JavaScript interviews are not the easiest, and many candidates spent countless hours grinding through scattered coding questions to build confidence. While lots of practice makes all the difference, it also matters how you practice. Having a structured plan will help you master all the fundamental and advanced concepts that interviewers expect.
To get you familiar with the interview prep process and the questions you are expected to know, we’ve compiled an organized list of the need-to-know concepts alongside practical coding solutions.
Today, we will cover:
Master JavaScript coding interview patterns with our hands-on course today.
With thousands of potential questions to account for, preparing for the coding interview can feel like an impossible challenge. Yet with a strategic approach, coding interview prep doesn’t have to take more than a few weeks. Stop drilling endless sets of practice problems, and prepare more efficiently by learning coding interview patterns. This course teaches you the underlying patterns behind common coding interview questions. By learning these essential patterns, you will be able to unpack and answer any problem the right way — just by assessing the problem statement. This approach was created by FAANG hiring managers to help you prepare for the typical rounds of interviews at major tech companies like Apple, Google, Meta, Microsoft, and Amazon. Before long, you will have the skills you need to unlock even the most challenging questions, grok the coding interview, and level up your career with confidence. This course is also available in JavaScript, Python, Go, and C++ — with more coming soon!
Coding interviews are notoriously rigorous, and many candidates feel stumped on how to prepare. The hard truth is that there is no silver bullet for acing your JavaScript interview; it all comes down to how much and how well you prepare. Educative is well versed in Coding Interview prep, so we want to walk you through this tried and tested strategy for preparation. Let’s jump in.
You already know what programming language you will be using, so now you need to research what aspects of that language will be tested. There are three facets to this step:
Know what level you’re interviewing for. It’s unlikely that a Junior Dev position will be expected to know a lot about concurrency. It’s probably expected that a senior dev role will test your system design capabilities. Knowing ahead of time what level you are interviewing at will best prepare you to focus on the important stuff. We all know that these levels can be arbitrary, but being aware of how the company organizes will empower you.
Know what skills are tested. JavaScript is a vast language with multiple versions and a seemingly unlimited number of capabilities. But not all of these will be tested in an interview. It’s also important to understand what technical skills are even included in an interview in general. As you may know, for example, data structures and algorithms come up frequently in interviews, whereas basic language questions are less common.
Know what the organization expects. Some companies will expect different skills than others. Some companies focus on certain JavaScript tools more than others. You need to know what is expected of you from the organization. You can do this research on their company sites, job descriptions, or resources like CodingInterview.com
Cater your preparation to the organization, and you’ll save time and energy.
Many people miss this step and end up preparing in a scattered way. Making a plan early on ensures that you cover everything you need to, and it keeps you more motivated. In general, you will need three months to prepare for a coding interview.
There’s a lot to cover, and you don’t want to cram anything last minute. Set aside enough time to do so, and make a plan at the beginning of that period.
We’ve put together a beloved three-month, definitive interview prep roadmap that you can use to make your plan. This roadmap begins with brushing up on the basics, then moves to data structures and algorithms before diving into more complex concepts.
It’s important not to skip the initial refresher or practice. Many people fail to review basic JavaScript principles and end up stumped when it comes to the interview. Build from the basics to the complex stuff, and your learning will be more focused and natural.
A coding interview will be accompanied by a behavioral interview. This is where the interviewers get to know you as a person and see if your values fit to their organization. These interviews are very important for making employment decisions. Many candidates forget to prepare for behavioral interviews and focus solely on technical skills.
This is a big mistake.
While behavioral interviewing takes less time to prepare, it is still vital to your success and should be accounted for in your plans.
Now that you have a sense of how to prepare for your interviews, let’s dive into the most important JavaScript questions you can expect in an interview.
function Add(){console.log(answer)var answer = 2};Add()
In JavaScript, we can declare a variable after it has been used because variable declarations that use var
are hoisted to the top of their functional scope at compile time. This means that a variable can be initialized before it is declared. Let’s look at another example.
