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How To Prepare For Javascript Interview

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Brush Up On Your Fundamentals

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Like any programming language, Javascript has its quirks and nuances that make it unique. Whats the difference between the different comparison operators? How does javascript work under the covers? Whats a promise? All of these and plenty of other JavaScript questions are available on plenty of different Github repos and are just a short google search away. These are generally good JavaScript questions to have ready under your belt to help you stay prepared and keep you on your guard.

What Are The Data Types Supported By Javascript

Ans. JavaScript variables are dynamically typed, which means there is a data type but it will not be bound to a particular type, For example, while initializing the variable it can be string type but later It can also assign to a numeric value.

There are two types of data types that are being supported which are primitive data types and non-primitive data types, below are some of the data types supported by JavaScript.

Difference Between Client Side Javascript And Server Side Javascript

Client-side JavaScript comprises the basic language and predefined objects which are relevant to running JavaScript in a browser. The client-side JavaScript is embedded directly by in the HTML pages. The browser interprets this script at runtime.

Server-side JavaScript also resembles client-side JavaScript. It has a relevant JavaScript which is to run in a server. The server-side JavaScript are deployed only after compilation.

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What Is The Need Of Tree Shaking

Tree Shaking can significantly reduce the code size in any application. i.e, The less code we send over the wire the more performant the application will be. For example, if we just want to create a Hello World Application using SPA frameworks then it will take around a few MBs, but by tree shaking it can bring down the size to just a few hundred KBs. Tree shaking is implemented in Rollup and Webpack bundlers.

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What Will Consolelog Consolelog Return

As a JavaScript Developer, How Can You Prepare For The Interview

You probably recognize these logical operators from an elementary math class years ago. However, they dont quite work the same in JavaScript.

The first statement returns true but the second returns false. This is because JavaScript reads from left to right, and evaluates expressions one-on-one.

For the first statement, it is true that one is less than two and two is less than one. Makes sense, right?

In the second statement, three is more than two, so youre still on the right track. But at this point, JavaScript returns the entire expression as true.

Now, the comparison is between true and one. Ironically, JavaScript does have a numerical value for true, which is equal to one.

However, one is definitely not more than one, making the entire statement false.

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What Is Strict Mode In Javascript

Strict mode is a feature of JavaScript ES5 to enforce stricter rules. The strict mode would cause code errors that would have been ignored or failed silently to generate errors. It is often good practice to use strict mode, though every use case is different. You, as the candidate should be able to explain the differences between using JavaScripts strict mode and not.

Strict mode is a mode in JavaScript to enforce stricter parsing and error handling on your JavaScript code.

The main benefit of using strict mode is catching errors early and making debugging your code easier. Common errors such as assigning a value to an undeclared variable would throw an error in strict mode alerting you that there is something wrong in your code.

You need to add the string use strict above the file to enable strict mode for that file.

Differences Between Declaring Variables Using Var Let And Const


Lets understand the differences with examples:

var variable1 =23 let variable2 =89 function catchValueswindow.variable1  // Returns the value 23window.variable2  // Returns undefined    

var vs let in functional scope

function varVsLetFunctionconsole.log  // Throws an errorconsole.log  // Throws an error

var let

console.log  // Outputs console.log  // Throws errorforconsole.log  // Throws errorforconsole.log // Outputs 2

var var let Const keyword const any variable declared with the const keyword cannot be reassigned.

const x =  x =   // Throws an ="Nikhil"  // No error is thrownconst y =23 y =44  // Throws an error


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Q41 What Is A Prompt Box In Javascript

A prompt box is a box which allows the user to enter input by providing a text box. The prompt method displays a dialog box that prompts the visitor for input. A prompt box is often used if you want the user to input a value before entering a page. When a prompt box pops up, the user will have to click either OK or Cancel to proceed after entering an input value.

How Do You Declare Strict Mode

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The strict mode is declared by adding “use strict” to the beginning of a script or a function.If declared at the beginning of a script, it has global scope.

"use strict" x=3.14 // This will cause an error because x is not declared

and if you declare inside a function, it has local scope

x=3.14 // This will not cause an error.myFunction functionmyFunction

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What Are The Properties Used To Get Size Of Window

You can use innerWidth, innerHeight, clientWidth, clientHeight properties of windows, document element and document body objects to find the size of a window. Let’s use them combination of these properties to calculate the size of a window or document,

varwidth=window.innerWidth||document.documentElement.clientWidth||document.body.clientWidth varheight=window.innerHeight||document.documentElement.clientHeight||document.body.clientHeight 

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What Is The Prototype Design Pattern

This is a different type of question compared to the others youve seen in this section. This is more conceptual, touching on design patterns, instead of discussing the features of JavaScript and how to perform a certain task. There are various design patterns used in software engineering, however since JavaScript is a prototypal language, a question about Prototype Design Pattern might be more relevant . An interviewer might ask this question to test your knowledge of design patterns in general and your familiarity with the prototype design pattern and how it could be used in the context of JavaScript.

