JavaScript Array Questions: A Comprehensive Guide for Interviews

JavaScript Array Interview Questions

If you're preparing for a JavaScript interview, having a strong understanding of JavaScript Arrays is essential. You might be asked to explain various Array Methods in JavaScript, their properties, and code implementations using JavaScript's built-in Array functionalities. Being familiar with array manipulation, iteration, filtering, and sorting, as well as understanding performance and immutability, is highly beneficial.

In this JavaScript tutorial, we’ll explore the top JavaScript Array interview questions with clear examples and explanations. By the end, you’ll be ready to confidently tackle any array-related question and apply these concepts in real-world scenarios.

What to Expect in JavaScript Array Interview Questions

JavaScript Array Interview Questions for Freshers

1. What does the concat() method do in JavaScript?

Ans: The concat() method is used to join two or more arrays together. It does not change the original arrays. Instead, it creates and returns a new array that contains all the elements of the arrays you passed to it.

let arr1 = [1, 2, 3];
let arr2 = [4, 5, 6];
let result = arr1.concat(arr2);
console.log(result);

Output

[1, 2, 3, 4, 5, 6]

2. What is the indexOf() method used for?

Ans: The indexOf() method helps you find the position of the first occurrence of a specified element in an array. If the element is not found, it will return -1.

let arr = [10, 20, 30, 40];
let index = arr.indexOf(30);
console.log(index);

Output

2

3. How does the push() method work?

Ans: The push() method adds one or more elements to the end of an array and gives you back the new length of the array.

let arr = [1, 2, 3];
arr.push(4, 5);
console.log(arr);

Output

[1, 2, 3, 4, 5]

4. What does the pop() method do?

Ans: The pop() method removes the last element from an array and gives you that element. The array's length will also decrease.

let arr = [10, 20, 30];
let lastElement = arr.pop();
console.log(lastElement);
console.log(arr);

Output

30
[10, 20]

5. What is the shift() method?

Ans: The shift() method removes the first element from an array and gives you that element. It also reduces the size of the array.

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

Output

1
[2, 3]

6. How does the unshift() method work?

Ans: The unshift() method adds one or more elements to the beginning of an array and gives back the new length of the array.

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

Output

[1, 2, 3, 4]

7. What is the slice() method?

Ans: The slice() method returns a new array that contains a portion of the original array. It does not modify the original array. You pass the start index and end index (exclusive) to select the portion you want.

let arr = [10, 20, 30, 40, 50];
let newArr = arr.slice(1, 4);
console.log(newArr);

Output

[20, 30, 40]

8. What does the splice() method do?

Ans: The splice() method changes the contents of an array. You can use it to remove, add, or replace elements at any position in the array. Unlike other methods, it directly modifies the original array.

let arr = [10, 20, 30, 40, 50];
arr.splice(2, 2, 100, 200);
console.log(arr);

Output

[10, 20, 100, 200, 50]

9. How does the reverse() method work?

Ans: The reverse() method reverses the order of the elements in the array. It changes the array in place and returns the reversed array.

let arr = [1, 2, 3, 4, 5];
arr.reverse();
console.log(arr);

Output

[5, 4, 3, 2, 1]

10. What is the sort() method used for?

Ans: The sort() method is used to arrange the elements of an array in ascending order. If you want a different order, you can provide a custom function to control how the sorting is done.

let arr = [40, 10, 100, 20];
arr.sort((a, b) => a - b);
console.log(arr);

Output

[10, 20, 40, 100]

11. What does the join() method do?

Ans: The join() method combines all elements of an array into a single string, using a separator like a comma, space, or custom string. If no separator is provided, it defaults to a comma.

let arr = [10, 20, 30];
let result = arr.join("-");
console.log(result);

Output

10-20-30

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

Ans: map() creates a new array by applying a function to each element, whereas forEach() just executes a function on each element without returning a new array.

