
user.lang = 'fr';
console.log(user.lang);  // setter used to set lang as fr

What is an error object

An error object is a built in error object that provides error information when an error occurs. It has two properties: name and message. For example, the below function logs error details:

try {
    greeting("Welcome");
} catch(err) {
    console.log(err.name + " / " + err.message);
}

// ReferenceError / greeting is not defined

When you get a syntax error

A SyntaxError is thrown if you try to evaluate code with a syntax error. For example, the below missing quote for the function parameter throws a syntax error:

try {
    eval("greeting('welcome)");  // Missing ' will produce an error
} catch(err) {
    console.log(err.name);
}

What are the different error names from error object

There are 6 different types of error names returned from error object:

  • EvalError - An error has occurred in the eval() function
  • RangeError - An error has occurred with a number "out of range"
  • ReferenceError - An error due to an illegal reference
  • SyntaxError - An error due to a syntax error
  • TypeError - An error due to a type error
  • URIError - An error due to encodeURI()

What are the various statements in error handling

Below are the list of statements used in an error handling:

  • try: This statement is used to test a block of code for errors
  • catch: This statement is used to handle the error
  • throw: This statement is used to create custom errors
  • finally: This statement is used to execute code after try and catch regardless of the result

What is call stack

Call Stack is a data structure for JS interpreters to keep track of function calls in the program. It has two major actions:

  • Whenever you call a function for its execution, you are pushing it to the stack.
  • Whenever the execution is completed, the function is popped out of the stack.

Let's take an example:

function hungry() {
    eatFruits();
}

function eatFruits() {
    return "I'm eating fruits";
}

// Invoke the 'hungry' function
hungry();

The above code processed in a call stack as below:

  1. Add the hungry() function to the call stack list and execute the code.
  2. Add the eatFruits() function to the call stack list and execute the code.
  3. Delete the eatFruits() function from our call stack list.
  4. Delete the hungry() function from the call stack list since there are no items anymore.

What is an unary operator

The unary (+) operator is used to convert a variable to a number. If the variable cannot be converted, it will still become a number but with the value NaN. Let's see this behavior in an action:

var x = "100";
var y = + x;
console.log(y);  // 100
console.log(typeof x, typeof y);  // string, number

var a = "Hello";
var b = + a;
console.log(b);  // NaN
console.log(typeof a, typeof b, b);  // string, number, NaN

How do you sort elements in an array

The sort() method is used to sort the elements of an array in place and returns the sorted array. The example usage would be as below:

var months = ["Aug", "Sep", "Jan", "June"];
months.sort();
console.log(months);  //  ["Aug", "Jan", "June", "Sep"]

What is the purpose of compareFunction while sorting arrays

The compareFunction is used to define the sort order. If omitted, the array elements are converted to strings, then sorted according to each character's Unicode code point value. Let's take an example to see the usage of compareFunction:

let numbers = [1, 2, 5, 3, 4];
numbers.sort((a, b) => b - a);
console.log(numbers);  // [5, 4, 3, 2, 1]

How do you reversing an array

You can use the reverse() method to reverse the elements in an array. This method is useful to sort an array in descending order. Let's see the usage of reverse() method in an example:

let numbers = [1, 2, 5, 3, 4];
numbers.sort((a, b) => b - a);
numbers.reverse();
console.log(numbers);  // [1, 2, 3, 4 ,5]

How do you find min and max value in an array

You can use Math.min and Math.max methods on array variables to find the minimum and maximum elements within an array. Let's create two functions to find the min and max value with in an array:

var marks = [50, 20, 70, 60, 45, 30];

function findMin(arr) {
    return Math.min.apply(null, arr);
}

function findMax(arr) {
    return Math.max.apply(null, arr);
}

console.log(findMin(marks));  // 20
console.log(findMax(marks));  // 70

How do you find min and max values without Math functions

You can write functions which loop through an array comparing each value with the lowest value or highest value to find the min and max values. Let's create those functions to find min and max values:

var marks = [50, 20, 70, 60, 45, 30];

function findMin(arr) {
    var length = arr.length
    var min = Infinity;

    while (length--) {
        if (arr[length] < min) {
            min = arr[length];
        }
    }

    return min;
}

function findMax(arr) {
    var length = arr.length
    var max = -Infinity;

