React useEffect: When to Run Effects

🧵 How JavaScript Does 10 Things at Once (While Being Single-Threaded) 🔄 Ever wondered how JavaScript handles API calls, timers, or async tasks without freezing the browser UI? The secret is the Event Loop. Even though JavaScript is single-threaded (it can do only one thing at a time), the Event Loop allows it to be asynchronous and non-blocking. Here’s a simple 4-step breakdown of how it works 👇 1️⃣ Call Stack   This is the “Now” zone. JavaScript executes synchronous code here.   If a function is in the stack, the engine is busy. 2️⃣ Web APIs / Node APIs   When you use `setTimeout`, `fetch`, or DOM events, JavaScript hands them off to the browser/Node environment.   This keeps the call stack free so the UI doesn’t freeze. 3️⃣ Callback Queue & Microtask Queue   Once async tasks complete, their callbacks wait here.   👉 Promises (Microtasks) always run before timers (`setTimeout`). 4️⃣ Event Loop   This is the coordinator.   It constantly checks: • Is the Call Stack empty?   • If yes → move the next task from the queue to the stack. 🔑 Golden Rule:   Avoid blocking the Event Loop with heavy synchronous code — otherwise users will experience laggy interfaces. Learning this really helped me understand async JavaScript better 🚀 #JavaScript #WebDevelopment #CodingTips #SoftwareEngineering #EventLoop #Programming

🧵 How JavaScript Does 10 Things at Once (While Being Single-Threaded) 🔄 Ever wondered how JavaScript handles API calls, timers, or async tasks without freezing the browser UI? The secret is the Event Loop. Even though JavaScript is single-threaded (it can do only one thing at a time), the Event Loop allows it to be asynchronous and non-blocking. Here’s a simple 4-step breakdown of how it works 👇 1️⃣ Call Stack This is the “Now” zone. JavaScript executes synchronous code here. If a function is in the stack, the engine is busy. 2️⃣ Web APIs / Node APIs When you use `setTimeout`, `fetch`, or DOM events, JavaScript hands them off to the browser/Node environment. This keeps the call stack free so the UI doesn’t freeze. 3️⃣ Callback Queue & Microtask Queue Once async tasks complete, their callbacks wait here. 👉 Promises (Microtasks) always run before timers (`setTimeout`). 4️⃣ Event Loop This is the coordinator. It constantly checks: • Is the Call Stack empty? • If yes → move the next task from the queue to the stack. 🔑 Golden Rule: Avoid blocking the Event Loop with heavy synchronous code — otherwise users will experience laggy interfaces. Learning this really helped me understand async JavaScript better 🚀 #JavaScript #WebDevelopment #CodingTips #SoftwareEngineering #EventLoop #Programming #javascript

🧠 Microtasks vs Macrotasks in JavaScript If you’ve ever wondered why Promise.then() runs before setTimeout(), this is the reason 👇 🔹 What are Macrotasks? Macrotasks are large tasks scheduled to run later. Examples: setTimeout setInterval setImmediate (Node.js) DOM events I/O operations setTimeout(() => { console.log("Macrotask"); }, 0); 🔹 What are Microtasks? Microtasks are high-priority tasks that run immediately after the current execution, before any macrotask. Examples: Promise.then / catch / finally queueMicrotask MutationObserver Promise.resolve().then(() => { console.log("Microtask"); }); 🔹 Execution Order (Very Important 🔥) console.log("Start"); setTimeout(() => console.log("Macrotask"), 0); Promise.resolve().then(() => console.log("Microtask")); console.log("End"); Output: Start End Microtask Macrotask 🔹 Why This Happens? JavaScript follows this rule: 1️⃣ Execute synchronous code 2️⃣ Run all microtasks 3️⃣ Run one macrotask 4️⃣ Repeat 👉 Microtask queue is always drained first 🔹 Common Interview Gotcha 😅 setTimeout(() => console.log("timeout"), 0); Promise.resolve() .then(() => console.log("promise 1")) .then(() => console.log("promise 2")); Output: promise 1 promise 2 timeout 💡 Takeaway Microtasks = urgent callbacks Macrotasks = scheduled callbacks Understanding this helps you: ✅ Debug async bugs ✅ Avoid UI freezes ✅ Write predictable async code If this cleared up the event loop for you, drop a 👍 #JavaScript #EventLoop #AsyncJS #FrontendDevelopment

