JavaScript interviews focus heavily on logic, string manipulation, array operations, recursion, and problem-solving.
Here are the top 50 JavaScript coding interview questions with solutions + outputs.

1. Reverse a String

function reverseString(str) {
  return str.split("").reverse().join("");
}

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

Output:

olleh

2. Check Palindrome

function isPalindrome(str) {
  return str === str.split("").reverse().join("");
}

console.log(isPalindrome("madonna"));
console.log(isPalindrome("madam"));

Output:

false
true

3. Reverse an Integer

function reverseInt(num) {
  return parseInt(num.toString().split("").reverse().join(""));
}

console.log(reverseInt(12345));

Output:

4. Factorial of a Number

function factorial(n) {
  return n === 0 ? 1 : n * factorial(n - 1);
}

console.log(factorial(5));

Output:

5. Largest Number in Array

console.log(Math.max(...[3, 10, 6, 2]));

Output:

6. Smallest Number in Array

console.log(Math.min(...[3, 10, 6, 2]));

Output:

7. Sum of Array Numbers

function sumArray(arr) {
  return arr.reduce((a, b) => a + b, 0);
}

console.log(sumArray([5, 5, 5]));

Output:

8. Remove Duplicates

console.log([...new Set([1, 2, 2, 3, 3, 4])]);

Output:

[1, 2, 3, 4]

9. Anagram Check

console.log(isAnagram("listen", "silent"));

function isAnagram(a, b) {
  return a.split("").sort().join("") === b.split("").sort().join("");
}

Output:

true

10. Character Count

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

function charCount(str) {
  return str.split("").reduce((acc, ch) => {
    acc[ch] = (acc[ch] || 0) + 1;
    return acc;
  }, {});
}

Output:

{ h: 1, e: 1, l: 2, o: 1 }

11. Fibonacci Sequence (Iterative)

function fibonacci(n) {
  const arr = [0, 1];
  for (let i = 2; i <= n; i++) {
    arr[i] = arr[i - 1] + arr[i - 2];
  }
  return arr;
}

console.log(fibonacci(7));

Output:

[0, 1, 1, 2, 3, 5, 8, 13]

12. Check Prime Number

function isPrime(n) {
  if (n < 2) return false;
  for (let i = 2; i <= Math.sqrt(n); i++) {
    if (n % i === 0) return false;
  }
  return true;
}

console.log(isPrime(11));
console.log(isPrime(20));

Output:

true
false

13. Find Second Largest Number

function secondLargest(arr) {
  const unique = [...new Set(arr)];
  unique.sort((a, b) => b - a);
  return unique[1];
}

console.log(secondLargest([10, 20, 30, 30, 5]));

Output:

14. Find Missing Number

function missingNumber(arr) {
  const n = arr.length + 1;
  const sum = (n * (n + 1)) / 2;
  return sum - arr.reduce((a, b) => a + b, 0);
}

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

Output:

15. Flatten Array

console.log([1, [2, [3, 4]]].flat(Infinity));

Output:

[1, 2, 3, 4]

16. Check if Arrays are Equal

function arraysEqual(a, b) {
  return JSON.stringify(a) === JSON.stringify(b);
}

console.log(arraysEqual([1,2,3], [1,2,3]));
console.log(arraysEqual([1,2,3], [3,2,1]));

Output:

true
false

17. Array Intersection

console.log([1,2,3,4].filter(x => [3,4,5].includes(x)));

Output:

[3, 4]

18. Count Vowels

function countVowels(str) {
  return str.match(/[aeiou]/gi)?.length || 0;
}

console.log(countVowels("javascript"));

Output:

19. Capitalize First Letter of Each Word

console.log("hello world".replace(/\b\w/g, c => c.toUpperCase()));

Output:

Hello World

20. Random Number (Range)

function random(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

console.log(random(10, 20));

Output Example:

21. Validate Email

console.log(/^\S+@\S+\.\S+$/.test("abc@gmail.com"));
console.log(/^\S+@\S+\.\S+$/.test("hello@com"));

Output:

true
false

22. Check Numeric String

console.log(/^\d+$/.test("12345"));
console.log(/^\d+$/.test("12a45"));

Output:

true
false

23. Convert to Title Case

function titleCase(str) {
  return str.toLowerCase().replace(/\b\w/g, c => c.toUpperCase());
}

console.log(titleCase("welcome to javascript"));

Output:

Welcome To Javascript

24. GCD (Greatest Common Divisor)

function gcd(a, b) {
  return b === 0 ? a : gcd(b, a % b);
}

console.log(gcd(20, 8));

Output:

25. LCM

function gcd(a, b) {
  return b === 0 ? a : gcd(b, a % b);
}
function lcm(a, b) {
  return (a * b) / gcd(a, b);
}

console.log(lcm(5, 10));

Output:

26. String Rotation Check

console.log(isRotation("abcde", "cdeab"));

function isRotation(a, b) {
  return a.length === b.length && (a + a).includes(b);
}

Output:

true

27. Longest Word

function longestWord(str) {
  return str.split(" ").reduce((a, b) => b.length > a.length ? b : a);
}

console.log(longestWord("JavaScript coding interview questions"));

Output:

interview

28. Remove All Spaces

console.log("a b  c d".replace(/\s+/g, ""));

Output:

abcd

29. Word Count

console.log("  JavaScript is awesome  ".trim().split(/\s+/).length);

Output:

30. Array to Object

console.log(Object.assign({}, ["a", "b", "c"]));

Output:

{ '0': 'a', '1': 'b', '2': 'c' }

31. Check Empty Object

console.log(Object.keys({}).length === 0);
console.log(Object.keys({a:1}).length === 0);

Output:

true
false

32. Merge Two Objects

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

Output:

{ a: 1, b: 2 }

33. Debounce Function

function debounce(fn, delay) {
  let timer;
  return function(...args) {
    clearTimeout(timer);
    timer = setTimeout(() => fn.apply(this, args), delay);
  }
}

const fn = debounce(() => console.log("Executed"), 500);
fn(); fn(); fn(); // Only last one runs

Output:

Executed

34. Throttle Function

function throttle(fn, delay) {
  let last = 0;
  return (...args) => {
    const now = Date.now();
    if (now - last >= delay) {
      last = now;
      fn.apply(this, args);
    }
  }
}

const fn = throttle(() => console.log("Called"), 1000);
fn(); fn(); fn();

Output:

Called

35. Shuffle Array

function shuffle(arr) {
  for (let i = arr.length - 1; i > 0; i--) {
    let j = Math.floor(Math.random() * (i + 1));
    [arr[i], arr[j]] = [arr[j], arr[i]];
  }
  return arr;
}

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

Output Example:

[3, 5, 1, 4, 2]

36. Object to Array

console.log(Object.entries({a:1, b:2}));

Output:

[ ['a',1], ['b',2] ]

37. Sum of Digits

function sumDigits(num) {
  return num.toString().split("")
    .reduce((a,b) => a + Number(b), 0);
}

console.log(sumDigits(987));

Output:

38. Same Digits Check

console.log(sameDigits(123, 321));
console.log(sameDigits(121, 112));

function sameDigits(a, b) {
  return a.toString().split("").sort().join("") === 
         b.toString().split("").sort().join("");
}

Output:

true
true

39. Clean Array (Remove Null/Empty)

console.log([0, null, undefined, 5, "", 3].filter(Boolean));

Output:

[5, 3]

40. Count Unique Values

console.log(new Set([1,2,2,3,4,4,5]).size);

Output:

41. Unique Elements

console.log([...new Set([1,2,2,3,4])]);

Output:

[1,2,3,4]

42. Find Duplicate Values

console.log([1,2,2,3,3,4].filter((v,i,arr)=>arr.indexOf(v)!==i));

Output:

[2,3]

43. Number to Binary

console.log((10).toString(2));

Output:

44. Binary to Number

console.log(parseInt("1010", 2));

Output:

45. Range Sum

function rangeSum(a,b){
  let sum=0;
  for(let i=a;i<=b;i++) sum+=i;
  return sum;
}

console.log(rangeSum(1,5));

Output:

46. Armstrong Number

function isArmstrong(num){
  const digits = num.toString().split("");
  const power = digits.length;
  return digits.reduce((a,b)=>a+Math.pow(b,power),0)==num;
}

console.log(isArmstrong(153));

Output:

true

47. Longest Repeating Character

function longestRepeat(str){
  let max=0, curr=1;
  for(let i=1;i<str.length;i++){
    if(str[i]==str[i-1]) curr++;
    else curr=1;
    max = Math.max(max, curr);
  }
  return max;
}

console.log(longestRepeat("aaabbccccdde"));

Output:

48. Median of Array

function median(arr){
  arr.sort((a,b)=>a-b);
  let mid = Math.floor(arr.length/2);
  return arr.length%2? arr[mid] : (arr[mid-1]+arr[mid])/2;
}

console.log(median([1,3,4,2,6]));

Output:

49. Balanced Parentheses

function isBalanced(str){
  const stack=[];
  const map={')':'(', ']':'[', '}':'{'};
  for(let ch of str){
    if("({[".includes(ch)) stack.push(ch);
    else if(map[ch]!==stack.pop()) return false;
  }
  return stack.length===0;
}

console.log(isBalanced("{[()]}"));
console.log(isBalanced("{[(])}"));

Output:

true
false

50. Add Without + Operator

function add(a,b){
  while(b!==0){
    let carry = (a & b) << 1;
    a = a ^ b;
    b = carry;
  }
  return a;
}

console.log(add(5,7));

Output:

1. Reverse a String

2. Check Palindrome

3. Reverse an Integer

4. Factorial of a Number

5. Largest Number in Array

6. Smallest Number in Array

7. Sum of Array Numbers

8. Remove Duplicates

9. Anagram Check

10. Character Count

11. Fibonacci Sequence (Iterative)

12. Check Prime Number

13. Find Second Largest Number

14. Find Missing Number

15. Flatten Array

16. Check if Arrays are Equal

17. Array Intersection

18. Count Vowels

19. Capitalize First Letter of Each Word

20. Random Number (Range)

21. Validate Email

22. Check Numeric String

23. Convert to Title Case

24. GCD (Greatest Common Divisor)

25. LCM

26. String Rotation Check

27. Longest Word

28. Remove All Spaces

29. Word Count

30. Array to Object

31. Check Empty Object

32. Merge Two Objects

33. Debounce Function

34. Throttle Function

35. Shuffle Array

36. Object to Array

37. Sum of Digits

38. Same Digits Check

39. Clean Array (Remove Null/Empty)

40. Count Unique Values

41. Unique Elements

42. Find Duplicate Values

43. Number to Binary

44. Binary to Number

45. Range Sum

46. Armstrong Number

47. Longest Repeating Character

48. Median of Array

49. Balanced Parentheses

50. Add Without + Operator

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