The Prototype Design Pattern, also known as Properties Pattern is a creational design pattern based on prototypal inheritance. When an object is created, it acts as a prototype for other objects. You can think of the prototype object as a blueprint for other objects the constructor creates the properties defined in the prototype object will also be present in the cloned object it creates.

The prototype model is mainly used for creating objects in performance-intensive situations. The prototype pattern helps eliminate the overhead of initializing an object.

Common applications of the prototype pattern are when you have a system independent of how its contents are created or when creating objects from a database whose values are copied to the newly created object.

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Basic Javascript Interview Questions

If you dont recognize the answer to some of these questions, think about taking a basic JavaScript class.

1. What do you understand by JavaScript?

JavaScript is the most popular web scripting language, used for both client-side and server-side development. Supporting object-oriented programming abilities, the JavaScript code can be inserted into HTML pages that can be understood and executed by web browsers.

2. How is JavaScript different from Java?

Java is a programming language, whereas JavaScript is essentially a scripting language. Developers use Java for building applications that can run in a virtual machine, an operating system, or inside a browser. Contrastingly, JS code is meant to run inside a browser only.

Nonetheless, standalone desktop applications can be built with JavaScript by using Electron and Node.js. Another important distinction between Java and JS code is that while the former requires compilation, the latter is available in a text format.

3. Please explain the various JavaScript data types.

There are a total of 7 basic data types supported by JavaScript. Each one of them is briefed up as follows:

4. Could you enumerate the various features of JavaScript?

Some important features of JavaScript are:

5. Can you differentiate between let and var?

6. What are the escape characters in JavaScript?

Six additional escape characters are also available in JavaScript:

  • \b Backspace
  • \t Horizontal tabulator
  • \v Vertical tabulator

What Is The Purpose Of The Array Splice Method

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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,

letarrayIntegersOriginal1= letarrayIntegersOriginal2= letarrayIntegersOriginal3= letarrayIntegers1=arrayIntegersOriginal1.splice // returns   original array: letarrayIntegers2=arrayIntegersOriginal2.splice // returns   original array: letarrayIntegers3=arrayIntegersOriginal3.splice //returns   original array: 

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

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What Are The Pros And Cons Of Promises Over Callbacks

Below are the list of pros and cons of promises over callbacks,


  • 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
  • Cons:

  • It makes little complex code
  • You need to load a polyfill if ES6 is not supported
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    Q40 What Is The Strict Mode In Javascript And How Can It Be Enabled

    Strict mode is a way to introduce better error-checking into your code.

    • When you use strict mode, you cannot use implicitly declared variables, or assign a value to a read-only property, or add a property to an object that is not extensible.
    • You can enable strict mode by adding use strict at the beginning of a file, a program, or a function.

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    Q46 What Will Be The Output Of The Following Code

    var X =   var Output =   )  console.log 

    The output would be undefined. The delete operator is used to delete the property of an object. Here, x is an object which has the property foo, and as it is a self-invoking function, we will delete the foo property from object x. After doing so, when we try to reference a deleted property foo, the result is undefined.

    How Do You Print The Contents Of Web Page

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    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,

    < inputtype="button" value="Print" onclick="window.print" /> 

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

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    Thoroughly Understand The Tech Stack On Your Resume

    Not all technical interviews will be on algorithms, many times the interviewer wants to be sure that youre a competent coder and youll need to build out a part of an application. For example, if you list that you know React on your resume, you better know how to build a component and understand how to use React hooks.

    Come to the interview prepared to answer questions about the programming language or framework. Also make sure to use the correct terminology for terms and concepts.