It's all about the timing. JavaScript is single-threaded, but it can still juggle multiple tasks at once - thanks to the Event Loop and Concurrency Model. This is key. The Event Loop acts like a conductor, coordinating between the Call Stack, Web APIs, and Task Queues to keep everything in sync. It's pretty simple: the Call Stack is where JavaScript keeps track of what's happening with function execution - a LIFO data structure, for those who care. But here's the thing: when you call functions like setTimeout, they aren't actually part of the JavaScript engine - they're Web APIs provided by the browser, which is a whole different story. So, how does it all work? Well, the Call Stack executes synchronous code, no problem. Then, when the Call Stack is empty, the Event Loop checks the Microtask Queue - which holds tasks like Promise callbacks, by the way. The Event Loop processes all these microtasks before moving on to the next macrotask, which is a different beast altogether. And that's where the Macrotask Queue comes in - holding tasks like setTimeout callbacks, for instance. It's worth noting: microtasks always run before the next macrotask. That's it. And, surprisingly, setTimeout(fn, 0) doesn't run immediately - it waits for the Call Stack and Microtask Queue to clear, which makes sense if you think about it. Also, React state updates are batched to optimize re-renders, which is a nice touch. So, always use functional updates in async callbacks to avoid stale closures - trust me on that one. Check out this article for more info: https://lnkd.in/gTYD4seC #JavaScript #EventLoop #ConcurrencyModel #WebDevelopment #Programming

Are you accidentally slowing down your JavaScript applications? It’s a common mistake I see in code reviews (and one I’ve made myself). When dealing with multiple independent asynchronous calls, it feels natural to just await them one by one. But as the image on the left illustrates, this creates a "waterfall" effect. Your code has to wait for the first operation to finish before it can even start the second one. ✅ The Better Way: Parallel Execution The solution, shown on the right, is Promise.all(). This function takes an array of promises and fires them off simultaneously. Instead of waiting for the sum of all request times (e.g., 2s + 2s = 4s), you only wait for the slowest single request (e.g., max(2s, 2s) = ~2s). This simple change can drastically improve the performance and user experience of your application. A quick rule of thumb: If the data from request A isn't needed to make request B, they should be running in parallel. Have you caught yourself making this mistake? What’s your favorite JS performance tip? Let me know in the comments! 👇 #JavaScript #WebDevelopment #FrontendDeveloper #CodingTips #SoftwareEngineering #PerformanceOptimization

Are you accidentally slowing down your JavaScript applications? It’s a common mistake I see in code reviews (and one I’ve made myself). When dealing with multiple independent asynchronous calls, it feels natural to just await them one by one. But as the image on the left illustrates, this creates a "waterfall" effect. Your code has to wait for the first operation to finish before it can even start the second one. ✅ The Better Way: Parallel Execution The solution, shown on the right, is Promise.all(). This function takes an array of promises and fires them off simultaneously. Instead of waiting for the sum of all request times (e.g., 2s + 2s = 4s), you only wait for the slowest single request (e.g., max(2s, 2s) = ~2s). This simple change can drastically improve the performance and user experience of your application. A quick rule of thumb: If the data from request A isn't needed to make request B, they should be running in parallel. Have you caught yourself making this mistake? What’s your favorite JS performance tip? Let me know in the comments! 👇 #JavaScript #WebDevelopment #FrontendDeveloper #CodingTips #SoftwareEngineering #PerformanceOptimization

🧠 99% of JavaScript devs get this event loop question wrong 👀 (Even seniors pause before answering) No frameworks. No libraries. Just how JavaScript actually schedules work. 🧩 Output-Based Question (Event Loop: sync vs microtasks vs macrotasks) console.log("start"); setTimeout(() => { console.log("timeout"); }, 0); Promise.resolve().then(() => { console.log("promise"); }); (async function () { console.log("async"); })(); console.log("end"); ❓ What will be printed — in the correct order? ❌ Don’t run the code 🧠 Think like the JavaScript engine A. start → async → end → promise → timeout B. start → end → async → promise → timeout C. start → async → promise → end → timeout D. start → promise → async → end → timeout 👇 Drop your answer in the comments (no cheating 😄) Why this question matters This tests whether you truly understand: • synchronous execution • the event loop • microtasks vs macrotasks • why Promise.then beats setTimeout(0) • async IIFEs vs promises Many developers “use” async code every day — but few understand when it actually runs. Good JavaScript developers don’t memorize outputs. They understand how the engine thinks. 💡 I’ll pin the full explanation after a few answers. #JavaScript #EventLoop #AsyncJavaScript #WebDevelopment #ProgrammingFundamentals #InterviewPrep #MCQ #DeveloperTips #CodeQuality

