Harish Kumar’s Post

🔍 JavaScript Behavior You Might Have Seen (Function Declaration vs Expression) You write this: sayHi(); function sayHi() { console.log("Hello"); } 👉 Works perfectly ✅ Now try this: sayHello(); const sayHello = function () { console.log("Hello"); }; 👉 ReferenceError ❌ Same idea… same function… Why different behavior? This happens because of Hoisting 📌 What’s happening here? 👉 Function Declarations are fully hoisted So internally: function sayHi() { console.log("Hello"); } sayHi(); 👉 That’s why it works even before definition But for Function Expressions: const sayHello = function () {} 👉 This is treated like a variable (const) So internally: const sayHello; // not initialized (TDZ) sayHello(); // ❌ error 📌 Key Difference: ✔ Function Declaration 👉 Hoisted completely (can call before definition) ✔ Function Expression 👉 Not hoisted like function 👉 Behaves like variable 📌 Bonus case 👇 var sayHello = function () { console.log("Hello"); }; sayHello(); 👉 Works (because var is hoisted) BUT: sayHello(); // before assignment 👉 TypeError ❌ (not a function yet) 💡 Takeaway: ✔ Function declarations → fully hoisted ✔ Function expressions → behave like variables ✔ let/const → TDZ error ✔ var → undefined (then TypeError if called) 👉 Same function… different behavior based on how you write it 🔁 Save this before it confuses you again 💬 Comment “function” if this clicked ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🔍 JavaScript Concept You Might Have Heard (First-Class Functions/Citizens) You write this: function greet() { return "Hello"; } const sayHi = greet; console.log(sayHi()); // ? 👉 Output: Hello Wait… 👉 We assigned a function to a variable? 👉 And it still works? Now look at this 👇 function greet() { return "Hello"; } function execute(fn) { return fn(); } console.log(execute(greet)); // ? 👉 Passing function as argument? 👉 And calling it later? This is why JavaScript functions are called First-Class Functions 📌 What does that mean? 👉 Functions are treated like any other value 📌 What can you do with them? ✔ Assign to variables ✔ Pass as arguments ✔ Return from another function ✔ Store inside objects/arrays Example 👇 function outer() { return function () { return "Inside function"; }; } console.log(outer()()); // ? 👉 Function returning another function 📌 Why do we need this? 👉 This is the foundation of: ✔ Callbacks ✔ Closures ✔ Functional programming ✔ Event handling 💡 Takeaway: ✔ Functions are just values in JavaScript ✔ You can pass them, store them, return them ✔ That’s why they are “first-class citizens” 👉 If you understand this, you understand half of JavaScript 🔁 Save this for later 💬 Comment “function” if this clicked ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

JavaScript prototypes clicked for me the moment I stopped thinking about copying and started thinking about linking. Here's the mental model that made it stick 👇 🔹 The core idea Every object in JavaScript has a hidden link to another object — its prototype. When you access a property, JS doesn't just look at the object itself. It walks the chain: Check the object Not found? Check its prototype Still not found? Check the prototype's prototype Keep going until null That's the prototype chain. Simple as that. 🔹 See it in action jsconst user = { name: "Aman" }; const admin = { isAdmin: true }; admin.__proto__ = user; console.log(admin.name); // "Aman" ✅ admin doesn't have name — but JS finds it one level up. 🔹 Why constructor functions use this jsfunction User(name) { this.name = name; } User.prototype.sayHi = function () { console.log(`Hi, I am ${this.name}`); }; Every User instance shares one sayHi method. Not copied — linked. That's free memory efficiency, built into the language. 🔹 Two things worth keeping straight prototype → a property on constructor functions __proto__ → the actual link on any object The modern, cleaner way to set that link: jsconst obj = Object.create(user); 🔹 Why bother understanding this? Because it shows up everywhere — class syntax, frameworks, unexpected undefined bugs, performance decisions. Prototypes aren't a quirk. They are the inheritance model. One line to remember: JS doesn't copy properties. It searches a chain of linked objects. #JavaScript #Frontend #WebDevelopment #Programming