Note: Only the declarations get hoisted to the top, not the initializations.
var temp = "hi"function display(){var tempconsole.log(temp)temp = "bye"}display()
Here, var temp = ‘bye’
is a function scoped variable. Its declaration var temp
is hoisted to the top of the display( )
function at compile time. Since the value bye
is after line 4, the output is undefined.
Checking for balanced parentheses is a common question asked during JavaScript interviews. You may be asked a question like this:
Write a Parentheses Checker function to determine if the input string’s opening and closing brackets are properly nested.
function balancedParentheses(str) {let stack = [];let map = {'(': ')','[': ']','{': '}'}for (let i = 0; i < str.length; i++) {// If character is an opening brace add it to a stackif (str[i] === '(' || str[i] === '{' || str[i] === '[' ) {stack.push(str[i]);}//if closing brace, pop from stackelse {let lastEle = stack.pop();//Return false if the element popped doesn’t match the corresponding closing brace in the mapif (str[i] !== map[lastEle]) {return false};}}//if stack not empty at end, return falseif (stack.length !== 0) {return false};return true;}console.log(balancedParentheses("{[]()}" ));console.log(balancedParentheses("{[(])}"));console.log(balancedParentheses("{[}"));
The solution to this problem is simpler than it looks. First, we declare a stack at line 2, which holds all the opening parentheses. Then, we declare an object map
at lines 3-6. This holds the three types of opening parentheses and their closing parentheses.
We then traverse the string expression passed into the function at line 9. If the current character is open, we push it to the stack. If it is a closing bracket, we pop
it from the stack. If that character does not match the starting bracket, we tell the program to return false
. When we reach the end, any remaining open brackets in the stack
will return false
.
A common challenge in a JavaScript interview asks you to remove the first two elements of an array using array destructuring. Let’s look at the solution.
function removeFirstTwo(list) {const [, , ...arr] = list;return arr;}var arrLiteral = [8,9,10,11,12]console.log("arr contains: " + removeFirstTwo(arrLiteral))
Destructing an array uses a similar syntax as an array literal. On the left of the equation, we define which values we want to retrieve from the right hand side.
const [value1, value2] = arrLiteral
Let’s take a look at how that is stored.
var arrLiteral = [8,9,10,11,12]const [value1, value2] = arrLiteralconsole.log("value1 is: " + value1)console.log("value2 is: " + value2)
value1
stores the value in arrLiteral
, 8
, and value2
stores its corresponding value in arrLiteral
, 9
. We can skip the first two values using a ,
comma operator. Thus, value1
and value2
store the next two values, 10
and 11
.
We can then use the rest parameter arr
to collect the remaining values into an array. We return arr
at the end. Take a look here.
function removeFirstTwo(list) {
const [, , ...arr] = list;
return arr; // line 3 of original code
}
This common challenge asks you to destructure undefined
in JavaScript. Let’s say you input in the point
variable, and we want the output values of name
and age
properties. Take a look.
function pointValues(point){const {name:n,age:a} = {...point}console.log(n)console.log(a)}pointValues({name:"jerry", age:2})pointValues(undefined)
If it is an object, the code displays name
and age
values of point
, which will give an error of undefined
. We need to spread the point
into an object before we can destructure it.
const {name:n,age:a} = {...point}
We do that using {...point}
, which creates a new object using the properties of point
. Now, the new object will contain all the same values as a kind of copy. The values undefined
and null
are ignored. When undefined
is spread, no value is stored in the new object, so no error is thrown. Thus, we see undefined
when we access name
and age
.
There are many other JavaScript interview questions to test your knowledge of language basics. Some of those include:
createPhoneNumber
, that accepts an array of 10 integers (from 0-9) and returns a string of those numbers in the form of a phone number.coins
, write a function to compute the number of ways you can make that amount
using those coins.Function.call
and Function.apply
methods?returnNthCat
that takes a variable n
and has an object state
defined in it. Return the name
value of the n
th cat object.var names = ["Tom","Anna",2,true]console.log(names instanceof String)console.log(names instanceof Number)console.log(names instanceof Object)console.log(names instanceof Array)
The instanceof
operator checks if an operand is an instance of the object passed on the right or an instance of any of its ancestors. The answer false false true true
is correct because we are using an array, names
that contains the values of string, number, and boolean types.
names
is not an instance of the string or number, but rather an array that is the instance of Array
. In JavaScript, Array
is an object, so names
is also an instance of Object
.