    while (length--) {
        if (arr[length] > max) {
            max = arr[length];
        }
    }

    return max;
}

console.log(findMin(marks));  // 20
console.log(findMax(marks));  // 70

What is a comma operator

The comma operator is used to evaluate each of its operands from left to right and returns the value of the last operand. This is totally different from comma usage within arrays, objects, and function arguments and parameters. For example, the usage for numeric expressions would be as below:

var x = 1;
x = (x++, x);

console.log(x);  // 2

What is the advantage of a comma operator

It is normally used to include multiple expressions in a location that requires a single expression. One of the common usages of this comma operator is to supply multiple parameters in a for loop. For example, the below for loop uses multiple expressions in a single location using comma operator:

for (var a = 0, b =10; a <= 10; a++, b--)

You can also use the comma operator in a return statement where it processes before returning:

function myFunction() {
    var a = 1;
    return (a += 10, a);  // 11
}

What is a constructor method

The constructor method is a special method for creating and initializing an object created within a class. If you do not specify a constructor method, a default constructor is used. The example usage of constructor would be as below:

class Employee {
    constructor() {
        this.name = "John";
    }
}

var employeeObject = new Employee();

console.log(employeeObject.name);  // John

What happens if you write constructor more than once in a class

The "constructor" in a class is a special method and it should be defined only once in a class. i.e, If you write a constructor method more than once in a class it will throw a SyntaxError error.

class Employee {
    constructor() {
        this.name = "John";
    }

    constructor() {  // Uncaught SyntaxError: A class may only have one constructor
        this.age = 30;
    }
}

var employeeObject = new Employee();

console.log(employeeObject.name);

How do you call the constructor of a parent class

You can use the super keyword to call the constructor of a parent class. Remember that super() must be called before using 'this' reference. Otherwise it will cause a reference error. Let's the usage of it:

class Square extends Rectangle {
    constructor(length) {
        super(length, length);
        this.name = 'Square';
    }

    get area() {
        return this.width * this.height;
    }

    set area(value) {
        this.area = value;
    }
}

What Is obfuscation in JS

Obfuscation is the deliberate act of creating obfuscated JS code (i.e, source or machine code) that is difficult for humans to understand. It is something similar to encryption, but a machine can understand the code and execute it. Let's see the below function before obfuscation:

function greeting() {
    console.log('Hello, welcome to JS world');
}

And after the code obfuscation, it would be appeared as below:

eval(function(p,a,c,k,e,d){e=function(c){return c};if(!''.replace(/^/,String)){while(c--){d[c]=k[c]||c}k=[function(e){return d[e]}];e=function(){return'\\w+'};c=1};while(c--){if(k[c]){p=p.replace(new RegExp('\\b'+e(c)+'\\b','g'),k[c])}}return p}('2 1(){0.3(\'4, 7 6 5 8\')}',9,9,'console|greeting|function|log|Hello|JS|to|welcome|world'.split('|'),0,{}))

Why do you need obfuscation

Below are the few reasons for obfuscation:

  • The Code size will be reduced. So data transfers between server and client will be fast
  • It hides the business logic from outside world and protects the code from others
  • Reverse engineering is highly difficult
  • The download time will be reduced

What is minification

Minification is the process of removing all unnecessary characters (empty spaces are removed) and variables will be renamed without changing it's functionality. It is also a type of obfuscation.

What are the advantages of minification

Normally it is recommended to use minification for heavy traffic and intensive requirements of resources. It reduces file sizes with below benefits:

  • Decreases loading times of a web page
  • Saves bandwidth usages

How do you perform form validation using JS

JavaScript can be used to perform HTML form validation. For example, if the form field is empty, the function needs to notify, and return false, to prevent the form being submitted. Lets' perform user login in an HTML form:

<form name="myForm" onsubmit="return validateForm()" method="post">
    User name: <input type="text" name="uname">
    <input type="submit" value="Submit">
</form>

And the validation on user login is below:

function validateForm() {
    var x = document.forms["myForm"]["uname"].value;

    if (x == "") {
        alert("The username shouldn't be empty");
        return false;
    }
}

How do you perform form validation without JS

You can perform HTML form validation automatically without using JS. The validation enabled by applying the required attribute to prevent form submission when the input is empty.