13. How does the filter() method work?

Ans: The filter() method creates a new array with elements that pass a test in a callback function. If no elements match, it returns an empty array.

let numbers = [1, 2, 3, 4, 5];
let result = numbers.filter(num => num % 2 === 0);
console.log(result);

Output

[2, 4]

14. What does the every() method do?

Ans: every() checks if all elements in an array satisfy a condition. It returns true if all match, otherwise false.

let numbers = [2, 4, 6];
let result = numbers.every(num => num % 2 === 0);
console.log(result);

Output

true

15. How does the some() method work?

Ans: some() checks if any element in the array satisfies a condition. It returns true if any match, otherwise false.

let numbers = [1, 3, 5];
let result = numbers.some(num => num % 2 === 0);
console.log(result);

Output

false

16. What is the reduce() method used for?

Ans: The reduce() method reduces an array to a single value by applying a function to an accumulator and each element.

let numbers = [1, 2, 3];
let sum = numbers.reduce((acc, num) => acc + num, 0);
console.log(sum);

Output

6

17. What is the reduceRight() method?

Ans: reduceRight() works like reduce(), but it processes the array from right to left.

let numbers = [1, 2, 3];
let result = numbers.reduceRight((acc, num) => acc - num, 0);
console.log(result);

Output

-4

18. What is an array-like object in JavaScript?

Ans: An array-like object has a length property and indexed elements but lacks array methods like push(). Examples include NodeList and the arguments object.

function example() {
    console.log(arguments);
}
example(1, 2, 3);

Output

[1, 2, 3]

19. What is the find() method used for?

Ans: The find() method returns the first element that satisfies a condition, or undefined if no match is found.

let numbers = [1, 2, 3, 4];
let result = numbers.find(num => num > 2);
console.log(result);

Output

3

20. How does the from() method work?

Ans: Array.from() creates a new array from an array-like or iterable object, such as a string or NodeList.

let str = "hello";
let result = Array.from(str);
console.log(result);

Output

["h", "e", "l", "l", "o"]

21. What is the difference between let and var?

Ans: let is block-scoped, while var is function-scoped. let prevents redeclaration in the same scope.

let a = 10;
{
    let a = 20;
    console.log(a); // 20
}
console.log(a); // 10

Output

20
10

22. What is the purpose of const in JavaScript?

Ans: const defines a variable whose value cannot be reassigned, though the contents of objects or arrays can still be modified.

const numbers = [1, 2, 3];
numbers.push(4); // Allowed
// numbers = [4, 5, 6]; // Error: Assignment to constant variable
console.log(numbers);

Output

[1, 2, 3, 4]

23. What is the bind() method in JavaScript?

Ans: The bind() method allows you to create a new function with its this context explicitly set to a specified object. This is particularly useful when passing functions as callbacks, ensuring that the function retains its original context regardless of how it is invoked.

let person = {
  firstName: 'John',
  lastName: 'Doe'
};

function greet() {
  console.log('Hello, ' + this.firstName + ' ' + this.lastName);
}

let greetPerson = greet.bind(person);
greetPerson();

Output

Hello, John Doe

24. What are arrow functions in JavaScript?

Ans: Arrow functions in JavaScript provide a shorter syntax for writing functions. They also handle the this keyword differently than regular functions, inheriting this from the surrounding context.

let sum = (a, b) => a + b;
console.log(sum(3, 4));

Output

7

25. What is a promise in JavaScript?

Ans: A promise is an object representing the eventual completion or failure of an asynchronous operation. It can be in one of three states: pending, fulfilled, or rejected. Promises are commonly used to handle asynchronous operations like API calls.

let promise = new Promise((resolve, reject) => {
  let success = true;
  if (success) {
    resolve('Operation was successful!');
  } else {
    reject('Operation failed');
  }
});

promise.then(result => console.log(result)).catch(error => console.log(error));

Output

Operation was successful!

