JavaScript interview questions & answers with code

Top JavaScript Interview Questions and Expert Answers with Code Examples

Flatten a Nested Object

Question : Write a function to flatten a nested JavaScript object.

Code :

function flattenObject(obj, prefix = '', res = {}) {

** for (let key in obj) {**

** const newKey = prefix ? ${prefix}.${key} : key;**

** if (typeof obj[key] === 'object' && obj[key] !== null) {**

** flattenObject(obj[key], newKey, res);**

** } else {**

** res[newKey] = obj[key];**

** }**

** }**

** return res;**

}

console.log(flattenObject({ a: { b: { c: 1 } }, d: 2 }));

// Output: { 'a.b.c': 1, d: 2 }

2. Find Duplicates in an Array

Question : Write a function to find duplicate values in an array.

Code :

function findDuplicates(arr) {

** const counts = {};**

** return arr.filter(item => counts[item] ? true : (counts[item] = 1, false));**

}

console.log(findDuplicates([1, 2, 2, 3, 4, 4, 5]));

// Output: [2, 4]

3. Implement debounce
Question : Write a debounce function that limits the rate a function is called.

Code :

function debounce(func, delay) {

** let timer;**

** return function (...args) {**

** clearTimeout(timer);**

** timer = setTimeout(() => func.apply(this, args), delay);**

** };**

}

const log = debounce(() => console.log('Logged after 1s'), 1000);

log();

4. Reverse a String Recursively

Question : Write a recursive function to reverse a string.

Code :

function reverseString(str) {

** if (str === "") return "";**

** return reverseString(str.slice(1)) + str[0];**

}

console.log(reverseString("hello"));

// Output: "olleh"

5. Check for Palindrome

Question : Write a function to check if a string is a palindrome.

Code :

function isPalindrome(str) {

** const cleaned = str.toLowerCase().replace(/[^a-z]/g, '');**

** return cleaned === cleaned.split('').reverse().join('');**

}

console.log(isPalindrome("A man, a plan, a canal, Panama"));

// Output: true

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