One of the most popular programming languages for making dynamic and interesting websites is JavaScript. Interviews for JavaScript can be difficult because of its broad reach. But these difficulties can be solved with careful planning. The interviewee ought to become acquainted with anything from the most fundamental ideas to the most intricate libraries and frameworks.
An interviewee can practise and prepare for their interview with the aid of these top 50 JavaScript interview questions and answers.
1. To find the sum of two numbers, write a JavaScript function.
The purpose of this question is to gauge the candidate’s foundational knowledge of JavaScript. They evaluate both their problem-solving abilities and comprehension of fundamental syntax. This aids in assessing the candidate’s attention to detail and coding style.
I would take two parameters and the following function can be used to calculate the sum of any 2 numbers that are passed as arguments.
function sumOfTwoNumbers(a, b) {
return a + b;
}
2. Create a JavaScript application that determines an array’s maximum value.
This question is used by a hiring manager to assess a candidate’s capacity for writing understandable and effective code. Candidates must demonstrate error-free code and walk through the code step-by-step.
function findMaxNumber(arr) {
return Math.max(…arr);
}
3. To determine whether a given string is a palindrome—one that reads the same way both forward and backward—write a JavaScript function.
The candidate’s knowledge of loop constructions, JavaScript string functions, and other fundamental JavaScript syntax is what the interviewer is searching for. Based on the method employed to answer the palindrome problem, they will assess the candidate’s abilities.
function isPalindrome(str) {
return str === str.split(”).reverse().join(”);
}
4. To reverse a given string, write a JavaScript program.
Employing managers are looking for a precise response that shows the candidate’s command of JavaScript.
const reverseString = (str) => str.split(”).reverse().join(”);
5. Create a JavaScript function that accepts a numeric array and outputs a new array containing only even integers.
Interviewers are searching for individuals who can demonstrate the ability to think rationally and clarify their thought processes in addition to being able to clearly describe the solution and the code.
By using the filter method on the array, I can check if each element is even or not by using the modulus operator (%) with 2. The element is even if the result is 0. This can be included in the new array.
function filterEvenNumbers(numbers) {
return numbers.filter(num => num % 2 === 0);
}
6. To find the factorial of a given number, write a JavaScript program.
Managers use this question to gauge a candidate’s grasp of JavaScript programming and algorithmic thinking. The interviewer anticipates that the candidate will show that they understand the factorial notion.
A factorial number is the product of all positive integers, which are equal to or less than the given number.
function factorial(number) {
if (number === 0 || number === 1) {
return 1;
} else {
return number * factorial(number – 1);
}
}
7. To determine whether a given integer is prime, write a JavaScript code.
The candidate’s understanding of mathematical ideas and JavaScript methods might be examined by interviewers. They anticipate that the applicant will convert a mathematical idea into useful code.
To check if a given number is prime, loop from 2 to the square root of the number. If any integer evenly divides it, the number is not prime.
function isPrime(num) {
if (num <= 1) return false;
for (let i = 2; i <= Math.sqrt(num); i++) {
if (num % i === 0) return false;
}
return true;
8. To determine the largest element in a nested array, write a JavaScript program.
Interviewers use this question to gauge a candidate’s proficiency with nested data structures and their application of conditional statements, arrays, and loops. Candidates are required to use what they have learnt in practical situations.
function findLargestElement(nestedArray) {
let largest = nestedArray[0][0];
for (let arr of nestedArray) {
for (let num of arr) {
if (num > largest) {
largest = num;
}
}
}
return largest;
}
9. Create a JavaScript function that, up to a specified number of words, returns the Fibonacci sequence.
Hiring managers can use this question to gauge an interviewee’s comprehension of basic JavaScript algorithms. They anticipate that the applicant will handle faults and take into account edge circumstances.
function fibonacciSequence(numTerms) {
if (numTerms <= 0) return [];
if (numTerms === 1) return [0];
let sequence = [0, 1];
while (sequence.length < numTerms) {
let nextNumber = sequence[sequence.length – 1] + sequence[sequence.length – 2];
sequence.push(nextNumber);
}
return sequence;
}
10. Create a JavaScript program that capitalises the initial letter of each word in a string to change it to title case.
Interviewers look at the candidate’s ability to deconstruct a problem into smaller, more manageable pieces and show that they understand looping, string manipulation, and the fundamentals of JavaScript.
function toTitleCase(str) {
return str.replace(/\b\w/g, l => l.toUpperCase());
}
Advanced JavaScript coding interview questions
Advanced JavaScript code involves a number of intricate ideas and methods. JavaScript interviews frequently test these fundamental ideas. Closure and scope, functional programming, prototypal inheritance, design patterns, memory management, ES6+ features, and many more are among the ideas.
1. Use JavaScript to implement a debounce function that restricts how frequently a function can execute when called repeatedly within a given time limit.