    Can I Redeclare Let And Const Variables

    No, you cannot redeclare let and const variables. If you do, it throws below error

    Uncaught SyntaxError: Identifier 'someVariable' has already been declared

    Explanation: The variable declaration with var keyword refers to a function scope and the variable is treated as if it were declared at the top of the enclosing scope due to hoisting feature. So all the multiple declarations contributing to the same hoisted variable without any error. Let’s take an example of re-declaring variables in the same scope for both var and let/const variables.

    varname="John" functionmyFuncmyFunc alert // John

    The block-scoped multi-declaration throws syntax error,

    letname="John" functionmyFuncmyFunc alert 

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    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,

    letarrayIntegers= letarrayIntegers1=arrayIntegers.slice // returns letarrayIntegers2=arrayIntegers.slice // returns letarrayIntegers3=arrayIntegers.slice //returns 

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

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    What Are Default Values In Destructuring Assignment

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    A variable can be assigned a default value when the value unpacked from the array or object is undefined during destructuring assignment. It helps to avoid setting default values separately for each assignment. Let’s take an example for both arrays and object use cases,

    Arrays destructuring:

    varx,y,z = console.log // 10console.log // 4console.log // 6

    Objects destructuring:

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    What Is A Strict Mode In Javascript

    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 javascript code in the Strict mode.

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    Top 100 Javascript Interview Questions And Answers For 2022

    To build a JavaScript programming career, candidates need to crack the interview. They are asked for variousJavaScript interview questions and answers.

    Following is a list of JavaScript interview questions and answers, which are likely to be asked during the interview. Candidates are likely to be asked basic JavaScriptinterview questions to advance JS interviewquestions depending on their experience and various other factors.

    The below list covers all the JavaScriptquestions for freshers and JavaScriptinterviewquestions for professional-level candidates. This JS interviewquestions guide will help you crack the interview and help you get your dream job for JavaScript Programming.

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    What Are Imports And Exports In Javascript

    Imports and exports help in writing modular code for our JavaScript applications. With the help of imports and exports, we can split a JavaScript code into multiple files in a project. This greatly simplifies the application source code and encourages code readability.


    This file exports two functions that calculate the squares and diagonal of the input respectively.


    import from “calc”

    console.log) // 16

    console.log) // 5

    Therefore, here we import those functions and pass input to those functions to calculate square and diagonal.

    What Is An Observable

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    An Observable is basically a function that can return a stream of values either synchronously or asynchronously to an observer over time. The consumer can get the value by calling subscribe method.Let’s look at a simple example of an Observable

    importfrom"rxjs" constobservable=newObservable=> ,3000) }) observable.subscribe=> console.log) 

    Note: Observables are not part of the JavaScript language yet but they are being proposed to be added to the language

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    Have Your Elevator Pitch Down Pat

    During the first few minutes of the interview youll have the opportunity to get to know your interviewer and exchange pleasantries. Theyre going to ask you about yourself and you should have a polished and well-versed response. Beyond just testing your technical skills, theyll want to get an overall feel of your personality.

    This is your opportunity to build a rapport with the interviewer and possibly find something in common. Once you have some kind of rapport, itll automatically make you feel more at ease.

    What Are The Terms Bom And Dom In Javascript

    • BOM- BOM is a Browser Object Model, in which the window object is supported by all the browsers. The JavaScript objects, variables, and functions also become members of the window object. This model deals with the objects of browsers like location, navigator, history, and screen.
    • DOM- DOM is a Document Object Model in JavaScript that helps to access document and HTML elements. When a web page is loaded the browser creates a DOM for the page. All the objects on the page all arranged in order.

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    Explain The Difference Between Objectfreeze Vs Const

    This explains your flexibility detailed understanding when it comes to handling variables and functions.

    Example:”const and Object.freeze are two different functions in JavaScript.const applies to bindings variables. It creates an immutable binding, which means it is not possible to assign a new value to the binding.

    < em> const < span> human< /span> = < span> human< /span> = animal // ERROR < span> “person”< /span> is read-only< /em>

    Object.freeze works on object values. It makes an object immutable, which means changing its properties is not possible.

    < span> < em> let< /em> < /span> < em> human = Object.freeze = < span> “Rohan”< /span> //TypeError: Cannot assign < span> to< /span> read < span> only< /span> property < span> ‘name'< /span> < span> of< /span> objectconsole.log < /em>

    What Are The Various Types Of Errors In Javascript

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    Every developer is bound to run into errors when programming in any language. JavaScript is no different. Debugging and resolving these errors are usually part of every developers day. Identifying the different error types can significantly help the developer narrow down the problem. The candidate should be able to identify and differentiate between the errors described below when asked this important JS interview question.

    There are three types of errors in JavaScript:

    • Syntax errors Errors that occur at interpretation time such as during the loading of a web page due to incorrect syntax in the code.
    • Runtime errors Errors that occur during the runtime of the program after it is interpreted by the compiler. Calling functions that dont exist is a common cause of this type of error.
    • Logical Errors Errors caused by the codes logic itself. They are syntactically correct and dont necessarily cause runtime errors. To think of this in terms of a sentence in the English language the vocabulary and grammar of the sentence are correct, however, the meaning of the sentence is incorrect.

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