JavaScript Hoisting: var vs let vs const

🔍 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 Hoisting Explained Simply Hoisting is one of those JavaScript concepts that can feel confusing — especially when your code behaves unexpectedly. Here’s a simple way to understand it 👇 🔹 What is Hoisting? Hoisting means JavaScript moves declarations to the top of their scope before execution. But there’s a catch 👇 🔹 Example with var console.log(a); var a = 10; Output: undefined Why? Because JavaScript internally treats it like: var a; console.log(a); a = 10; 🔹 What about let and const? console.log(b); let b = 20; This throws a ReferenceError. Because "let" and "const" are hoisted too — but they stay in a “temporal dead zone” until initialized. 🔹 Function hoisting Functions are fully hoisted: sayHello(); function sayHello() { console.log("Hello"); } This works because the function is available before execution. 🔹 Key takeaway • "var" → hoisted with "undefined" • "let/const" → hoisted but not initialized • functions → fully hoisted 💡 One thing I’ve learned: Many “weird” JavaScript bugs come from not understanding hoisting properly. Curious to hear from other developers 👇 Did hoisting ever confuse you when you started learning JavaScript? #javascript #frontenddevelopment #webdevelopment #reactjs #softwareengineering #developers

🔍 JavaScript Bug You Might Have Seen (setTimeout vs Promise) You write this code: console.log("Start"); setTimeout(() => { console.log("Timeout"); }, 0); Promise.resolve().then(() => { console.log("Promise"); }); console.log("End"); What do you expect? Start Timeout Promise End But actual output is: Start End Promise Timeout This happens because of the Event Loop 📌 What is the Event Loop? 👉 The event loop is the mechanism that decides which task runs next in JavaScript’s asynchronous execution. 📌 Priority order (very important): 1️⃣ Call Stack (synchronous code) 2️⃣ Microtask Queue 3️⃣ Macrotask Queue 📌 What’s inside each queue? 👉 Microtask Queue (HIGH priority): ✔ Promise.then / catch / finally ✔ queueMicrotask ✔ MutationObserver 👉 Macrotask Queue (LOWER priority): ✔ setTimeout ✔ setInterval ✔ setImmediate ✔ I/O tasks ✔ UI rendering events Execution flow: ✔ Step 1: Run all synchronous code 👉 Start → End ✔ Step 2: Execute ALL microtasks 👉 Promise ✔ Step 3: Execute macrotasks 👉 setTimeout So final order becomes: Start End Promise Timeout 💡 Takeaway: ✔ Microtasks run before macrotasks ✔ Promises > setTimeout ✔ setTimeout(fn, 0) is NOT immediate 👉 Understand queues = master async JS 🔁 Save this for later 💬 Comment “event loop” if this made sense ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🔍 JavaScript Bug You Might Have Seen (typeof null === "object") You write this: console.log(typeof null); // ? What do you expect? 👉 "null" But you get: 👉 "object" 🤯 Wait… null is NOT an object… So why is JavaScript saying this? This happens because of a historical bug in JavaScript 📌 What’s going on? In the early days of JavaScript: 👉 Values were stored in a low-level format 👉 Objects were identified by a specific type tag Unfortunately… 👉 null was given the same tag as objects So: typeof null === "object" 📌 Important point: 👉 This is NOT correct behavior 👉 But it was never fixed (for backward compatibility) 📌 So how do you check for null? ❌ Don’t do this: typeof value === "null" ✔ Do this instead: value === null 💡 Takeaway: ✔ typeof null returns "object" (bug) ✔ It’s a legacy behavior in JavaScript ✔ Always check null using === null 👉 Not everything in JavaScript makes sense… some things just stayed for history 😄 🔁 Save this before it confuses you again 💬 Comment “null” if this surprised you ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

🔍 JavaScript Bug You Might Have Seen (typeof null === "object") You write this: console.log(typeof null); // ? What do you expect? 👉 "null" But you get: 👉 "object" 🤯 Wait… null is NOT an object… So why is JavaScript saying this? This happens because of a historical bug in JavaScript 📌 What’s going on? In the early days of JavaScript: 👉 Values were stored in a low-level format 👉 Objects were identified by a specific type tag Unfortunately… 👉 null was given the same tag as objects So: typeof null === "object" 📌 Important point: 👉 This is NOT correct behavior 👉 But it was never fixed (for backward compatibility) 📌 So how do you check for null? ❌ Don’t do this: typeof value === "null" ✔ Do this instead: value === null 💡 Takeaway: ✔ typeof null returns "object" (bug) ✔ It’s a legacy behavior in JavaScript ✔ Always check null using === null 👉 Not everything in JavaScript makes sense… some things just stayed for history 😄 🔁 Save this before it confuses you again 💬 Comment “null” if this surprised you ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer

Day 5: The Shortest JavaScript Program — What happens when you write NOTHING? 📄✨ Today I learned that even if you create a totally empty .js file and run it in a browser, JavaScript is already working hard behind the scenes. 🕵️♂️ The "Shortest Program" If your file has zero lines of code, the JavaScript Engine still does three major things: Creates a Global Execution Context. Creates a window object (in browsers). Creates the this keyword. 🪟 What is window? The window is a massive object created by the JS engine that contains all the built-in methods and properties (like setTimeout, localStorage, or console) provided by the browser environment. 🧭 The this Keyword At the global level, JavaScript sets this to point directly to the window object. 👉 Proof: If you type console.log(this === window) in an empty file, it returns true! 🌐 The Global Space I also explored the Global Space—which is any code you write outside of a function. If you declare var x = 10; in the global space, it automatically gets attached to the window object. You can access it using x, window.x, or this.x. They all point to the same memory location! 💡 Key Takeaway: Anything not inside a function sits in the Global Memory Space. Keeping this space clean is vital for performance and avoiding variable name collisions in large apps! It’s fascinating to see that even before we write our first line of code, JavaScript has already set up the entire "universe" for us to work in. #JavaScript #WebDevelopment #NamasteJavaScript #ExecutionContext #WindowObject #JSFundamentals #CodingJourney #FrontendEngineer

⚡ Day 7 — JavaScript Event Loop (Explained Simply) Ever wondered how JavaScript handles async tasks while being single-threaded? 🤔 That’s where the Event Loop comes in. --- 🧠 What is the Event Loop? 👉 The Event Loop manages execution of code, async tasks, and callbacks. --- 🔄 How it works: 1. Call Stack → Executes synchronous code 2. Web APIs → Handle async tasks (setTimeout, fetch, etc.) 3. Callback Queue / Microtask Queue → Stores callbacks 4. Event Loop → Moves tasks to the stack when it’s empty --- 🔍 Example: console.log("Start"); setTimeout(() => { console.log("Timeout"); }, 0); Promise.resolve().then(() => { console.log("Promise"); }); console.log("End"); --- 📌 Output: Start End Promise Timeout --- 🧠 Why? 👉 Microtasks (Promises) run before macrotasks (setTimeout) --- 🔥 One-line takeaway: 👉 “Event Loop decides what runs next in async JavaScript.” --- If you're learning async JS, understanding this will change how you debug forever. #JavaScript #EventLoop #WebDevelopment #Frontend #100DaysOfCode 🚀

What will happen if you call a variable before initialization? 🤔 That is called Hoisting 👉 "JavaScript moves declarations to the top of their scope before execution" Sounds confusing? Let’s break it down 👇 When you create variables or functions, JavaScript runs your code in 2 phases: 1️⃣ Memory Creation Phase Before execution, JavaScript scans your code and allocates memory Example (mentally): var a → undefined let b → uninitialized (Temporal Dead Zone) 2️⃣ Execution Phase Now JavaScript runs your code line by line 👉 If you access variables before initialization: var → returns undefined let / const → ReferenceError Why does this happen? Because: var is initialized with undefined in memory let and const are hoisted but stay in the Temporal Dead Zone (TDZ) until the line where they are declared Simple way to remember: var => “exist, but don’t have a value yet” let / const => “Don’t touch before declaration” ⚡ Bonus: Function declarations are fully hoisted, so you can call them before defining them Curious how functions behave in hoisting? 🤔 Go Google function vs function expression in JavaScript — it’ll surprise you 👀 That’s hoisting in JavaScript 🚀 #javascript #webdevelopment #coding #frontend #learninpublic #hoisting

🔍 JavaScript Bug You Might Have Seen (null vs undefined) You write this: let a; console.log(a); // ? let b = null; console.log(b); // ? Both look similar… But they are NOT the same. 💥 Output: undefined null Now here’s where it gets confusing 👇 console.log(null == undefined); // ? console.log(null === undefined); // ? 💥 Output: true false Wait… WHAT? 🤯 This happens because of how JavaScript treats them. 📌 What is undefined? 👉 A variable that is declared but NOT assigned a value 📌 What is null? 👉 A value that represents “intentional absence of value” (You explicitly set it) 📌 Then why this? null == undefined // true Because == does loose comparison 👉 It treats them as equal BUT: null === undefined // false Because === checks: ✔ Value ✔ Type 💡 Takeaway: ✔ undefined → JS gives it ✔ null → YOU assign it ✔ == → treats them equal ✔ === → they are different 👉 Use undefined for default/unassigned 👉 Use null when you intentionally want “no value” 🔁 Save this before it confuses you again 💬 Comment “null” if this clicked ❤️ Like for more JavaScript deep dives #javascript #frontend #codingtips #webdevelopment #js #developer