🧠 Day 22 — JavaScript Array Methods (map, filter, reduce) If you work with arrays, these 3 methods are a must-know 🚀 --- ⚡ 1. map() 👉 Transforms each element 👉 Returns a new array const nums = [1, 2, 3]; const doubled = nums .map(n => n * 2); console.log(doubled); // [2, 4, 6] --- ⚡ 2. filter() 👉 Filters elements based on condition const nums = [1, 2, 3, 4]; const even = nums.filter(n => n % 2 === 0); console.log(even); // [2, 4] --- ⚡ 3. reduce() 👉 Reduces array to a single value const nums = [1, 2, 3]; const sum = nums.reduce((acc, curr) => acc + curr, 0); console.log(sum); // 6 --- 🧠 Quick Difference map → transform filter → select reduce → combine --- 🚀 Why it matters ✔ Cleaner & functional code ✔ Less loops, more readability ✔ Widely used in React & real apps --- 💡 One-line takeaway: 👉 “map transforms, filter selects, reduce combines.” --- Master these, and your JavaScript will feel much more powerful. #JavaScript #ArrayMethods #WebDevelopment #Frontend #100DaysOfCode 🚀

🔍 JavaScript Behavior You Might Have Seen (Prototype Chain) You write this: const arr = [1, 2, 3]; console.log(arr.toString()); // ? 👉 Did you define toString on this array? 👉 Did you even define it anywhere? No. Still it works. Now think step by step 👇 When you do: 👉 arr.toString JavaScript checks: Inside arr → ❌ not found Inside Array.prototype → ❌ not found Inside Object.prototype → ✔ found This step-by-step lookup is called the Prototype Chain 📌 What is Prototype Chain? 👉 It’s the process JavaScript uses to find a property by searching up through prototypes. 📌 How it works? Every object is linked to another object (its prototype) So lookup happens like this: 👉 arr → Array.prototype → Object.prototype → null Same with strings 👇 const str = "hello"; console.log(str.hasOwnProperty("length")); // ? 👉 hasOwnProperty is not in string JS finds it here: 👉 str → String.prototype → Object.prototype 📌 Important point: 👉 JavaScript keeps searching until it finds the property or reaches null 💡 Takeaway: ✔ Prototype Chain = lookup mechanism ✔ JS searches step by step ✔ That’s how all built-in methods work 👉 If you understand this, you’ll understand how JavaScript really works 💬 Comment “chain” if this clicked 🔁 Save this for later ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🔍 JavaScript Behavior You Might Have Seen (Hoisting) You write this: console.log(a); var a = 10; You expect: 👉 ReferenceError But you get: 👉 undefined 🤯 Now try this: console.log(b); let b = 20; 👉 This time you get: ReferenceError ❌ Same with: console.log(c); const c = 30; 👉 Error again ❌ Now look at functions 👇 sayHi(); function sayHi() { console.log("Hello"); } 👉 This works ✅ But this doesn’t: sayHello(); const sayHello = function () { console.log("Hello"); }; 👉 ReferenceError ❌ Why different behavior? This happens because of Hoisting 📌 What is Hoisting? 👉 Hoisting is JavaScript’s behavior of moving declarations to the top of their scope before execution. 📌 What’s actually happening? ✔ var: var a; console.log(a); // undefined a = 10; 👉 Hoisted + initialized ✔ let / const: 👉 Hoisted but NOT initialized 👉 Access before declaration → ReferenceError 👉 (Temporal Dead Zone) ✔ Function declarations: function sayHi() {} 👉 Fully hoisted (can be called before definition) ✔ Function expressions: const sayHello = function () {} 👉 Behave like variables 👉 Not usable before declaration 💡 Takeaway: ✔ var → hoisted + initialized ✔ let / const → hoisted but restricted (TDZ) ✔ Function declarations → fully hoisted ✔ Function expressions → NOT hoisted like functions 👉 Same concept… different rules 🔁 Save this before hoisting confuses you again 💬 Comment “hoisting” if this made sense ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🔁 JavaScript Tip: Convert Object → Array Easily! Working with objects in JavaScript? Sometimes you need to transform them into arrays for better handling — especially in loops, UI rendering, or API data processing. Here are 3 powerful methods you should know: ✅ Object.keys() → Get all keys ✅ Object.values() → Get all values ✅ Object.entries() → Get key-value pairs 💡 Example: const zoo = { lion: "🦁", panda: "🐼" }; 👉 "Object.keys(zoo)" → ['lion', 'panda'] 👉 "Object.values(zoo)" → ['🦁', '🐼'] 👉 "Object.entries(zoo)" → [['lion', '🦁'], ['panda', '🐼']] 🚀 These methods are super useful in React, API handling, and data transformations. #JavaScript #WebDevelopment #Frontend #ReactJS #CodingTips #Developers #100DaysOfCode

