Javascript interview questions and answers with code

To start a series for the JavaScript interview questions which may help junior to mid level developers.

Common JavaScript interview questions with examples.

1. What is let and const in JavaScript?

» Let and const are block-scoped declarations in JavaScript, used to declare variables. let allows you to reassign the value of the variable, while const creates a read-only reference to a value.

Example:

let message = "Hello, World!";

message = "Hello, JavaScript!";

console.log(message); // Output: "Hello, JavaScript!"

const PI = 3.14;

PI = 3.14159; // TypeError: Assignment to constant variable.

2. What is arrow function in JavaScript?

» Arrow functions are a shorthand for anonymous functions in JavaScript. They are also known as “fat arrow” functions.

Example:

let add = (a, b) => a + b;

console.log(add(1, 2)); // Output: 3

let numbers = [1, 2, 3, 4, 5];

let doubledNumbers = numbers.map(num => num * 2);

console.log(doubledNumbers); // Output: [2, 4, 6, 8, 10]

3. What is default parameter in JavaScript?

» Default parameters allow function parameters to be initialized with default values if no values are passed or if the values are undefined.

Example:

function greet(name = "Atharv") {

  return `Hello, ${name}`;

}

console.log(greet()); // Output: "Hello, "Atharv

console.log(greet("Atharv")); // Output: "Hello, Atharv"

4. What is spread operator in JavaScript?

» The spread operator allows an iterable to be expanded into its elements.

Example:

let numbers1 = [1, 2, 3];

let numbers2 = [4, 5, 6];

let numbers3 = [...numbers1, ...numbers2];

console.log(numbers3); // Output: [1, 2, 3, 4, 5, 6]

let person = { name: "Atharv", age: 30 };

let newPerson = { ...person, city: "Manipal" };

console.log(newPerson); // Output: { name: "Atharv", age: 30, city: "Manipal" }

5. What is destructuring in JavaScript?

» Destructuring is a convenient way to extract values from arrays or objects into variables.

Example:

let numbers = [1, 2, 3];

let [a, b, c] = numbers;

console.log(a, b, c); // Output: 1 2 3

let person = { name: "Atharv", age: 30 };

let { name, age } = person;

console.log(name, age); // Output: Atharv 30

6. What is a rest parameter in JavaScript?

» A rest parameter is a way to represent an indefinite number of arguments as an array. It allows a function to take a variable number of arguments. The rest parameter is represented by three dots (…).

Example:

function sum(...args) {

  return args.reduce((a, b) => a + b);

}

console.log(sum(1, 2, 3, 4, 5));

// Output: 15

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

» Var is function scoped and its value can be changed, while let and const are block scoped and their values cannot be changed once they are assigned with const.

Example:

var name = "Atharv";

var name = "Gyan";

console.log(name); // Output: "Gyan"

let age = 30;

// age = 31;

console.log(age); // Output: 30

const city = "Manipal";

// city = "Manipal";

console.log(city); // Output: "Manipal"

8. What is closure in JavaScript?

» A closure is a function that has access to variables in its outer scope, even after the outer function has returned.

Example:

function outerFunction(counter) {

  return function innerFunction() {

    return ++counter;

  };

}

let counter = 0;

let increment = outerFunction(counter);

console.log(increment()); // Output: 1

console.log(increment()); // Output: 2

9. What is a promise in JavaScript?

» A promise is an object representing the eventual completion or failure of an asynchronous operation.

Example:

let promise = new Promise((resolve, reject) => {

  setTimeout(() => {

    resolve("Hello, World!");

  }, 2000);

});

promise.then(result => {

  console.log(result); // Output: "Hello, World!"

});

10. What is async/await in JavaScript?

» Async/await is a way to write asynchronous code that looks and behaves like synchronous code.

Example:

async function getData() {

  let response = await fetch("https://atharvgyan.com/home");

  let data = await response.json();

  return data;

}

getData().then(data => {

  console.log(data);

});

11. What is a template literal in JavaScript?

» A template literal is a string that is delimited by backticks (`) instead of single or double quotes.

Example:

let name = "Atharv";

let message = `Hello, ${name}`;

console.log(message); // Output: "Hello, Atharv"

12. What is a class in JavaScript?

» A class is a blueprint for creating objects that defines a set of properties and methods.

Example:

class Person {

  constructor(name, age) {

    this.name = name;

    this.age = age;

  }

greet() {

    return `Hello, my name is ${this.name} and I am ${this.age} years old.`;

  }

}

let person = new Person("Atharv", 30);

console.log(person.greet()); // Output: "Hello, my name is Atharv and I am 30 years old."

13. What is a constructor in JavaScript?

» A constructor is a special method that is called when an object is created from a class. It is used to initialize the object’s properties.

Example:

class Person {

  constructor(name, age) {

    this.name = name;

    this.age = age;

  }

greet() {

    return `Hello, my name is ${this.name} and I am ${this.age} years old.`;

  }

}

let person = new Person("John", 30);

console.log(person.greet()); // Output: "Hello, my name is Atharv and I am 30 years old."

14. What is the difference between == and == in JavaScript?

» == Performs type coercion, meaning it converts the operands to the same type before making the comparison, while == does not perform type coercion and  returns false if the operands have different types.

Example:

console.log(1 == "1"); // Output: true

console.log(1 === "1"); // Output: false

15. What is the difference between null and undefined in JavaScript?

» Undefined means a variable has been declared but has not been assigned a value, while null is explicitly set to represent no value.

Example:

let name;

console.log(name === undefined); // Output: true

let city = null;

console.log(city === null); // Output: true

16. What is the difference between let and const when declaring a constant?

» Both let and const can be used to declare a constant, but the value of a const cannot be changed after it is declared, while the value of a let can be changed.

Example:

const PI = 3.14;

// PI = 3.15;

console.log(PI); // Output: 3.14

let name = "Atharv";

name = "Atharv";

console.log(name); // Output: "Atharv"

17. What is a spread operator in JavaScript?

» The spread operator (...) allows an expression to be expanded in places where multiple elements or variables are expected.

Example:

let numbers = [1, 2, 3];

let newNumbers = [...numbers, 4, 5];

console.log(newNumbers); // Output: [1, 2, 3, 4, 5]

let person = { name: "Atharv", age: 30 };

let newPerson = { ...person, city: "Manipal" };

console.log(newPerson); // Output: { name: "Atharv", age: 30, city: "Manipal" }

18. What is a rest parameter in JavaScript?

» A rest parameter is used to represent an indefinite number of arguments as an array. It is prefixed with ... in the function definition.

Example:

function sum(...numbers) {

  return numbers.reduce((total, num) => total + num, 0);

}

console.log(sum(1, 2, 3)); // Output: 6

19. What is a destructuring assignment in JavaScript?

» Destructuring assignment is a way to extract data from arrays or objects into separate variables.

Example:

let person = { name: "Atharv", age: 30, city: "Manipal" };

let { name, age } = person;

console.log(name); // Output: "Atharv"

console.log(age); // Output: 30

let numbers = [1, 2, 3];

let [a, b] = numbers;

console.log(a); // Output: 1

console.log(b); // Output: 2

20. What is a higher-order function in JavaScript?

» A higher-order function is a function that takes one or more functions as arguments or returns a function as a result.

Example:

function multiplyBy(factor) {

  return function (number) {

    return number * factor;

  };

}

let double = multiplyBy(2);

console.log(double(5)); // Output: 10

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