A common problem tested looks like this:
Implement a function check
that takes an object and determines if it is an array or not. It should return either true
or false
.
Take a look at the answer:
function check(obj) {if (Object.prototype.toString.call(obj) === "[object Array]") {return true;} else {return false;}}console.log(check(123));console.log(check("cat"));console.log(check([1, 2, 3, 4]));
To check if an object is an array, we must use the Object.prototype.toString
method, which returns a string in the form of [object Type]
. If we call it on 123
, we return the string [object Number
. If we call it on {}
, we will return [object Object]
. If we call it on an array, we should return [object Array]
.
On line 2, see how we compare the returned string from Object.prototype.toString
with [object Array]
. If they match, we return true
, otherwise, false
.
This question, which tests your knowledge of instanceof
, is a bit trickier. Take a look at the code below:
function check(){var tempFunc = function () {}return new tempFunc instanceof Array;}console.log(check())
The check
function contains the definition of tempFunc
, and on line 4, we are checking if an instance of tempFunc
is an instance of Array
. Your task is to write code on line 3 so that the statement returns true
. Let’s break down the solution.
function check(){var tempFunc = function () {}tempFunc.prototype = Array.prototypereturn new tempFunc instanceof Array;}console.log(check())
We need to modify the code so that the new object becomes an instance of the array. We need to make a modification so that Array.prototype
is present in the prototype chain of tempFunc
.
tempFunc.prototype = Array.prototype //line 3
Now, we set the tempFunc.prototype
as equal to Array.prototype
. It should now return true
.
There are many other JavaScript interview questions to test your knowledge of type coercion. basics. Some of those include:
console.log(Object.prototype.toString.call(new (function Custom(){})));
typeof
operator?typeof
operator in JavaScriptprototype.toString
and arrow functionsfunction isPrototype(){var obj1 = {x: 1};var obj2;console.log(obj1.isPrototypeOf(obj2));}isPrototype()
You would be asked to write code for obj2
so that the statement returns true
on line 5. We need obj1
to become part of obj2
’s prototype chain. We can do this using the Object.create
function, which creates an object to store in variable obj2
.
Object.create
takes it as a parameter and returns an object with a prototype property that points to obj1
. This makes obj1
the prototype chain, so Object.getPrototypeOf(obj2)
will return as obj1
.
function isPrototype(){var obj1 = {x: 1};var obj2 = Object.create(obj1)console.log(obj1.isPrototypeOf(obj2));}isPrototype()
Master JavaScript coding interview patterns with our hands-on course today.
With thousands of potential questions to account for, preparing for the coding interview can feel like an impossible challenge. Yet with a strategic approach, coding interview prep doesn’t have to take more than a few weeks. Stop drilling endless sets of practice problems, and prepare more efficiently by learning coding interview patterns. This course teaches you the underlying patterns behind common coding interview questions. By learning these essential patterns, you will be able to unpack and answer any problem the right way — just by assessing the problem statement. This approach was created by FAANG hiring managers to help you prepare for the typical rounds of interviews at major tech companies like Apple, Google, Meta, Microsoft, and Amazon. Before long, you will have the skills you need to unlock even the most challenging questions, grok the coding interview, and level up your career with confidence. This course is also available in JavaScript, Python, Go, and C++ — with more coming soon!
ES6 skills are important in JavaScript interviews, as they show you are up to date with your JavaScript skills. Say you’re given the following code:
function Cat (name) {this.name = name}Cat.meow = function () {console.log(this.name + ' says meow')}let catty = new Cat('catty')catty.meow()
You would be asked to modify the code to resolve its error, but you can only use ES6 classes. The error in this code is because meow
is not defined on the prototype Cat
, so it is not inherited by the object catty
. We need to redefine it as a prototype but replacing the keyword function
with class
.
