React State Mutation Causes Invisible Bugs

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 17/30 I wrote an API call inside `useEffect`… and React showed a warning 😐 useEffect(async () => { const res = await fetch("/api/users") }, []) The mistake: `useEffect` should NOT be async. React expects the effect to return either: nothing, or a cleanup function. But async always returns a Promise. Fix 👇 Create an async function INSIDE the effect and call it. useEffect(() => { async function fetchUsers() { ... } fetchUsers() }, []) Small change. Correct lifecycle behavior. Day 18 tomorrow 👀 #30DaysOfCode #reactjs #javascript #frontend #webdevelopment

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 18/30 I clicked “Add Item”… nothing happened 😐 No error. No warning. Button worked. My code: items.push("New Task") setItems(items) The array DID change. But React didn’t update UI. Why? Because React doesn’t check contents. React checks references. Same array reference = React thinks nothing changed. Fix 👇 setItems([...items, "New Task"]) Create a NEW array instead of modifying the old one. This single mistake causes many: “React state not updating” moments. Save this — you will hit this bug in a real project. #30DaysOfCode #reactjs #javascript #frontend #codeinuse

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 7/30 I typed inside an input… clicked “Add new item”… and the text moved to another field 😳 The bug? key={index} I used array index as React key. React does not track elements. React tracks keys. When list order changes, React reuses DOM nodes, so your input becomes a different item. Fix 👇 key={item.id} Stable key = stable UI. React warnings are not decoration… they are future bugs. Day 8 tomorrow 👀 #30DaysOfCode #reactjs #javascript #frontend #webdevelopment #codeinuse

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 9/30 I built a form in React… and used: document.getElementById("name").value It worked. But later validation broke, reset didn’t work, and UI went out of sync 😐 Because React was not controlling the input. The DOM was. Correct way 👇 Use controlled input with state. <input value={name} onChange={e => setName(e.target.value)} /> In React: State should control UI not the DOM. This is why React forms become predictable and easier to debug. Day 10 tomorrow 👀 #30DaysOfCode #reactjs #javascript #frontend #webdevelopment #codeinuse

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 15/30 I changed the state… but the input field didn’t update 😐 <input defaultValue={name} /> `defaultValue` only sets the initial value. After that, the DOM controls the input — not React. So even when state changed, UI didn’t. Fix 👇 <input value={name} onChange={e => setName(e.target.value)} /> Now React state controls the input. In React: Uncontrolled input → unpredictable Controlled input → reliable Day 16 tomorrow 👀 #30DaysOfCode #reactjs #javascript #frontend #webdevelopment #codeinuse

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 4/30 I returned a value inside `forEach()`… but my array became `undefined` 😐 const doubled = numbers.forEach(n => n * 2); The mistake: `forEach()` does NOT return anything. It only runs a loop. Correct 👇 const doubled = numbers.map(n => n * 2); `map()` creates a brand new array with transformed values. Simple rule: forEach → do something map → create something This is especially important in React when rendering lists. Day 5 tomorrow 👀 #30DaysOfCode #javascript #reactjs #frontend #webdevelopment #codeinuse

Learning React made me realize something — frontend isn’t about “changing elements.” It’s about controlling state and thinking in systems. Once that clicked, everything started making sense. Still early in the journey, but the foundation is getting stronger every day. Next stop: advanced hooks and performance optimization. Building > consuming. #ReactJS #JavaScript #FrontendDeveloper #WebDevelopment #CodingJourney #LearnInPublic #FullStackPath

📚 Today’s Learning – React Strict Mode 1.It runs additional checks and warnings for unsafe lifecycle methods, unexpected side effects, and deprecated APIs. 2.One interesting behavior is that components may render twice in development to help detect side effects. 💡 Key takeaway: Strict Mode doesn’t affect production builds, but it helps catch bugs early during development and encourages writing safer, cleaner React code. #ReactJS #FrontendDevelopment #WebDevelopment #JavaScript #CodingJourney #LearnInPublic

🚀 React.js Basics: Understanding Props vs State While learning and building projects in React.js, one of the most important concepts to understand is the difference between Props and State. 🔹 State Managed inside the component Can change over time Used for dynamic data like counters, forms, UI updates 🔹 Props Passed from parent to child components Read-only (cannot be modified by the child) Used to share data between components 💡 Simple way to remember: State = Internal data of a component Props = Data passed from parent component Understanding these two concepts clearly helps in building clean, reusable, and scalable React components. I created this visual to make the concept easier for beginners who are learning React UI Development. 🌐 Explore my work: https://allconverthub.com https://lnkd.in/g4Hnzt9Z #ReactJS #FrontendDevelopment #UIDeveloper #JavaScript #WebDevelopment #ReactLearning #FrontendEngineer #Coding

🚀 30 Days — 30 Coding Mistakes Beginners Make Day 16/30 My component received a new userId… but UI still showed the old user 😐 The reason? I wrote: useEffect(() => { fetchUser(userId) }, []) Empty dependency array means: run only once on mount. So when userId changed, React never fetched new data. Fix 👇 useEffect(() => { fetchUser(userId) }, [userId]) Dependencies tell React WHEN to re-run the effect. Missing dependency = stale UI data. Day 17 tomorrow 👀 #30DaysOfCode #reactjs #javascript #frontend #webdevelopment #codeinuse