🔍 JavaScript Behavior You Might Have Seen (Array methods: mutate vs not) You write this: const arr = [1, 2, 3]; const newArr = arr.push(4); console.log(arr); // ? console.log(newArr); // ? You expect: 👉 newArr to be a new array But you get: 👉 arr → [1, 2, 3, 4] 👉 newArr → 4 This happens because of mutating array methods 📌 What does “mutate” mean? 👉 It means changing the original array In this case: 👉 push() modifies arr directly 👉 And returns the new length (not a new array) Now look at this 👇 const arr = [1, 2, 3]; const newArr = arr.map(x => x * 2); console.log(arr); // ? console.log(newArr); // ? 👉 arr → [1, 2, 3] 👉 newArr → [2, 4, 6] This is a non-mutating method 👉 It creates a new array 👉 Original array stays unchanged 📌 Common mutating methods: ✔ push ✔ pop ✔ shift ✔ unshift ✔ splice ✔ sort ✔ reverse 📌 Common non-mutating methods: ✔ map ✔ filter ✔ slice ✔ concat ✔ reduce 📌 Real problem: You think you’re creating a new array… 👉 But you accidentally modify the original one 💡 Takeaway: ✔ Mutating methods change original array ✔ Non-mutating methods return a new array ✔ Always be careful when updating state (especially in React) 👉 One wrong method can break your logic silently 🔁 Save this for later 💬 Comment “array” if this made sense ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🔍 JavaScript Behavior You Might Have Seen (Promise Chaining) You write this: Promise.resolve(1) .then((res) => { console.log(res); return res + 1; }) .then((res) => { console.log(res); return res + 1; }); 👉 Output: 1 2 Now try this 👇 Promise.resolve(1) .then((res) => { console.log(res); }) .then((res) => { console.log(res); }); 👉 Output: 1 undefined 🤯 Same chain… different result… why? This happens because of Promise Chaining 📌 What is Promise Chaining? 👉 It’s the process of passing the result from one .then() to the next 📌 What’s happening here? ✔ If you return a value: 👉 It goes to the next .then() ✔ If you don’t return anything: 👉 undefined is passed So this: .then((res) => { console.log(res); }) 👉 is actually: .then((res) => { console.log(res); return undefined; }) 📌 Important rule: 👉 Every .then() returns a new Promise 📌 Bonus case 👇 .then((res) => { return Promise.resolve(res + 1); }) 👉 Next .then() waits for it automatically 💡 Takeaway: ✔ Return value → passed to next .then() ✔ No return → undefined ✔ Returning Promise → waits automatically 👉 Promise chain is all about “what you return” 🔁 Save this for later 💬 Comment “promise” if this clicked ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🧠 Day 6 — Closures in JavaScript (Explained Simply) Closures are one of the most powerful (and frequently asked) concepts in JavaScript — and once you understand them, a lot of things start to click 🔥 --- 🔐 What is a Closure? 👉 A closure is when a function “remembers” variables from its outer scope even after that scope has finished executing. --- 🔍 Example: function outer() { let count = 0; return function inner() { count++; console.log(count); }; } const counter = outer(); counter(); // 1 counter(); // 2 --- 🧠 What’s happening? inner() still has access to count Even after outer() has finished execution This happens because of lexical scoping --- 🚀 Why Closures Matter ✔ Data privacy (like encapsulation) ✔ Used in callbacks & async code ✔ Foundation of React hooks (useState) ✔ Helps create reusable logic --- ⚠️ Common Pitfall for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1000); } 👉 Output: 3 3 3 ✔ Fix: for (let i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1000); } --- 💡 One-line takeaway: 👉 “A closure remembers its outer scope even after it’s gone.” --- If you’re learning JavaScript fundamentals, closures are a must-know — they show up everywhere. #JavaScript #Closures #WebDevelopment #Frontend #100DaysOfCode 🚀