Top Web Developer Interview Questions

What is the difference between var, let, and const in JavaScript?

What is a closure in JavaScript?

What is the difference between null and undefined in JavaScript?

What is a promise in JavaScript?

What is the difference between synchronous and asynchronous programming?

What is a callback function in JavaScript?

What is the purpose of the 'this' keyword in JavaScript?

What is a higher-order function in JavaScript?

What is the difference between a function declaration and a function expression in JavaScript?

What is a JavaScript module?

What is the purpose of the 'use strict' directive in JavaScript?

What is a JavaScript framework?

What is the difference between a library and a framework in JavaScript?

What is a single-page application (SPA) in JavaScript?

What is a RESTful API in JavaScript?

What is a JSON object in JavaScript?

What is the difference between JSON and JavaScript objects?

What is a web storage in JavaScript?

What is the difference between local storage and session storage in JavaScript?

What is a cookie in JavaScript?

What is the difference between a cookie and local storage in JavaScript?

Write a JavaScript function to reverse a string.

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

Write a JavaScript function to find the maximum value in an array.

function findMax(arr) {
  return Math.max.apply(null, arr);
}

Write a JavaScript function to check if a string is a palindrome.

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

Write a JavaScript function to find the first duplicate in an array.

function findFirstDuplicate(arr) {
  let seen = new Set();
  for (let i = 0; i < arr.length; i++) {
    if (seen.has(arr[i])) {
      return arr[i];
    }
    seen.add(arr[i]);
  }
  return null;
}

Write a JavaScript function to find the missing number in an array of consecutive integers.

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

Write a JavaScript function to check if a binary tree is balanced.

function isBalanced(root) {
  if (root === null) {
    return true;
  }
  let leftHeight = getHeight(root.left);
  let rightHeight = getHeight(root.right);
  return (
    Math.abs(leftHeight - rightHeight) <= 1 &&
    isBalanced(root.left) &&
    isBalanced(root.right)
  );
}
function getHeight(node) {
  if (node === null) {
    return 0;
  }
  return 1 + Math.max(getHeight(node.left), getHeight(node.right));
}

Write a JavaScript function to find the lowest common ancestor of two nodes in a binary tree.

function findLCA(root, p, q) {
  if (root === null) {
    return null;
  }
  if (root === p || root === q) {
    return root;
  }
  let left = findLCA(root.left, p, q);
  let right = findLCA(root.right, p, q);
  if (left !== null && right !== null) {
    return root;
  }
  return left !== null ? left : right;
}

Write a JavaScript function to check if a string contains all unique characters.

function hasUniqueChars(str) {
  let charSet = new Set();
  for (let char of str) {
    if (charSet.has(char)) {
      return false;
    }
    charSet.add(char);
  }
  return true;
}

Write a JavaScript function to find the first non-repeating character in a string.

function findFirstNonRepeatingChar(str) {
  let charCount = {};
  for (let char of str) {
    if (charCount[char] === undefined) {
      charCount[char] = 1;
    } else {
      charCount[char]++;
    }
  }
  for (let char of str) {
    if (charCount[char] === 1) {
      return char;
    }
  }
  return null;
}

Write a JavaScript function to check if two strings are anagrams of each other.

function areAnagrams(str1, str2) {
  return str1.split('').sort().join('') === str2.split('').sort().join('');
}