40+ Essential JavaScript Coding Challenges for Interviews and Practice

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 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 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 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 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];

Apr 28, 2025 - 14:47

40+ Essential JavaScript Coding Challenges for Interviews and Practice

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.