JavaScript Interview Questions

Stay in the know with the frequently asked JavaScript interview questions along with the best answers that will help you crack your next JavaScript interview and land your dream career as a JavaScript Developer. We have also compiled the questions on various topics like different ways to create an object in JS, Hoisting, callback, anonymous function and advanced topics like strict mode, closures, IIFE, etc. which you should be well versed in. Take your JavaScript proficiency to the next best level well with these expert-curated JavaScript interview questions and answers.

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Beginner

Boolean.

Null.

Undefined.

Number.

String.

Symbol (new in ECMAScript 6)

  • Using (.) dot operator - this operator is used when the property name is known at compile time or statically resolved
var hotel = {name:"Taj",city:"Delhi"}
hotel.name
  • Using [] square brackets - this syntax is used to access object property dynamically based on expression.
var hotel = {name:"Taj",city:"Delhi"}
var propertyName="city"
hotel[propertyName]
  •  Using object literal
  • Using constructor function
  • Using Object constructor
  • Using class (new in ES6)

 Yes. JS is a dynamically typed language and the array can contain mixed datatype based values as it elements.

var mixed=[1,true,"mix",[],{}]

 Yes. It is legal. As JS is dynamically typed language

Only one. Global scope is created as soon as JS process starts and lives till JS process dies. It is shared scope and must be used with caution

 Variables created with in function are local scoped variables and cannot be used outside function

As you see above xLocal is created inside a function and cannot be accessed outside.

Hoisting is a process where all declarations are moved to the top of innermost scope. Variables declarations and function declarations hoist in JavaScript 

hoisted(); 
function hoisted(){}

As you can see above-hoisted function is executed even before it is created. This works due to hoisting.

An anonymous function assigned to a variable is called a function expression.

var functionExpression = function() {
    console.log("this is anonymous")
}

 Wrap function in setInterval and pass the time argument as milliseconds.

setInterval(function(){
    console.log(new Date().toLocaleString())
},1000*60*5)

Callback is a function that is passed to another function and invoked in another function. In JS use callbacks to defer the function execution.

var callback = function(){
    console.log("I am callback")
}
setTimeout(callback,1000);

 setTimeout is a function that has received another function named callback as a parameter. And this function named callback will be executed by setTimeout after a second

== checks for value 

=== checks for value and type

1 < 10 && 10

Result is 10 

 && logical operator is short-handed which means in a complex operation it stops expression execution when it sees false. In above expressions is 1 < 10 is true and it continues to execute and sees 10 which true but it looks more like return value hence it is 10

Both undefined and null represents non-existence of data, whereas undefined is assigned by JavaScript when a variable is just declared but null  is used by programmers to set the non-existence of data

Using break statement you can quit the loop

Using type of operator.

typeof "this is string"
"string" 

A function created without a name is an anonymous function.

function(){
    console.log(new Date().toLocaleString())
}

Use Number function and isNaN functions together. Number functions create a number with a passed parameter. 

Example:

Number(10) will result in 10 whereas Number(“10a”) will result in NaN. 

isNaN(Number("10")) - false

when above expression results in false then given input is not a number

isNaN(Number("10a")) - true

when above expression results in false then given input is a number

One. Since exit condition check happens at the end of the loop even for an initial false condition for a do..while execution happens once.

  • > Greater than
  • < Less than
  • <= Less than or equal to
  • >= Greater than or equal to
  • == Equal to
  • != Not Equal to
  • === Equal to with datatype check
  • !== Not equal to with datatype check

Result: 10. Since asterisk (*) operator is used for multiplying numerics, JS will perform an implicit cast to convert strings to numbers. Since both strings can be converted to numbers it results in 10

Using delete keyword we can remove the properties from an object

Description

JavaScript is a very powerful client-side scripting language. It is mainly used for magnifying the interaction of a user with the webpage. You can also make your webpage more lively and interactive with the help of JavaScript. It is also being used widely in Mobile application development and Game development.
 

Over the last few years, JavaScript has taken over the world of web development and became the primary language of choice for many new developers.  It’s not just because JavaScript provides a superior way to make web pages interactive- but also because of the countless job opportunities it has created.
 

According to the US Bureau of Labor Statistics, web developer jobs are targeted to grow by 20% in 2012-2022.
 

According to Daxx.com, the average salary of a JavaScript Developer is $88,360 per year. Top companies around the world who are using JavaScript and JavaScript Integration are Airbnb, Instagram, Reddit, Slack, etc.
 

The possibility of these JavaScript Interview Questions being asked in a JavaScript interview is high. How will one be able to crack a JavaScript interview? How to get through the interview? Here, discussed below are the small steps that are needed to be taken at least to face a JavaScript Interview. These JavaScript interview questions and answers for experienced as well as freshers will help you attain the skills and confidence required to ace the interview.  Practice them well.
 

These common interview questions for JavaScript will help you to get through the JavaScript Interview and will help you in cracking your JavaScript Interview to acquire a dream career as a JavaScript Developer.

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