JavaScript is a cornerstone language for frontend and backend development. Whether you're preparing for coding interviews, improving your algorithmic thinking, or simply honing your skills, mastering the basics through coding challenges is vital.
In this blog, we'll walk through 40+ JavaScript problems and solutions, ranging from string manipulation to algorithms like sorting and recursion. Each problem is explained in simple language and provides clean, optimized code snippets.
Let's dive in!
1. Reverse a String
function reverseString(str) {
return str.split('').reverse().join('');
}
console.log(reverseString('Hello')); // Output: 'olleH'
Splits the string into an array, reverses it, and joins it back.
2. Check for Palindrome
function isPalindrome(str) {
const reversed = str.split('').reverse().join('');
return (str === reversed) ? "Yes, it is" : "No, it isn't";
}
console.log(isPalindrome("121")); // Output: "Yes, it is"
console.log(isPalindrome("911")); // Output: "No, it isn't"
A palindrome reads the same backward as forward.
3. Generate Fibonacci Series
function fibonacci(n) {
const sequence = [0, 1];
for (let i = 2; i < n; i++) {
sequence.push(sequence[i-1] + sequence[i-2]);
}
return sequence.slice(0, n);
}
console.log(fibonacci(10));
Generates the first n numbers of the Fibonacci sequence.
4. Find Factorial of a Number
function factorial(n) {
if (n < 0) return "Factorial undefined for negatives.";
let result = 1;
for (let i = 1; i <= n; i++) {
result *= i;
}
return result;
}
console.log(factorial(5)); // Output: 120
Factorial grows rapidly; use recursion cautiously.
5. Check for Prime Number
function isPrime(num) {
if (num < 2) return false;
for (let i = 2; i <= Math.sqrt(num); i++) {
if (num % i === 0) return false;
}
return true;
}
console.log(isPrime(17)); // Output: true
Only divisible by 1 and itself.
6. Count Vowels and Consonants
function countLetters(str) {
const vowels = str.match(/[aeiou]/gi) || [];
const consonants = str.match(/[bcdfghjklmnpqrstvwxyz]/gi) || [];
return `Vowels: ${vowels.length}, Consonants: ${consonants.length}`;
}
console.log(countLetters("Ganesh205"));
7. Sort an Array Numerically
function sortArray(arr) {
return arr.sort((a, b) => a - b);
}
console.log(sortArray([10, 1, 4, 2, 3]));
8. Merge Two Arrays
function mergeArrays(arr1, arr2) {
return [...arr1, ...arr2];
}
console.log(mergeArrays([1, 2, 3], [4, 5, 6]));
9. Find Largest Element
function findLargest(arr) {
return Math.max(...arr);
}
console.log(findLargest([1, 2, 15, 4, 5]));
10. Remove Duplicates from Array
function removeDuplicates(arr) {
return [...new Set(arr)];
}
console.log(removeDuplicates([1, 2, 2, 3, 4, 4, 5]));
11. Check Armstrong Number
function isArmstrong(n) {
const digits = n.toString().split('');
const sum = digits.reduce((acc, digit) => acc + Math.pow(+digit, digits.length), 0);
return (sum === n) ? "Yes, it is" : "It is not";
}
console.log(isArmstrong(153));
12. Reverse a Number
function reverseNumber(n) {
const reversed = parseInt(n.toString().split('').reverse().join('')) * Math.sign(n);
return reversed;
}
console.log(reverseNumber(119)); // Output: 911
13. Calculate GCD
function gcd(a, b) {
if (!b) return a;
return gcd(b, a % b);
}
console.log(gcd(12, 8)); // Output: 4
14. Anagram Checker
function isAnagram(str1, str2) {
const normalize = str => str.replace(/\s/g, '').toLowerCase().split('').sort().join('');
return normalize(str1) === normalize(str2);
}
console.log(isAnagram("sIl e nt", "listen"));
15. Count Digits
function countDigits(num) {
return num.toString().replace('-', '').length;
}
console.log(countDigits(911));
16. Prime Numbers in a Range
function primesInRange(start, end) {
const primes = [];
for (let i = start; i <= end; i++) {
if (isPrime(i)) primes.push(i);
}
return primes;
}
console.log(primesInRange(10, 30));
17. Second Largest Element
function secondLargest(arr) {
const unique = [...new Set(arr)];
unique.sort((a, b) => b - a);
return unique[1];
}
console.log(secondLargest([1, 3, 5, 8, 2, 9]));
18. Pascal's Triangle
function generatePascal(n) {
const triangle = [];
for (let row = 0; row < n; row++) {
triangle[row] = [1];
for (let col = 1; col < row; col++) {
triangle[row][col] = triangle[row - 1][col - 1] + triangle[row - 1][col];
}
if (row) triangle[row].push(1);
}
return triangle.map(row => row.join(' ')).join('\n');
}
console.log(generatePascal(5));
19. Find Missing Number in Array
function findMissing(arr, n) {
const expectedSum = (n * (n + 1)) / 2;
const actualSum = arr.reduce((acc, num) => acc + num, 0);
return expectedSum - actualSum;
}
console.log(findMissing([1, 2, 4, 5, 6], 6)); // Output: 3
Uses the formula for the sum of the first n natural numbers.
20. Check if Two Strings Are Isomorphic
function isIsomorphic(s, t) {
if (s.length !== t.length) return false;
const mapS = new Map();
const mapT = new Map();
for (let i = 0; i < s.length; i++) {
if (!mapS.has(s[i])) mapS.set(s[i], t[i]);
if (!mapT.has(t[i])) mapT.set(t[i], s[i]);
if (mapS.get(s[i]) !== t[i] || mapT.get(t[i]) !== s[i]) return false;
}
return true;
}
console.log(isIsomorphic("egg", "add")); // Output: true
Maintains a one-to-one mapping between characters.