JavaScript – Execution Context Before JavaScript runs even a single line of your code, it prepares a working space for it. That space is called an Execution Context. In simple words: Execution Context is where JavaScript remembers things and runs your code Whenever JavaScript runs: ● Your whole program → one big Execution Context ● Every function call → a new small Execution Context Each one is created in two steps: 1️⃣ Memory Phase ● Variables are created → undefined ● Functions are stored fully 2️⃣ Execution Phase ● Code runs line by line ● Variables get real values ● Functions are executed Example: *** var a = 5; function show() { var b = 3; console.log(a + b); } show(); *** How JavaScript works: ● Creates a global context ◦ a → undefined ◦ show → saved ● Runs code ◦ a = 5 ◦ show() is called → new context is created ● Inside show() ◦ b = 3 ◦ Prints 8 JavaScript manages these contexts using a Call Stack: ● Global goes first ● Each function goes on top ● When a function finishes, it is removed This is why understanding Execution Context helps you: ● Understand hoisting ● Read call stack errors ● Master scope & closures ● Debug with confidence This is how JavaScript thinks before it acts. #Day1 #JavaScript #Frontend #WebDevelopment #LearningInPublic #React #Developers #CareerGrowth

⏳ Who Runs First in JavaScript — Promise or Timer? If fetch() and setTimeout() both finish at the same time — which one goes into the call stack first? 🤔 To answer this, let' see how JavaScript actually schedules async work. JavaScript does one thing at a time. When something takes time (API calls, timers, promises), JavaScript delegates the work and continues executing without blocking. So… where does this delegated work go, and how does it come back? 👇 Let's dig more... Call Stack → Executes code one step at a time. Whatever is on top, runs first. Web APIs → Timers (setTimeout, setInterval), fetch, DOM events, console these are not part of JavaScript itself, but are provided by the JS runtime environment (browser / Node). Callback Queue / Microtask Queue → When async work completes: • setTimeout → callback is pushed to Callback Queue • Promises → callback is pushed to Microtask Queue Event Loop → The real hero, its only job is to keep checking: 👉 Is the call stack empty? If yes → move tasks from the queue to the stack (based on priority). What Priority and what about the question? If fetch() (promise) and setTimeout() complete at the same time 👉 Promise callbacks (Microtask Queue) always get priority over timers (Callback Queue). #JavaScript #FrontendDevelopment #WebDevelopment

🔄 JavaScript is single-threaded. So how does it handle multiple tasks at once? Meet the Event Loop. It's the unsung hero that keeps your browser from freezing while waiting for an API call. Many developers use setTimeout or fetch daily without knowing what happens under the hood. Here is the architecture that makes non-blocking I/O possible: 1️⃣ Call Stack (The Boss): JavaScript does one thing at a time. It executes functions LIFO (Last In, First Out). If the stack is busy, nothing else happens. 2️⃣ Web APIs (The Assistants): When you call setTimeout or fetch, the browser takes that task out of the Call Stack and handles it in the background (Web APIs). This frees up the stack to keep running your code! 3️⃣ Callback Queue (The Waiting Room): Once the background task is done (e.g., the timer finishes), the callback function is moved to the Queue. It waits there patiently. 4️⃣ The Event Loop (The Traffic Controller): This is the infinite loop. It constantly checks: - "Is the Call Stack empty?" - "Is there something in the Queue?" - If Stack is Empty AND Queue has items 👉 Move item to Stack. Understanding this flow is the key to debugging weird async behavior. Did you know setTimeout(fn, 0) is a hack to defer execution until the stack is clear? #JavaScript #WebDevelopment #EventLoop #CodingInterview #Frontend #SoftwareEngineering