This content originally appeared on DEV Community and was authored by Sidali Assoul
Introduction
JavaScript is a fundamental technology for web development, and mastering it is crucial for any aspiring developer. Whether you're preparing for a job interview or looking to sharpen your skills, understanding the key concepts and techniques in JavaScript can make a significant difference. This listicle covers 50 essential JavaScript interview questions that will help you showcase your expertise and confidence in any interview setting.
What is JavaScript and how is it different from other programming languages?
JavaScript is a high-level, interpreted programming language primarily used for web development to create interactive and dynamic web pages. Unlike other languages such as Java or C++, JavaScript is lightweight and can run in the browser, enabling real-time user interaction without the need for server-side processing.
Explain the difference between var, let, and const.
- var: Function-scoped, can be re-declared, and hoisted.
- let: Block-scoped, cannot be re-declared within the same scope, not hoisted to the top.
- const: Block-scoped, cannot be re-assigned or re-declared, not hoisted.
What are the data types supported by JavaScript?
JavaScript supports several data types, including:
- Primitive types: Number, String, Boolean, Null, Undefined, Symbol, and BigInt.
- Non-primitive types: Object (including arrays and functions).
How do you declare a variable in JavaScript?
Variables can be declared using var, let, or const. For example:
var x = 10;
let y = 20;
const z = 30;
What is hoisting in JavaScript?
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope. This means variable and function declarations are processed before any code is executed.
Explain the concept of closures in JavaScript.
A closure is a function that retains access to its lexical scope even when the function is executed outside that scope. Closures are useful for creating private variables and functions.
What are JavaScript functions? How do you define a function?
JavaScript functions are blocks of code designed to perform a particular task. They can be defined using function declarations or function expressions.
- Function Declaration: function greet() { console.log(\"Hello, World!\"); }
- Function Expression: const greet = function() { console.log(\"Hello, World!\"); };
What is the difference between function declarations and function expressions?
Function declarations are hoisted to the top of their scope, meaning they can be called before they are defined. Function expressions are not hoisted and cannot be called until they are defined.
Explain arrow functions and their advantages.
Arrow functions provide a more concise syntax for writing functions and do not have their own this context, making them useful for maintaining the context of this in callbacks and other functions.
const add = (a, b) => a + b;
What is the difference between == and ===?
- \==: Loose equality operator that performs type coercion before comparison.
- \===: Strict equality operator that checks both value and type without performing type coercion.
Objects and Arrays
How do you create an object in JavaScript?
Objects can be created using object literals or the new Object() syntax.
- Object Literal: const person = { name: \"John\", age: 30 };
- Using new Object(): const person = new Object(); person.name = \"John\"; person.age = 30;
What is the difference between an array and an object?
An array is an ordered collection of elements indexed by numeric keys, while an object is a collection of key-value pairs where the keys can be strings or symbols.
How do you iterate over an array?
You can iterate over an array using various methods like for, forEach, for...of, and more.
const arr = [1, 2, 3];
arr.forEach(item => console.log(item));
What are the various methods to manipulate arrays in JavaScript?
Common methods include push(), pop(), shift(), unshift(), splice(), slice(), map(), filter(), reduce(), and many more.
Explain the concept of this in JavaScript.
this refers to the object from which the function was called. Its value depends on the context in which the function is called.
How can you merge two or more objects in JavaScript?
You can merge objects using the Object.assign() method or the spread operator (...).
- Object.assign():javascriptCopy codeconst obj1 = { a: 1 }; const obj2 = { b: 2 }; const merged = Object.assign({}, obj1, obj2);
- Spread Operator:javascriptCopy codeconst merged = { ...obj1, ...obj2 };
What is the prototype chain?
The prototype chain is a series of links between objects used to inherit properties and methods. Each object has a prototype object, and this chain continues until an object with a null prototype is reached.
Explain how inheritance works in JavaScript.
Inheritance in JavaScript is achieved through prototypes. An object can inherit properties and methods from another object via its prototype.
What is the difference between call, apply, and bind?
- call(): Invokes a function with a specified this context and arguments provided individually.
- apply(): Invokes a function with a specified this context and arguments provided as an array.
- bind(): Creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments.
How do you clone an object in JavaScript?
You can clone an object using the spread operator, Object.assign(), or JSON.parse(JSON.stringify()).
- Spread Operator: const clone = { ...original };
Promises and Asynchronous Programming
What is a Promise in JavaScript?
A Promise is an object representing the eventual completion or failure of an asynchronous operation. It provides methods like then(), catch(), and finally() to handle the result.
Explain the difference between async and await.
async is used to declare an asynchronous function, while await is used to pause the execution of an async function until a Promise is resolved.
How does the event loop work in JavaScript?
The event loop continuously checks the message queue and the call stack, processing events and executing code, thereby enabling asynchronous operations.
What is the difference between synchronous and asynchronous code?
Synchronous code is executed sequentially, blocking further execution until the current operation completes. Asynchronous code allows other operations to run while waiting for an operation to complete.
How do you handle errors in Promises?
Errors in Promises can be handled using the catch() method.
promise
.then(result => console.log(result))
.catch(error => console.error(error));
Explain the concept of callback functions.