Interviewers anticipate that the candidate will demonstrate their ability to articulate the debounce function’s function and how it is used in situations where function calls must be managed. They are trying to find someone who can easily explain technical ideas.
By delaying the execution of the debounce function until the specified time frame has passed, the frequency can be limited.
function debounce(func, delay) {
let timer;
return function() {
clearTimeout(timer);
timer = setTimeout(func, delay);
};
}
2. Create a function that accepts a key and an array of objects, then returns a new array that is arranged in ascending order according to the key’s values.
Hiring managers assess a candidate’s ability to explain the sorting algorithm and its time complexity by asking this question. Additionally, candidates must show that their code is reliable.
The following function takes an array of objects and a key to sort the array based on the values in ascending order.
function sortByKey(arr, key) {
return arr.sort((a, b) => a[key] – b[key]);
}
3. Use JavaScript to implement a deep clone function that copies an array or nested object without referencing the original.
Employers seek to evaluate the candidate’s ability to manage intricate coding assignments and comprehend the idea of avoiding reference problems during cloning.
By using two methods together and creating a deep clone, I can serialize the object to a JSON string. I would then parse it back into a new object, thereby removing any reference to the original object.
function deepClone(obj) {
return JSON.parse(JSON.stringify(obj));
}
4. To determine the factorial of a given number, write a recursive function.
The candidate should create a succinct recursive function that manages edge cases, according to interviewers. In order to prevent infinite loops and stack overflow issues, candidates must demonstrate their comprehension of recursion.
function factorial(num) {
if (num <= 1) return 1;
return num * factorial(num – 1);
}
5. Without utilising any built-in sorting functions, create a function that takes two sorted arrays and combines them into a single sorted array.
Interviewers use this question to gauge candidates’ proficiency with algorithms and their ability to handle sorted data efficiently. They also search for the capacity to come up with and carry out the right answer.
I can implement a function that can efficiently merge two sorted arrays.
function mergeSortedArrays(arr1, arr2) {
return […arr1, …arr2].sort((a, b) => a – b);
}
6. Create a function that determines whether a given string is a palindrome by ignoring case and only taking into account alphanumeric characters.
Interviewers examine the interviewee’s manner of running code and show that they are comfortable with regular expressions, JavaScript string methods, case-sensitive and alphanumeric checks, and more.
function isPalindrome(str) {
const cleanStr = str.replace(/[^a-zA-Z0-9]/g, ”).toLowerCase();
const reversedStr = cleanStr.split(”).reverse().join(”);
return cleanStr === reversedStr;
7. Write a JavaScript class for a linked list that has methods to add nodes at the start, finish, or specified location, as well as methods to remove nodes from a specified location.
This question allows interviewers to assess a candidate’s problem-solving abilities as well as how well they can create and implement a class for a linked list.
I would implement a linked list with methods to insert a node at the beginning, end, and at specific positions. Then, I would delete a node from a given position.
8. Create a JavaScript method that flattens a nested array into a single-level array.
Managers are able to assess a candidate’s capacity for handling complex data structures and logical reasoning. Interviewees should show that they understand arrays, recursion, and loops.
const flattenArray = (nestedArray) => {
return nestedArray.flat(Infinity);
};
9. Create an algorithm that checks to see if two strings are anagrams of one another.
The purpose of this question is to gauge the candidate’s proficiency with proper string-related techniques and accurate anagram identification.
function areAnagrams(str1, str2) {
return str1.split(“”).sort().join(“”) === str2.split(“”).sort().join(“”);
}
10. Write a JavaScript function that, for optimal speed, uses memoization to return the Fibonacci sequence up to a specified integer.
Interviewees should demonstrate their mastery of OOP as well as their knowledge of memoization and recursion. They can also assess how well the candidate organises code and pays attention to details in class design.
By creating a function that uses an array to store the computed values, a Fibonacci sequence can be generated.
function fibonacciWithMemoization(n) {
let memo = [0, 1];
for (let i = 2; i <= n; i++) {
memo[i] = memo[i – 1] + memo[i – 2];
}
return memo;
}
Complex questions about JavaScript coding
Managers can evaluate critical thinking, JavaScript principles, and problem-solving abilities by posing challenging JavaScript code problems. These require the candidate to think creatively and logically in order to solve difficulties, and they go beyond syntactic knowledge.
1. Without utilising the built-in reverse() technique, create a function that flips a sentence’s word order.
In addition to evaluating candidates’ inventiveness, this question gives hiring managers insight into how effectively a candidate can provide a clear and intelligible reply.
function reverseSentence(sentence) {
const words = sentence.split(‘ ‘);
const reversedWords = words.reverse();
return reversedWords.join(‘ ‘);
}
2. Create a function that ignores punctuation and whitespace to determine whether a given text is a palindrome, or one that reads the same way both forward and backward.