We can then define a constructor
inside the class to take name
as an argument. Once we do that, we can define a new method, meow
, inside the new class, so meow
is part of Cat
. It will then inherit all its methods, so catty.meow()
will not result in an error.
class Cat {constructor (name) {this.name = name}meow () {console.log(this.name + ' says meow')}}let catty = new Cat('catty')catty.meow()
Since JavaScript is a prototype-based inheritance language, you can expect questions like these to test your skills. Say you are given the following code:
function Human(name, age) {this.name = name;this.age = age;};function Man(name) {};function check(){var obj = new Man("Tommy Tan");console.log(obj.name)console.log(obj instanceof Human)}check()
You would be asked to implement inheritance between the class Human
and Man
. The code should compile successfully. Let’s look at the solution.
function Human(name, age) {this.name = name;this.age = age;};function Man(name,age) {Human.call(this, name, age);};Man.prototype = Object.create(Human.prototype);Man.prototype.constructor = Man;function check(){var obj = new Man("Tommy Tan",20);console.log(obj.name)console.log(obj instanceof Human)}check()
Human
is the parent, which the child Man
inherits. In order for Man
to inherit its properties, we must call the constructor of the parent in Man
’s constructor function. This will initialize members desired from the parent, name
and age
.
This process enables obj
to access name
property, so you will see name
instead of undefined
. Line 10 creates a prototype chain, which means that any Man
will have Human
as its prototype. Now, obj instanceof Human
will return true
.
Once we set Man.prototype
to Human
, its constructor pointer will point to it. We correct this by setting the constructor
property to point to Man on line 11, but this is optional to preserve the constructor property.
There are many other JavaScript interview questions to test your knowledge of Object Oriented programming JavaScript. Some of those include:
this
keywordsuper
keywordIf you want to learn more about Object Oriented programming in JavaScript, check out this guide for an introduction or refresher.
const addAndPrint = (a, b) => {const sum = a+b;console.log(`The sum is ${sum}`);return sum;};const ans = addAndPrint(4,5)
First, you must figure out why the function is impure. The addAndPrint
appears to have no negative side effects on an external state. However, the console.log
statement has a side effect that violates the rule that a result should not cause a side effect, such as a mutation of mutable objects. We must convert it to a pure function by removing console.log
.
Remember that any work a function performs that isn’t directly related to calculating a final output is a side effect!
const addAndPrint = (a, b) => {const sum = a+b;return sum;};const ans = addAndPrint(4,5)console.log("The sum is " + ans)
You can expect questions about shallow copying in JavaScript. Say you’re given the following code and asked for its output.
const girl = {name: 'Anna',info: { age: 20, number: 123 }};const newGirl = { ...girl };newGirl.info.age = 30;console.log(girl.info.age, newGirl.info.age);
The correct output is 30 30
. Why? The object girl
has properties name
and info
. We copy the properties of that object into newGirl
using the spread operator, which creates a shallow copy. Both share a reference because objects are passed through reference in JavaScript.
This means that when you assign an object to a variable, it is linked to that object’s identity. When we assign girl
to newGirl
, newGirl
points to the same object as girl
, so their properties will both change if altered.
Master JavaScript coding interview patterns with our hands-on course today.
With thousands of potential questions to account for, preparing for the coding interview can feel like an impossible challenge. Yet with a strategic approach, coding interview prep doesn’t have to take more than a few weeks. Stop drilling endless sets of practice problems, and prepare more efficiently by learning coding interview patterns. This course teaches you the underlying patterns behind common coding interview questions. By learning these essential patterns, you will be able to unpack and answer any problem the right way — just by assessing the problem statement. This approach was created by FAANG hiring managers to help you prepare for the typical rounds of interviews at major tech companies like Apple, Google, Meta, Microsoft, and Amazon. Before long, you will have the skills you need to unlock even the most challenging questions, grok the coding interview, and level up your career with confidence. This course is also available in JavaScript, Python, Go, and C++ — with more coming soon!
