JavaScript Array Interview Questions and Answers

🔹 Easy Level

1. How do you create an array in JavaScript?

let arr = [1, 2, 3, 4, 5];
let arr2 = new Array(1, 2, 3, 4, 5);

2. How do you find the length of an array?

let arr = [1, 2, 3];
console.log(arr.length); // 3

3. How do you check if a value is an array?

console.log(Array.isArray([1, 2, 3])); // true
console.log(Array.isArray({ key: "value" })); // false

4. How do you add an element to the end of an array?

let arr = [1, 2, 3];
arr.push(4);
console.log(arr); // [1, 2, 3, 4]

5. How do you remove the last element from an array?

let arr = [1, 2, 3];
arr.pop();
console.log(arr); // [1, 2]

6. How do you add an element at the beginning of an array?

let arr = [2, 3, 4];
arr.unshift(1);
console.log(arr); // [1, 2, 3, 4]

7. How do you remove the first element from an array?

let arr = [1, 2, 3];
arr.shift();
console.log(arr); // [2, 3]

8. What is the difference between map() and forEach()?

• map() returns a new array by applying a function to each element.
• forEach() iterates over elements but does not return a new array.

let arr = [1, 2, 3];
let squared = arr.map(num => num * num);
console.log(squared); // [1, 4, 9]

arr.forEach(num => console.log(num * num)); // Logs 1, 4, 9 (No new array created)

9. How do you convert an array to a string?

let arr = ["apple", "banana", "cherry"];
console.log(arr.join(", ")); // "apple, banana, cherry"

10. How do you reverse an array?

let arr = [1, 2, 3];
arr.reverse();
console.log(arr); // [3, 2, 1]

---

🔹 Medium Level

11. How do you remove duplicates from an array?

let arr = [1, 2, 2, 3, 4, 4];
let uniqueArr = [...new Set(arr)];
console.log(uniqueArr); // [1, 2, 3, 4]

12. How do you merge two arrays?

let arr1 = [1, 2];
let arr2 = [3, 4];
let merged = [...arr1, ...arr2];
console.log(merged); // [1, 2, 3, 4]

13. How do you find the intersection of two arrays?

let arr1 = [1, 2, 3, 4];
let arr2 = [3, 4, 5, 6];
let intersection = arr1.filter(num => arr2.includes(num));
console.log(intersection); // [3, 4]

14. How do you shuffle an array?

let arr = [1, 2, 3, 4, 5];
arr.sort(() => Math.random() - 0.5);
console.log(arr);

15. How do you check if all elements in an array satisfy a condition?

let arr = [10, 20, 30];
console.log(arr.every(num => num > 5)); // true
console.log(arr.every(num => num > 15)); // false

---

🔹 Hard Level

16. How do you implement quicksort on an array?

function quickSort(arr) {
    if (arr.length <= 1) return arr;
    let pivot = arr[arr.length - 1];
    let left = arr.filter(num => num < pivot);
    let right = arr.filter(num => num > pivot);
    return [...quickSort(left), pivot, ...quickSort(right)];
}

console.log(quickSort([4, 2, 7, 1, 9])); // [1, 2, 4, 7, 9]

17. How do you find the maximum sum subarray using Kadane’s Algorithm?

function maxSubArraySum(arr) {
    let maxSum = arr[0], currentSum = arr[0];
    for (let i = 1; i < arr.length; i++) {
        currentSum = Math.max(arr[i], currentSum + arr[i]);
        maxSum = Math.max(maxSum, currentSum);
    }
    return maxSum;
}

console.log(maxSubArraySum([-2, 1, -3, 4, -1, 2, 1, -5, 4])); // 6

---

This covers Arrays. Next, I'll add Strings section!