The interviewee’s ability to use punctuation and whitespace effectively while adhering to the palindrome-checking logic can be evaluated by the interviewer. Candidates are required to demonstrate their understanding of regular expressions or any other effective strategy.
function isPalindrome(str) {
const cleanedStr = str.replace(/[^\w]/g, ”).toLowerCase();
const reversedStr = cleanedStr.split(”).reverse().join(”);
return cleanedStr === reversedStr;
}
3. Create a function that returns the greatest difference between any two numerical values in an array of integers.
In order to manage edge cases and invalid inputs, candidates should show how they determine the greatest difference between the array items.
function largestDifference(arr) {
let min = arr[0];
let maxDiff = 0;
for (let i = 1; i < arr.length; i++) {
if (arr[i] < min) {
min = arr[i];
}
else {
const diff = arr[i] – min;
if (diff > maxDiff) {
maxDiff = diff;
}
}
}
return maxDiff;
}
4. Put in place a function that eliminates duplicates from an array, leaving only the elements that are unique.
Interviewers can assess a candidate’s proficiency with algorithmic efficiency and their ability to explain code in an understandable manner.
function removeDuplicates(arr) {
return arr.filter((item, index) => arr.indexOf(item) === index);
}
5. Create a function that takes an integer and returns its factorial; for example, 5 x 4 x 3 x 2 x 1 is the factorial of 5.
During the interview, hiring managers might use this question to gauge a candidate’s aptitude for handling numerical computations. If necessary, they can also assess the interviewee’s ability to focus on managing edge cases.
function factorial(num) {
if (num === 0 || num === 1) {
return 1;
} else {
return num * factorial(num – 1);
}
}
6. Put into practice a method that creates a single-dimensional array from a nested array.
Interviewers anticipate that candidates will show that they can work with intricate data structures and apply the right methods to complete assignments.
function flattenArray(arr) {
return arr.flat();
}
7. Create a function that determines whether a given text with the same characters in a different order is an anagram of another string.
Candidates should demonstrate their proficiency with intricate logic and algorithms. Knowledge of loop constructs, data structures, and string techniques is specifically sought after by interviewers.
function isAnagram(str1, str2) {
const sortedStr1 = str1.split(”).sort().join(”);
const sortedStr2 = str2.split(”).sort().join(”);
return sortedStr1 === sortedStr2;
}
8. Put into practice a method that determines an array of integers’ second-smallest element.
Interviewers can assess a candidate’s ability to solve problems and their knowledge of arrays, loops, and conditional statements.
function secondSmallest(arr) {
const sortedArr = arr.sort((a, b) => a – b);
return sortedArr[1];
}
9. Create a function that, given a length, produces a random alphanumeric string.
Interviewers can determine how effectively a candidate can guarantee that the function generates a dependable and consistent random output by posing this question.
function generateRandomString(length) {
const characters = ‘ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789’;
let result = ”;
for (let i = 0; i < length; i++) {
const randomIndex = Math.floor(Math.random() * characters.length);
result += characters.charAt(randomIndex);
}
return result;
}
10. Put into practice a function that changes a number to its equivalent in Roman numerals.
Employers can assess a candidate’s aptitude for developing an effective algorithm and putting coding solutions into practice.
function toRomanNumeral(number) {
// Implement your code here
}
Tips to prepare for a JavaScript coding interview
When attending a JavaScript coding interview, a candidate should have the following five points in mind:
Learn the fundamentals of JavaScript: Applicants must be well-versed in the language’s variables, loops, conditionals, objects, and data types. It’s also critical to practise your coding abilities.
Examine well-known frameworks and libraries: Understanding the most popular JavaScript frameworks and libraries, such as Vue.js, React, etc., aids in understanding important ideas.
Whiteboard practice Java coding: Whiteboard coding is another way to hone your Java coding abilities. Candidates should solve coding puzzles and vocally describe their ideas. They ought to prioritise efficiency, clarity, and simplicity.
Test code: After creating Java solutions, make sure they function properly and can handle edge cases by testing them with a range of inputs. Aim for efficiency while taking the temporal complexity of the solutions into account.
Examine JavaScript projects: It’s crucial to bring up JavaScript projects throughout the interview. Interviewees should provide a comprehensive explanation of their strategy and how they handled the difficulties.
Warning Signs
During a JavaScript coding interview, hiring managers should be aware of the following warning signs:
- Ignorance of the fundamentals of JavaScript
- Offering ineffective fixes
- Lack of ability to solve problems in the real world
- Insufficient familiarity with asynchronous programming
- Pasting code
- Incapacity to clearly explain their approach to the issue or to express their thought processes
- Lack of Knowledge about Current JavaScript
- Ignoring edge cases and failing to offer error handling
Candidates are mostly judged on how well they comprehend the fundamental ideas and abilities of JavaScript. Candidates can ace their interview by learning the basics and practising. Additionally, it’s critical to concentrate on being truthful, precise, and transparent throughout the interview.
