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JavaScript is a powerful and versatile programming language used for web development, server-side scripting, and even desktop/mobile applications. Mastering these core concepts will enhance your coding skills and prepare you for technical interviews.
- What is JavaScript, and how does it work?
JavaScript is a single-threaded, interpreted scripting language that runs in web browsers and server environments (Node.js). It manipulates the DOM, handles events, and enables asynchronous operations.
2. Difference between `var`, `let`, and `const`
var: Function-scoped, hoisted, can be redeclared.let: Block-scoped, can be reassigned but not redeclared.const: Block-scoped, cannot be reassigned or redeclared.
3. Hoisting in JavaScript
JavaScript moves variable and function declarations to the top of their scope before execution.
console.log(a); // undefined (not ReferenceError) var a = 5;
4. Closures
A function that remembers its lexical scope even when executed outside it.
function outer() {
let x = 10;
return function inner() {
console.log(x);
};
}
const closureFunc = outer();
closureFunc(); // 10
5. Event Loop
JavaScript uses an event loop to handle asynchronous callbacks. It processes the call stack, then checks the callback queue.
6. Synchronous vs. Asynchronous JavaScript
- Synchronous: Code runs sequentially.
- Asynchronous: Non-blocking operations (e.g.,
setTimeout, Promises).
7. `null` vs. `undefined`
null: Explicitly assigned “no value.”undefined: Variable declared but not assigned.
8. Type Coercion
Automatic type conversion (e.g., "5" + 2 = "52").
9. Higher-Order Functions
Functions that take other functions as arguments or return them.
const numbers = [1, 2, 3]; const doubled = numbers.map(num => num 2);
10. `map`, `filter`, `reduce`
map: Transforms each element.filter: Returns elements matching a condition.reduce: Aggregates values into a single result.
11. Currying
Converting a function with multiple arguments into a sequence of functions.
const add = a => b => a + b; add(2)(3); // 5
12. Deep Copy vs. Shallow Copy
- Shallow Copy: Copies references (e.g.,
Object.assign). - Deep Copy: Creates a new object (e.g.,
JSON.parse(JSON.stringify(obj))).
13. Debouncing & Throttling
- Debouncing: Delays execution until after a pause (e.g., search input).
- Throttling: Limits execution rate (e.g., scroll events).
14. `call`, `apply`, `bind`
call: Invokes a function with a given `this` and arguments.apply: Similar, but takes arguments as an array.bind: Returns a new function with a boundthis.
15. Prototype Inheritance
Objects inherit properties/methods from prototypes.
function Person(name) { this.name = name; }
Person.prototype.greet = function() { console.log(<code>Hello, ${this.name}</code>); };
16. ES5 vs. ES6
- ES5:
var, function-based syntax. - ES6:
let/const, arrow functions, classes, modules.
17. Arrow Functions
Shorter syntax, no `this` binding.
const square = x => x x;
18. Promises
Handle asynchronous operations with states: Pending, Fulfilled, Rejected.
fetch('url').then(res => res.json()).catch(err => console.log(err));
19. `async/await`
Syntactic sugar for Promises.
async function fetchData() {
const res = await fetch('url');
const data = await res.json();
}
20. `localStorage`, `sessionStorage`, Cookies
localStorage: Persistent storage.sessionStorage: Cleared on tab close.- Cookies: Sent with HTTP requests, limited size.
21. JavaScript Modules
Organize code using `import`/`export`.
// module.js
export const PI = 3.14;
// app.js
import { PI } from './module.js';
22. `WeakMap` & `WeakSet`
Collections with weak references (keys are objects).
23. `==` vs. `===`
==: Loose equality (type coercion).===: Strict equality (no coercion).
24. `this` in JavaScript
Refers to the execution context (varies in functions, arrow functions, methods).
25. Function Declaration vs. Expression
- Declaration: Hoisted (
function foo() {}). - Expression: Not hoisted (
const foo = function() {}).
26. Event Delegation
Attaching a single event listener to a parent instead of multiple children.
27. `forEach` vs. `map`
forEach: Executes a function for each element (no return).map: Returns a new array.
28. Spread (`…`) & Rest (`…`) Operators
- Spread: Expands iterables (
[...arr]). - Rest: Collects arguments into an array (
function(...args) {}).
29. Template Literals
String interpolation with backticks.
const name = 'Alice';
console.log(<code>Hello, ${name}!</code>);
30. Generators
Functions that can pause/resume execution (`function`).
function idGenerator() {
let id = 0;
while (true) yield id++;
}
You Should Know:
- Debugging JavaScript: Use
console.log(),debugger, and Chrome DevTools. - Performance Optimization: Minimize reflows, use
requestAnimationFrame. - Security: Sanitize inputs to prevent XSS, use HTTPS for APIs.
What Undercode Say:
Mastering JavaScript fundamentals is crucial for modern web development. Practice these concepts with real-world projects, contribute to open-source, and explore frameworks like React and Node.js.
Expected Output:
A well-structured JavaScript guide with practical examples, debugging tips, and security best practices.
References:
Reported By: Sumit Yadav – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
