Top 10 Javascript (ES6+) Interview Questions with sample code

Welcome to the world of JavaScript interview questions! If you're preparing for a job interview as a JavaScript developer, you've come to the right place. We've compiled a list of the most common questions that you may encounter during your interview, along with some tips and sample answers to help you showcase your skills and knowledge. Whether you're an experienced developer or just starting out, this guide will provide you with a comprehensive understanding of the concepts and best practices of JavaScript. So, grab a cup of coffee, get comfortable, and let's dive into the world of JavaScript interview questions! 

1. What is the difference between let and var in JavaScript? var is function scoped, whereas let is block scoped. This means that a variable declared with var can be accessed within the entire function, whereas a variable declared with let can only be accessed within the block in which it was declared. Example: //var function testVar() { var x = 1; if (true) { var x = 2; // x is 2 console.log(x); // 2 } console.log(x); // 2 } testVar(); //let function testLet() { let x = 1; if (true) { let x = 2; // x is 2 console.log(x); // 2 } console.log(x); // 1 } testLet();

2. Define arrow functions in ES6?

• Arrow functions, also known as "fat arrow" functions, are a shorthand notation for writing anonymous functions in JavaScript. They are defined using the => syntax and do not have their own this context, instead they inherit the this context of their parent scope.
• Example: // traditional function function sayHello(name) { return "Hello, " + name; } // arrow function let sayHello = (name) => { return "Hello, " + name; } // concise arrow function (single line) let sayHello = (name) => "Hello, " + name;

3. How do you use template literals in JavaScript?

• Template literals are a way to create string literals that can include expressions. They are defined using backticks (`) instead of single or double quotes and expressions can be included within ${} .
• Example: let name = "John"; let age = 30; console.log(`My name is ${name} and I am ${age} years old.`); //output: My name is John and I am 30 years old.

4. What is destructuring in ES6?

• Destructuring is a way to extract data from arrays or objects and assign them to variables. It allows you to extract data from nested objects and arrays easily.
• Example: // destructuring array let numbers = [1, 2, 3]; let [a, b, c] = numbers; console.log(a); // 1 console.log(b); // 2 console.log(c); // 3 // destructuring object let person = { name: "John", age: 30 }; let {name, age} = person; console.log(name); // John console.log(age); // 30

5. How do you use the spread operator in JavaScript?

• The spread operator (...) is used to spread the elements of an iterable object (such as an array) into a new array or to spread the properties of an object into a new object. It can also be used to combine multiple arrays or objects into a single array or object.
• Example: // spread operator for arrays let numbers1 = [1, 2, 3]; let numbers2 = [4, 5, 6]; let numbers3 = [...numbers1, ...numbers2]; console.log(numbers3); // [1, 2, 3, 4, 5, 6] // spread operator for objects let person1 = { name: "John", age: 30 }; let person2 = { ...person1, location: "New York" }; console.log(person2); // {name: "John", age: 30, location: "New York" }

6. Explain the concept of classes in ES6?

• Classes in JavaScript are a way to create objects using a class-based syntax. They are similar to classes in other object-oriented programming languages and provide a way to create objects with similar properties and methods.
• Example: class Person { constructor(name, age) { this.name = name; this.age = age; } sayHello() { console.log(`Hello, my name is ${this.name}`); } } let john = new Person("John", 30); console.log(john.name); // "John" console.log(john.age); // 30 john.sayHello(); // "Hello, my name is John"

7. How do you use the 'for of' loop in JavaScript?

• The for...of loop is used to iterate over the values of an iterable object, such as an array or a string. It is similar to the for...in loop, but it only iterates over the values of the object and not its properties.
• Example: let numbers = [1, 2, 3, 4, 5]; for (let number of numbers) { console.log(number); } // Output: 1 2 3 4 5
  

8. Can you explain the concept of modules in ES6?

• Modules in JavaScript are a way to organize and reuse code. They allow you to define variables, functions and classes in one file, and then export them to be used by other files. In ES6, the import and export keywords are used to work with modules. Example: // myModule.js export let x = 1; export function add(a, b) { return a + b; } // main.js import { x, add } from './myModule'; console.log(x); // 1 console.log(add(1, 2)); // 3

9. How do you use the Map and Set objects in JavaScript?

• Map and Set are new objects introduced in ES6. Map is a collection of key-value pairs, and Set is a collection of unique values. They are useful for working with data that needs to be stored in a specific order or for data that needs to be unique. 
• Example:
• // Map
• let map = new Map(); map.set("name", "John"); map.set("age", 30); console.log(map.get("name")); // John console.log(map.get("age")); // 30 // Set let set = new Set(); set.add(1); set.add(2); set.add(3); console.log(set.has(1)); // true console.log(set.has(4)); // false

10. Explain Promises in JavaScript?

• Promises are a way to handle asynchronous operations in JavaScript. They represent a value that may not be available yet but will be at some point in the future. Promises provide a way to register callbacks for when the value is available, as well as methods for handling errors.
• Example: let promise = new Promise((resolve, reject) => { setTimeout(() => { resolve("Hello, World!"); }, 1000); }); promise.then(value => { console.log(value); // "Hello, World!" });