Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is the let keyword used for in ES6?
The let keyword is used to declare block-scoped variables.
Example:
let x = 10; if (true) { let x = 20; console.log(x); } console.log(x);
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Ques 2. What are template literals in ES6?
Template literals are a way to create strings with embedded expressions. They are enclosed by backticks (`).
Example:
let name = 'John'; let greeting = `Hello, ${name}!`;
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Ques 3. Explain the arrow functions in ES6.
Arrow functions are a concise way to write functions. They do not have their own 'this' and 'arguments'.
Example:
const add = (a, b) => a + b;
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Ques 4. What is the purpose of the 'class' keyword in ES6?
The 'class' keyword is used to create classes in JavaScript for object-oriented programming.
Example:
class Person { constructor(name) { this.name = name; } }
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Ques 5. What is the purpose of the 'map' function in ES6?
The 'map' function is used to transform each element of an array and create a new array with the results.
Example:
const numbers = [1, 2, 3]; const doubled = numbers.map(num => num * 2);
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Ques 6. Explain the 'filter' function in ES6.
The 'filter' function is used to create a new array with elements that satisfy a given condition.
Example:
const numbers = [1, 2, 3, 4, 5]; const evens = numbers.filter(num => num % 2 === 0);
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Ques 7. Explain the 'find' function in ES6.
The 'find' function is used to find the first element in an array that satisfies a given condition.
Example:
const numbers = [1, 2, 3, 4, 5]; const result = numbers.find(num => num > 2);
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Ques 8. What are the rest parameters in ES6?
Rest parameters allow a function to accept an indefinite number of arguments as an array.
Example:
function sum(...numbers) { return numbers.reduce((acc, num) => acc + num, 0); }
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Ques 9. What is the 'default parameter value' feature in ES6?
Default parameter values allow you to specify default values for function parameters if no value or undefined is passed.
Example:
function greet(name = 'Guest') { console.log(`Hello, ${name}!`); }
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Ques 10. Explain the 'Object.assign()' method in ES6.
Object.assign() is used to copy the values of all enumerable own properties from one or more source objects to a target object.
Example:
const target = {}; const source = { a: 1, b: 2 }; Object.assign(target, source);
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Ques 11. What is the 'Array.from()' method in ES6?
Array.from() is used to create a new shallow-copied array from an array-like or iterable object.
Example:
const arrayLike = { 0: 'a', 1: 'b', length: 2 }; const newArray = Array.from(arrayLike);
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Ques 12. Explain the 'Array.of()' method in ES6.
Array.of() is used to create a new array with a variable number of arguments, regardless of their types.
Example:
const newArray = Array.of(1, 'hello', true);
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Ques 13. Explain the 'Array.includes()' method in ES6.
Array.includes() is used to check if an array includes a certain element, returning true or false.
Example:
const numbers = [1, 2, 3, 4, 5]; const includesThree = numbers.includes(3);
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Ques 14. What is the 'Object.entries()' method in ES6?
Object.entries() is used to return an array of a given object's own enumerable string-keyed property [key, value] pairs.
Example:
const obj = { a: 1, b: 2 }; const entries = Object.entries(obj);
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Ques 15. Explain the 'Object.keys()' method in ES6.
Object.keys() is used to return an array of a given object's own enumerable string-keyed property names.
Example:
const obj = { a: 1, b: 2 }; const keys = Object.keys(obj);
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Ques 16. What is the purpose of the 'Object.values()' method in ES6?
Object.values() is used to return an array of a given object's own enumerable string-keyed property values.
Example:
const obj = { a: 1, b: 2 }; const values = Object.values(obj);
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Ques 17. Explain the concept of the 'for...of' loop in ES6.
The 'for...of' loop is used to iterate over values in an iterable object, such as an array, string, or Map.
Example:
const arr = [1, 2, 3]; for (const num of arr) { console.log(num); }
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Intermediate / 1 to 5 years experienced level questions & answers
Ques 18. Explain the differences between let, const, and var.
let and const are block-scoped, while var is function-scoped. const is used for constants and cannot be reassigned.
Example:
const PI = 3.14; let x = 10; var y = 5;
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Ques 19. What is destructuring assignment in ES6?
Destructuring assignment allows you to extract values from arrays or objects and assign them to variables.
Example:
const person = { name: 'John', age: 30 }; const { name, age } = person;
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Ques 20. Explain the concept of spread/rest operator in ES6.
The spread operator (...) is used to spread elements of an array or object, while the rest operator is used to collect elements into an array.
Example:
const arr = [1, 2, 3]; const newArr = [...arr, 4, 5];
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Ques 21. Explain the concept of promises in ES6.
Promises are used for asynchronous programming. They represent a value that may be available now, or in the future, or never.
Example:
const fetchData = new Promise((resolve, reject) => { /* async operation */ });
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Ques 22. What is the 'async/await' feature in ES6?
Async/await is a syntax sugar for working with promises. It makes asynchronous code look and behave more like synchronous code.
Example:
async function fetchData() { const result = await fetch('https://example.com'); }
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Ques 23. Explain the concept of modules in ES6.
Modules allow you to split your code into multiple files. You can export and import functionality between modules.
Example:
export const PI = 3.14; import { PI } from './math';
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Ques 24. What is the purpose of the 'reduce' function in ES6?
The 'reduce' function is used to accumulate the elements of an array into a single value.
Example:
const numbers = [1, 2, 3, 4, 5]; const sum = numbers.reduce((acc, num) => acc + num, 0);
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Ques 25. Explain the concept of the 'Symbol' data type in ES6.
Symbols are a new primitive data type in ES6, used to create unique identifiers.
Example:
const mySymbol = Symbol('description');
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Ques 26. What is the purpose of the 'Set' data structure in ES6?
A Set is a collection of values with no duplicate entries. It is iterable and can store various types of values.
Example:
const uniqueNumbers = new Set([1, 2, 3, 2, 1]);
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Ques 27. What is the purpose of the 'Array.findIndex()' method in ES6?
Array.findIndex() is used to find the index of the first element in an array that satisfies a given condition.
Example:
const numbers = [1, 2, 3, 4, 5]; const index = numbers.findIndex(num => num > 2);
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Experienced / Expert level questions & answers
Ques 28. Explain the 'WeakMap' and 'WeakSet' data structures in ES6.
WeakMap and WeakSet are similar to Map and Set, respectively, but they allow for garbage collection of keys and values that are not used elsewhere.
Example:
let weakMap = new WeakMap(); let obj = {}; weakMap.set(obj, 'some value');
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Ques 29. What is the 'Proxy' object in ES6?
The Proxy object is used to define custom behavior for fundamental operations (e.g., property lookup, assignment, enumeration, etc.).
Example:
const handler = { get: function(target, prop) { return prop in target ? target[prop] : 'Not found'; } }; const proxy = new Proxy({}, handler);
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Ques 30. Explain the concept of the 'Reflect' object in ES6.
The Reflect object provides methods for interceptable JavaScript operations. It is used for method invocation, property manipulation, etc.
Example:
Reflect.has(obj, 'property');
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