21. Move Zeros to End
function moveZeros(arr) {
const nonZero = arr.filter(x => x !== 0);
const zeros = arr.filter(x => x === 0);
return [...nonZero, ...zeros];
}
console.log(moveZeros([0,1,0,3,12])); // Output: [1,3,12,0,0]
22. Find Intersection of Two Arrays
function arrayIntersection(arr1, arr2) {
return arr1.filter(value => arr2.includes(value));
}
console.log(arrayIntersection([1,2,2,1], [2,2])); // Output: [2,2]
23. Find First Non-Repeating Character
function firstNonRepeatingChar(str) {
for (let char of str) {
if (str.indexOf(char) === str.lastIndexOf(char)) return char;
}
return null;
}
console.log(firstNonRepeatingChar("aabbcdd")); // Output: 'c'
24. Sum of Elements in Array
function arraySum(arr) {
return arr.reduce((sum, num) => sum + num, 0);
}
console.log(arraySum([1,2,3,4,5])); // Output: 15
25. Check if Number is Power of Two
function isPowerOfTwo(n) {
return n > 0 && (n & (n - 1)) === 0;
}
console.log(isPowerOfTwo(16)); // Output: true
Bit manipulation trick: only powers of two have exactly one bit set.
26. Find Duplicate Number
function findDuplicate(nums) {
const set = new Set();
for (let num of nums) {
if (set.has(num)) return num;
set.add(num);
}
}
console.log(findDuplicate([1,3,4,2,2])); // Output: 2
27. Longest Word in a String
function findLongestWord(str) {
return str.split(' ').sort((a, b) => b.length - a.length)[0];
}
console.log(findLongestWord("The quick brown fox jumped over the lazy dog"));
28. Capitalize First Letter of Each Word
function capitalizeWords(str) {
return str.split(' ').map(word => word.charAt(0).toUpperCase() + word.slice(1)).join(' ');
}
console.log(capitalizeWords("hello world"));
29. Flatten a Nested Array
function flattenArray(arr) {
return arr.flat(Infinity);
}
console.log(flattenArray([1, [2, [3, [4]]]])); // Output: [1,2,3,4]
30. Find Majority Element
function majorityElement(nums) {
const count = {};
const n = nums.length;
for (let num of nums) {
count[num] = (count[num] || 0) + 1;
if (count[num] > n/2) return num;
}
}
console.log(majorityElement([2,2,1,1,2,2])); // Output: 2
31. Find Subsets of an Array
function subsets(arr) {
const result = [[]];
for (let num of arr) {
const len = result.length;
for (let i = 0; i < len; i++) {
result.push([...result[i], num]);
}
}
return result;
}
console.log(subsets([1,2]));
32. Debounce Function
function debounce(func, delay) {
let timer;
return function(...args) {
clearTimeout(timer);
timer = setTimeout(() => func.apply(this, args), delay);
};
}
const processChange = debounce(() => console.log('Saved!'), 300);
Debouncing delays execution until a pause happens.
33. Throttle Function
function throttle(func, limit) {
let inThrottle;
return function(...args) {
if (!inThrottle) {
func.apply(this, args);
inThrottle = true;
setTimeout(() => inThrottle = false, limit);
}
};
}
const handleScroll = throttle(() => console.log('Scrolling!'), 200);
34. Deep Clone an Object
function deepClone(obj) {
return JSON.parse(JSON.stringify(obj));
}
const original = { a: 1, b: { c: 2 } };
const clone = deepClone(original);
console.log(clone);
35. Group Anagrams
function groupAnagrams(words) {
const map = new Map();
for (let word of words) {
const sorted = word.split('').sort().join('');
map.set(sorted, [...(map.get(sorted) || []), word]);
}
return Array.from(map.values());
}
console.log(groupAnagrams(["eat","tea","tan","ate","nat","bat"]));
36. Validate Brackets
function isValidBrackets(s) {
const stack = [];
const map = {')':'(', ']':'[', '}':'{'};
for (let char of s) {
if (['(','{','['].includes(char)) {
stack.push(char);
} else {
if (stack.pop() !== map[char]) return false;
}
}
return stack.length === 0;
}
console.log(isValidBrackets("()[]{}")); // Output: true
37. Find All Permutations
function permutations(str) {
if (str.length <= 1) return [str];
let result = [];
for (let i = 0; i < str.length; i++) {
const char = str[i];
const perms = permutations(str.slice(0, i) + str.slice(i + 1));
for (let perm of perms) {
result.push(char + perm);
}
}
return result;
}
console.log(permutations("abc"));
38. Implement Currying
function curry(fn) {
return function curried(...args) {
if (args.length >= fn.length) {
return fn(...args);
} else {
return (...nextArgs) => curried(...args, ...nextArgs);
}
};
}
function add(a, b, c) {
return a + b + c;
}
const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)(3)); // Output: 6
39. Find Min and Max in Array
function findMinMax(arr) {
return { min: Math.min(...arr), max: Math.max(...arr) };
}
console.log(findMinMax([1,2,3,4,5]));
40. Remove Falsy Values
function removeFalsy(arr) {
return arr.filter(Boolean);
}
console.log(removeFalsy([0, "", false, 1, "hello", null, undefined]));
Conclusion
Practicing JavaScript problems consistently builds strong problem-solving skills, which are critical for frontend interviews, backend development, and even system design.
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