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Closure + setTimeout Interview Questions

 A closure in JavaScript is a function that retains access to its lexical scope, even when the function is executed outside that scope. This means that a closure can remember and access variables from its outer function even after that function has finished executing. citeturn0search0 Key Characteristics of Closures: Lexical Scoping: Functions in JavaScript form closures by capturing variables from their surrounding lexical environment. This allows inner functions to access variables defined in their outer functions. Persistent State: Closures enable functions to maintain a persistent state. Since the inner function has access to the outer function's variables, it can remember and modify these variables across multiple invocations. Practical Applications of Closures: Data Encapsulation: Closures allow for the creation of private variables, enabling data hiding and encapsulation. This is particularly useful in module patterns where certain data should not be expo...
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Promises | Creating Promises | Chaining | Error Handling and Async/Await in JavaScript

1. Why Promises and Async/Await Came into Existence a. The Problem with Callbacks Callback Hell: Nested callbacks made code difficult to read and maintain. fetchData((data1) => { processData(data1, (data2) => { saveData(data2, (data3) => { displayData(data3, (data4) => { console.log("Final result:", data4); }); }); }); }); Inversion of Control: Callbacks rely on external functions to execute them, leading to trust issues. Error Handling: Handling errors in nested callbacks is cumbersome. b. The Need for a Better Solution Promises were introduced to handle asynchronous operations in a more structured and readable way. Async/await was later introduced to make working with Promises even simpler and more intuitive. 2. Promises in JavaScript a. What is a Promise? A Promise is an object representing the eventual completion (or failure) of an asynchronous operation. It has three states: Pending: Initial state, neither fulfilled nor rejec...
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Callback Hell | Inversion of Control | Bad and Good Practices

 1. Introduction to Callbacks A callback is a function passed as an argument to another function and is executed after some operation is completed. Callbacks are commonly used in asynchronous operations like handling API requests, file I/O, or timers. Example: function fetchData(callback) {   setTimeout(() => {     const data = "Some data";     callback(data);   }, 1000); } fetchData((data) => {   console.log(data); // "Some data" }); 2. What is Callback Hell? Callback Hell (also known as the Pyramid of Doom) occurs when multiple nested callbacks are used to handle asynchronous operations, making the code difficult to read and maintain. Example of Callback Hell: fetchData((data1) => {   processData(data1, (data2) => {     saveData(data2, (data3) => {       displayData(data3, (data4) => {         console.log("Final result:", data4);       });     });  ...
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Promise APIs + Interview Questions | all | allSettled | race | any

1. Introduction to Promises A Promise in JavaScript is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises have three states: Pending: Initial state, neither fulfilled nor rejected. Fulfilled: The operation completed successfully. Rejected: The operation failed.  2. Promise APIs a. Promise.all() Purpose: Takes an iterable of promises and returns a single promise that resolves when all of the promises in the iterable have resolved, or rejects if any of the promises reject. Use Case: Useful when you want to wait for multiple asynchronous operations to complete successfully. Example: const p1 = Promise.resolve(1); const p2 = Promise.resolve(2); const p3 = Promise.resolve(3); Promise.all([p1, p2, p3])   .then(values => console.log(values)) // [1, 2, 3]   .catch(error => console.error(error)); Behavior: If all promises resolve, Promise.all resolves with an array of results. If any promise rejects,...
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Javascript Interview Questions ( Var, Let and Const ) - Hoisting, Scoping, Shadowing

  1. var , let , and const Declarations : var : Introduced in ES5, var is function-scoped and can be redeclared within the same scope. Variables declared with var are hoisted to the top of their scope but are initialized with undefined . let : Introduced in ES6, let is block-scoped and cannot be redeclared within the same scope. Variables declared with let are hoisted but are not initialized, leading to a "temporal dead zone" until the declaration is encountered. const : Also introduced in ES6, const is block-scoped and must be initialized at the time of declaration. Like let , const declarations are hoisted but not initialized, resulting in a temporal dead zone. Variables declared with const cannot be reassigned. 2. Hoisting Hoisting refers to the JavaScript engine's behavior of moving variable and function declarations to the top of their scope before code execution. var Hoisting : Variables declared with var are hoisted and initialized with undefined . This m...
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