A callback function is a function passed as an argument to another function, to be executed once the latter completes its operation.
What is the purpose of setTimeout and setInterval?
- setTimeout: Executes a function after a specified delay.
- setInterval: Repeatedly executes a function at specified intervals.
How can you cancel a setTimeout?
You can cancel a setTimeout using the clearTimeout() method.
const timeoutID = setTimeout(() => { /* ... */ }, 1000);
clearTimeout(timeoutID);
What are async iterators?
Async iterators allow you to iterate over data that comes asynchronously, using for await...of loops.
Explain the difference between microtasks and macrotasks.
Microtasks are smaller tasks that run after the currently executing script, before rendering updates. Examples include Promises and process.nextTick(). Macrotasks are larger tasks that run after rendering updates, such as setTimeout and setInterval.
DOM Manipulation and Events
What is the DOM?
The Document Object Model (DOM) is a programming interface for HTML and XML documents. It represents the structure of a document as a tree of nodes.
How do you select elements in the DOM?
Elements can be selected using methods like getElementById(), getElementsByClassName(), getElementsByTagName(), querySelector(), and querySelectorAll().
How do you add an event listener in JavaScript?
You can add an event listener using the addEventListener() method.
element.addEventListener('click', function() {
// Event handler code
});
What is event delegation?
Event delegation is a technique for handling events at a higher level in the DOM rather than attaching event listeners to individual elements. This is achieved by taking advantage of event bubbling.
Explain the difference between event.target and event.currentTarget.
- event.target: Refers to the element that triggered the event.
- event.currentTarget: Refers to the element to which the event listener is attached.
How do you create and dispatch events?
Events can be created using the Event constructor and dispatched using the dispatchEvent() method.
const event = new Event('customEvent');
element.dispatchEvent(event);
How can you prevent the default behavior of an event?
The default behavior can be prevented using the preventDefault() method.
event.preventDefault();
What is the purpose of stopPropagation?
stopPropagation() prevents the event from bubbling up the DOM tree, stopping any parent handlers from being notified of the event.
How do you manipulate CSS classes using JavaScript?
CSS classes can be manipulated using methods like classList.add(), classList.remove(), classList.toggle(), and classList.contains().
element.classList.add('new-class');
Explain the concept of bubbling and capturing in event handling.
- Bubbling: Events start from the target element and bubble up to the root of the DOM tree.
- Capturing: Events start from the root of the DOM tree and capture down to the target element.
ES6 and Beyond
What are template literals?
Template literals are string literals that allow embedded expressions. They are enclosed by backticks () and can include placeholders indicated by ${expression}\`.
const name = \"World\";
const greeting =
`
Explain destructuring assignment.
Destructuring assignment allows you to unpack values from arrays or properties from objects into distinct variables.
`
const [a, b] = [1, 2];
const {name, age} = {name: \"John\", age: 30};
`
What are rest and spread operators?
- Rest operator (...): Collects all remaining elements into an array.javascriptCopy codefunction sum(...args) { return args.reduce((a, b) => a + b); }
- Spread operator (...): Expands an array into individual elements.javascriptCopy codeconst arr = \[1, 2, 3\]; const newArr = \[...arr, 4, 5\];
How do you use the import and export statements?
import and export are used for modular programming to share code between files.
- Export:javascriptCopy codeexport const name = \"John\"; export function greet() { return \"Hello\"; }
- Import:javascriptCopy codeimport { name, greet } from './module';
What are JavaScript modules and why are they useful?
JavaScript modules allow you to encapsulate code into separate files and reuse them across your application, improving code organization and maintainability.
Explain the concept of generators.
Generators are special functions that can be paused and resumed, allowing you to handle sequences of values or perform asynchronous tasks.
`
function* generator() {
yield 1;
yield 2;
yield 3;
}
const gen = generator();
console.log(gen.next().value);
`
What are symbols in JavaScript?
Symbols are unique and immutable primitive values used to create unique object property keys.
`
const sym = Symbol('description');
const obj = { [sym]: 'value' };
`
How do you use default parameters in functions?
Default parameters allow you to specify default values for function parameters if no arguments are provided.
function greet(name = \"World\") {
return
}
`
What is the purpose of let and const in block scope?
let and const provide block-level scoping, helping to avoid issues with variable hoisting and ensuring variables are only accessible within the block they are defined.
Explain the concept of promises chaining.
Promise chaining allows you to execute multiple asynchronous operations in sequence, with each operation starting after the previous one completes.
`
promise
.then(result => {
return anotherPromise(result);
})
.then(newResult => {
console.log(newResult);
});
`
Conclusion
Mastering these essential JavaScript concepts will significantly enhance your understanding and confidence, whether you're preparing for an interview or working on a project. Remember, practice is key to becoming proficient in JavaScript. Keep exploring, keep coding, and stay curious!
This content originally appeared on DEV Community and was authored by Sidali Assoul
Sidali Assoul | Sciencx (2024-07-15T13:51:08+00:00) 50 Essential JavaScript Interview Questions You Need to Know. Retrieved from https://www.scien.cx/2024/07/15/50-essential-javascript-interview-questions-you-need-to-know-2/
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