Questions about higher order functions are important for standing out in your JavaScript interview. You could expect a question like this.
Study the code below. Is func1
a higher order function?
const func1 = function(num){return function(){if(typeof num == 'NaN'){return "Not a number"}else{return typeof(num)}}}
Higher-order functions are those that accept functions as parameters or return a function as their output. When we look at the code, we can see that func1
does not take a function as a parameter. If we look harder at line 2, however, we see that it does return a function as its output. Therefore, yes, it is a higher order function.
There are many other questions in a JavaScript interview that test your knowledge of functional programming. Some of those questions and concepts may be:
maxCookies
that returns the maximum number of whole cookies that can be cooked from a recipeTo test your knowledge of JavaScript and DOM skills, you may be asked questions like,
What is the difference between feature detection and feature inference?
We can use feature detection and feature inference to determine if a web technology exists in the browser. Feature detection figures out if a feature exists in a specific browser. Feature inference assumes that if a feature is available in one browser, it is available in others. This question may be asked in a junior-level interview.
You could also be asked about attributes and properties. An attribute is written in the HTML document, in which the HTML tags may have attributes. When that document is covered to a DOM object model, the attributes are converted to properties.
You will also be expected to demonstrate your ability to implement DOM related functions, such as to hide and show text. For example, you would be asked to implement text hiding functionality, where clicking on the button hides the text Hello World!
. Take a look at the solution.
function hideShow() {var ele = document.getElementById("hideDiv");if (ele.style.display === "none") {ele.style.display = "block";} else {ele.style.display = "none";}}
We create a button
that executes the hideShow
function, and a div
with the text. We use getElementById
to access that div
element. On line 3, we check if the style.display
property is none
(aka. hidden). If true
, we show the element by setting the display
property to block
. If false
, we hide it by setting the display
value to none.
There are many other JavaScript interview questions to test your knowledge of DOM and web browser skills in JavaScript. Some of those include:
getElementByAttribute
Data structures and algorithms are a key part of all coding interviews. Interviewers will expect you to demonstrate mastery over these concepts and apply them to real-world challenges. Let’s go over a few of the most popular JavaScript data structures that you need to know.
Problem statement: Implement a function removeEven(arr)
, which takes an array arr in its input and removes all the even elements from a given array.
Input: An array of random integers
[1,2,4,5,10,6,3]
Output: an array containing only odd integers
[1,5,3]
function removeEven(arr) {var odds = []for (let number of arr) {if (number % 2 != 0) // Check if the item in the list is NOT even ('%' is the modulus symbol!)odds.push(number) //If it isn't even append it to the empty list}return odds // Return the new list}console.log(removeEven([3, 2, 41, 3, 34]))
This approach starts with the first element of the array. If that current element is not even, it pushes this element into a new array. If it is even, it will move to the next element, repeating until it reaches the end of the array. In regards to time complexity, since the entire array has to be iterated over, this solution is in O(n)O(n).
Problem statement: Implement the isBalanced()
function to take a string containing only curly {}
, square []
, and round ()
parentheses. The function should tell us if all the parentheses in the string are balanced. This means that every opening parenthesis will have a closing one. For example, {[]}
is balanced, but {[}]
is not.
Input: A string consisting solely of (
, )
, {
, }
, [
and ]
exp = "{[({})]}"
Output: Returns False
if the expression doesn’t have balanced parentheses. If it does, the function returns True
.
True
To solve this problem, we can simply use a stack of characters. Look below at the code to see how it works.
"use strict";module.exports = class Stack {constructor() {this.items = [];this.top = null;}getTop() {if (this.items.length == 0)return null;return this.top;}isEmpty() {return this.items.length == 0;}size() {return this.items.length;}push(element) {this.items.push(element);this.top = element;}pop() {if (this.items.length != 0) {if (this.items.length == 1) {this.top = null;return this.items.pop();} else {this.top = this.items[this.items.length - 2];return this.items.pop();}} elsereturn null;}}
This process will iterate over the string one character at a time. We can determine that the string is unbalanced based on two factors:
If either of these conditions is true, we return False
.