<form method="post">
    <input type="text" name="uname" required>
    <input type="submit" value="Submit">
</form>

Note: Automatic form validation does not work in Internet Explorer 9 or earlier.

Is enums feature available in JS

No, JS does not natively support enums. But there are different kinds of solutions to simulate them even though they may not provide exact equivalents. For example, you can use freeze or seal on object:

var DaysEnum = Object.freeze({"monday":1, "tuesday":2, "wednesday":3, ...});

What is an enum

An enum is a type restricting variables to one value from a predefined set of constants. JavaScript has no enums but TypeScript provides built-in enum support:

enum Color {
    RED, GREEN, BLUE
}

How do I modify the url without reloading the page

The window.location.url property will be helpful to modify the url but it reloads the page. HTML5 introduced the history.pushState() and history.replaceState() methods, which allow you to add and modify history entries, respectively. For example, you can use pushState as below:

window.history.pushState('page2', 'Title', '/page2.html');

Does JavaScript supports namespace

JavaScript doesn't support namespace by default. So if you create any element (function, method, object, variable) then it becomes global and pollutes the global namespace. Let's take an example of defining two functions without any namespace:

function func1() {
    console.log("This is a first definition");
}

function func1() {
    console.log("This is a second definition");
}

func1();  // This is a second definition

It always calls the second function definition. In this case, namespace will solve the name collision problem.

How do you load CSS and JS files dynamically

You can create both link and script elements in the DOM and append them as child to head tag. Let's create a function to add script and style resources as below:

function loadAssets(filename, filetype) {
    if (filetype == "css") {  // External CSS file
        var fileReference = document.createElement("link")
        fileReference.setAttribute("rel", "stylesheet");
        fileReference.setAttribute("type", "text/css");
        fileReference.setAttribute("href", filename);
    } else if (filetype == "js") {  // External JavaScript file
        var fileReference = document.createElement('script');
        fileReference.setAttribute("type", "text/javascript");
        fileReference.setAttribute("src", filename);
    }

    if (typeof fileReference != "undefined") {
        document.getElementsByTagName("head")[0].appendChild(fileReference);
    }
}

Why do we call JS as dynamic language

JavaScript is a loosely typed or a dynamic language because variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned/reassigned with values of all types.

let age = 50;  // age is a number now
age  = 'old';  // age is a string now
age  = true;   // age is a boolean

How to set the cursor to wait

The cursor can be set to wait in JavaScript by using the property "cursor". Let's perform this behavior on page load using the below function.

function myFunction() {
    window.document.body.style.cursor = "wait";
}

and this function invoked on page load:

<body onload="myFunction()">

How do you create an infinite loop

You can create infinite loops using for and while loops without using any expressions. The for loop construct or syntax is better approach in terms of ESLint and code optimizer tools:

for (;;) {}

while(true) {}

What are template literals

Template literals or template strings are string literals allowing embedded expressions. These are enclosed by the back-tick (`) character instead of double or single quotes. In E6, this feature enables using dynamic expressions as below:

var greeting = `Welcome to JS World, Mr. ${firstName} ${lastName}.`;

In ES5, you need break string like below:

var greeting = 'Welcome to JS World, Mr. ' + firstName + ' ' + lastName.`;

Note: You can use multi-line strings and string interpolation features with template literals.

What is the output of below spread operator array

[...'John Resig']

The output of the array is ['J', 'o', 'h', 'n', '', 'R', 'e', 's', 'i', 'g']

Explanation: The string is an iterable type and the spread operator within an array maps every character of an iterable to one element. Hence, each character of a string becomes an element within an array.

What is the difference between a parameter and an argument

Parameter is the variable name of a function definition, whereas an argument represents the value given to a function when it is invoked. Let's explain this with a simple function:

function myFunction (parameter1, parameter2, parameter3) {
    console.log(arguments[0]);  // "argument1"
    console.log(arguments[1]);  // "argument2"
    console.log(arguments[2]);  // "argument3"
}

myFunction("argument1", "argument2", "argument3");

What is nullish coalescing operator (??)

It is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand. This can be contrasted with the logical OR (||) operator, which returns the right-hand side operand if the left operand is any falsy value, not only null or undefined.

console.log(null ?? true);  // true
console.log(false ?? true);  // false
console.log(undefined ?? true);  // true