JavaScript coding questions to help prepare for interviews

1. Basic JavaScript Questions

• Reverse a String:

function reverseString(str) {
  return str.split('').reverse().join('');
}
console.log(reverseString("hello")); // Output: "olleh"

Check if a String is a Palindrome:

function isPalindrome(str) {
  const reversed = str.split('').reverse().join('');
  return str === reversed;
}
console.log(isPalindrome("madam")); // Output: true

Find the Largest Number in an Array:

function findMax(arr) {
  return Math.max(...arr);
}
console.log(findMax([1, 5, 3, 9, 2])); // Output: 9

FizzBuzz Problem: Print numbers from 1 to 100, but for multiples of 3 print "Fizz", for multiples of 5 print "Buzz", and for multiples of both, print "FizzBuzz".

for (let i = 1; i <= 100; i++) {
  let output = '';
  if (i % 3 === 0) output += 'Fizz';
  if (i % 5 === 0) output += 'Buzz';
  console.log(output || i);
}

Remove Duplicates from an Array:

function removeDuplicates(arr) {
  return [...new Set(arr)];
}
console.log(removeDuplicates([1, 2, 2, 3, 4, 4])); // Output: [1, 2, 3, 4]

2. Intermediate JavaScript Questions

• Find Factorial of a Number (Recursive):

function factorial(n) {
  return n === 0 ? 1 : n * factorial(n - 1);
}
console.log(factorial(5)); // Output: 120

Flatten a Nested Array:

function flattenArray(arr) {
  return arr.reduce((acc, val) => Array.isArray(val) ? acc.concat(flattenArray(val)) : acc.concat(val), []);
}
console.log(flattenArray([1, [2, [3, 4], 5], 6])); // Output: [1, 2, 3, 4, 5, 6]

Check for Anagram:

function isAnagram(str1, str2) {
  return str1.split('').sort().join('') === str2.split('').sort().join('');
}
console.log(isAnagram('listen', 'silent')); // Output: true

Implement Array.prototype.map():

Array.prototype.customMap = function(callback) {
  const result = [];
  for (let i = 0; i < this.length; i++) {
    result.push(callback(this[i], i, this));
  }
  return result;
};

Find Missing Number in an Array:

function findMissing(arr, n) {
  const expectedSum = (n * (n + 1)) / 2;
  const actualSum = arr.reduce((a, b) => a + b, 0);
  return expectedSum - actualSum;
}
console.log(findMissing([1, 2, 4, 5, 6], 6)); // Output: 3

3. Advanced JavaScript Questions

• Deep Clone an Object:

function deepClone(obj) {
  return JSON.parse(JSON.stringify(obj));
}

Debounce Function:

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

Throttle Function:

function throttle(func, limit) {
  let lastFunc;
  let lastRan;
  return function(...args) {
    const context = this;
    if (!lastRan) {
      func.apply(context, args);
      lastRan = Date.now();
    } else {
      clearTimeout(lastFunc);
      lastFunc = setTimeout(function() {
        if (Date.now() - lastRan >= limit) {
          func.apply(context, args);
          lastRan = Date.now();
        }
      }, limit - (Date.now() - lastRan));
    }
  };
}