If the parenthesis is an opening parenthesis, it is pushed into the stack. If by the end all are balanced, the stack will be empty. If it is not empty, we return False
. Since we traverse the string exp only once, the time complexity is O(n).
Problem statement: Implement a function findBin(n)
, which will generate binary numbers from 1
to n
in the form of a string using a queue.
Input: A positive integer n
n = 3
Output: Returns binary numbers in the form of strings from 1
up to n
result = ["1","10","11"]
The easiest way to solve this problem is using a queue to generate new numbers from previous numbers. Let’s break that down.
"use strict";const Queue = require('./Queue.js');function findBin(n) {let result = [];let myQueue = new Queue();var s1, s2;myQueue.enqueue("1");for (var i = 0; i < n; i++) {result.push(myQueue.dequeue());s1 = result[i] + "0";s2 = result[i] + "1";myQueue.enqueue(s1);myQueue.enqueue(s2);}return result;}console.log(findBin(10))
The key is to generate consecutive binary numbers by appending 0 and 1 to previous binary numbers. To clarify,
Once we generate a binary number, it is then enqueued to a queue so that new binary numbers can be generated if we append 0 and 1 when that number will be enqueued. Since a queue follows the First-In First-Out property, the enqueued binary numbers are dequeued so that the resulting array is mathematically correct.
Look at the code above. On line 7, 1
is enqueued. To generate the sequence of binary numbers, a number is dequeued and stored in the array result
. On lines 11-12, we append 0
and 1
to produce the next numbers. Those new numbers are also enqueued at lines 14-15. The queue will take integer values, so it converts the string to an integer as it is enqueued.
The time complexity of this solution is in O(n)O(n) since constant-time operations are executed for n times.
Problem statement: Write the reverse
function to take a singly linked list and reverse it in place.
Input: a singly linked list
LinkedList = 0->1->2->3-4
Output: a reverse linked list
LinkedList = 4->3->2->1->0
The easiest way to solve this problem is by using iterative pointer manipulation. Let’s take a look.
"use strict";module.exports = class Node {constructor(data) {this.data = data;this.nextElement = null;}}
We use a loop to iterate through the input list. For a current
node, its link with the previous
node is reversed. then, next
stores the next node in the list. Let’s break that down by line.
current
node’s nextElement
in next
current
node’s nextElement
to previous
current
node the new previous
for the next iterationnext
to go to the next nodehead
pointer to point at the last nodeSince the list is traversed only once, the algorithm runs in O(n).
Problem statement: Use the findMin(root)
function to find the minimum value in a Binary Search Tree.
Input: a root node for a binary search tree
bst = {
6 -> 4,9
4 -> 2,5
9 -> 8,12
12 -> 10,14
}
where parent -> leftChild,rightChild
Output: the smallest integer value from that binary search tree
2
Let’s look at an easy solution for this problem
This solution begins by checking if the root is null
. It returns null
if so. It then moves to the left subtree and continues with each node’s left child until the left-most child is reached.
"use strict";module.exports = class Node {constructor(value) {this.val = value;this.leftChild = null;this.rightChild = null;}}
You can also expect to see questions that deal with security, even handling, and asynchronous callbacks in JavaScript. Let’s look at a few common concepts you will be expected to know.
func is not a function
errorcheckName
function to implement error checksCannot read property ‘text’ of undefined
error.then
and .catch
statements
print the numbers 0-9 in a sequential manner, so that each number is printed after a random waiting time.JavaScript interview prep takes a lot of time and hard work, but it’s worth it in the end! Javascript continues to be one of the most popular and high paying programming languages.
To help you prepare for interviews, Educative has created the unique course Grokking Coding Interview Patterns in JavaScript. Available in multiple languages, this course takes you through 100+ real-world questions like these. After each project, you’ll better understand what techniques you just applied.
This is coding interview prep reimagined, with your needs in mind.
Happy learning!
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