26. What is the async/await syntax in JavaScript?

Ans: The async/await syntax is used to work with asynchronous code in a more readable way. asyncIt makes a function return a promise and await pauses the function execution until the promise is resolved.

async function fetchData() {
  let response = await fetch('https://api.example.com/data');
  let data = await response.json();
  console.log(data);
}

fetchData();

27. What is the this keyword in JavaScript?

Ans: The this keyword refers to the context in which a function is called. In regular functions, this refers to the global object (or undefined in strict mode), while in object methods, it refers to the object itself.

28. What is event delegation in JavaScript?

Ans: Event delegation is a technique for handling events at a higher level in the DOM rather than attaching event listeners to individual elements. This improves performance, especially when dealing with dynamically added elements.

document.getElementById('parent').addEventListener('click', function(event) {
  if (event.target && event.target.matches('button.classname')) {
    console.log('Button clicked!');
  }
});

29. What is a closure in JavaScript?

Ans: A closure is a function that has access to its own scope, the outer function's scope, and the global scope. This allows functions to remember and access variables from their lexical environment even after the outer function has finished execution.

function outer() {
  let counter = 0;
  return function inner() {
    counter++;
    console.log(counter);
  };
}

let increment = outer();
increment();
increment();

Output

1
2

30. What is the difference between == and ===?

Ans: == is the equality operator that compares values, allowing type coercion. === is the strict equality operator that compares both values and types, without allowing type conversion.

31. What is the Array.prototype.map() method?

Ans: The map() method creates a new array populated with the results of calling a provided function on every element in the calling array. It doesn’t change the original array.

let numbers = [1, 2, 3, 4];
let squares = numbers.map(num => num * num);
console.log(squares);

Output

[1, 4, 9, 16]

32. What is the Array.prototype.filter() method?

Ans: The filter() method creates a new array with all elements that pass the test implemented by the provided function. It doesn’t change the original array.

let numbers = [1, 2, 3, 4, 5];
let evenNumbers = numbers.filter(num => num % 2 === 0);
console.log(evenNumbers);

Output

[2, 4]

33. What is the Array.prototype.reduce() method?

Ans: The reduce() method applies a function against an accumulator and each element in the array to reduce it to a single value.

let numbers = [1, 2, 3, 4];
let sum = numbers.reduce((acc, num) => acc + num, 0);
console.log(sum);

Output

10

34. What is the setTimeout() function?

Ans: The setTimeout() function is used to execute a function or code snippet after a specified delay (in milliseconds).

setTimeout(() => console.log('Hello, world!'), 2000);

Output

Hello, world! (after 2 seconds)

35. What is the setInterval() function?

Ans: The setInterval() function calls a function repeatedly with a fixed time delay between each call.

setInterval(() => console.log('Repeating message'), 1000);

Output

Repeating message (every second)

JavaScript Array Interview Questions for Experienced

// file1.js
export const greeting = 'Hello World';

// file2.js
import { greeting } from './file1.js';
console.log(greeting);

Hello World

function multiplyBy(factor) {
  return function (num) {
    return num * factor;
  };
}

const double = multiplyBy(2);
console.log(double(5)); // 10

10

let map = new Map();
map.set('name', 'John');
map.set('age', 30);
console.log(map.get('name')); // John

John

function animate() {
  // Animation code
  console.log('Animating...');
  requestAnimationFrame(animate);
}

requestAnimationFrame(animate);

Animating... (calls indefinitely)

try {
  let result = riskyFunction();
} catch (error) {
  console.error('Error:', error.message);
}

Error: [Error message here]

let obj = { name: 'John' };
Object.freeze(obj);
obj.name = 'Jane'; // This will not change the name
console.log(obj.name); // John

John

if (array.length > 0) {
    console.log("Array has values");
} else {
    console.log("Array is empty");
}

Take our Javascript skill challenge to evaluate yourself!

In less than 5 minutes, with our skill challenge, you can identify your knowledge gaps and strengths in a given skill.

GET FREE CHALLENGE