Lightning Web Components (LWC) power modern Salesforce applications with lightning-fast UI and modular design. But here’s the secret sauce 👇 — understanding Lifecycle Hooks is what separates a basic component from a production-ready one.

Lifecycle hooks let you control what happens when a component is created, rendered, updated, and destroyed. If you know when to use which hook, you’ll avoid bugs, reduce performance issues, and write clean, scalable code.

---

🚀 LWC Lifecycle Stages (Quick Recap)

1. Creation – Component is born, constructor runs.
2. Render – Template is rendered.
3. Re-render – Updates after reactive property changes.
4. Destruction – Component is removed from DOM.

---

🔑 Lifecycle Hooks in Action

1️⃣ constructor() – Initialization

📍 When it runs: When component is created, before it hits the DOM.
💡 Use Case: Initialize default values. Avoid DOM access here.

constructor() {
    super();
    this.filterCriteria = 'Active Students';
}

✅ Great for setting internal state, NOT for fetching data.

---

2️⃣ connectedCallback() – DOM Entry Point

📍 When it runs: When component is inserted into the DOM.
💡 Use Case: Fetch data, call Apex, subscribe to events.

connectedCallback() {
    getStudents()
        .then(result => {
            this.students = result;
        })
        .catch(error => {
            this.error = error;
        });
}

✅ Perfect for Apex calls, pub-sub setup, listeners.

---

3️⃣ renderedCallback() – After Every Render

📍 When it runs: Every time DOM is rendered.
💡 Use Case: DOM manipulations, scroll handling, third-party library init.

renderedCallback() {
    if (!this.isRendered) {
        this.template.querySelector('lightning-input').focus();
        this.isRendered = true;
    }
}

⚠️ Use carefully — avoid infinite loops.

---

4️⃣ disconnectedCallback() – Cleanup Time

📍 When it runs: When component is removed from DOM.
💡 Use Case: Unsubscribe, clear intervals, disconnect observers.

disconnectedCallback() {
    unsubscribe(this.subscription);
    this.subscription = null;
}

✅ Prevents memory leaks.

---

5️⃣ errorCallback(error, stack) – Error Handling

📍 When it runs: If error occurs in lifecycle methods or child components.
💡 Use Case: Debugging + fallback UI.

errorCallback(error, stack) {
    console.error('Error in LWC:', error, stack);
    this.errorMessage = 'Something went wrong! Please try again later.';
}

✅ Helps you fail gracefully instead of breaking UI.

---

📊 Quick Reference Table

Lifecycle Hook	Timing	Common Use Case
constructor()	On creation	Initialize state, default values
connectedCallback()	On DOM insertion	Fetch data, setup subscriptions
renderedCallback()	After every render	DOM manipulation, libraries
disconnectedCallback()	On DOM removal	Cleanup, unsubscribe
errorCallback()	On error in component	Graceful error handling

---

🎓 Real-Life Business Example: Student Attendance Dashboard

Imagine a Student Attendance Dashboard in Salesforce:

• constructor() → Default filter: “Today’s Attendance”
• connectedCallback() → Fetch attendance records from Apex
• renderedCallback() → Auto-scroll to latest student entry
• disconnectedCallback() → Unsubscribe from live attendance updates
• errorCallback() → Show fallback message if chart rendering fails

This ensures a smooth user experience, without memory leaks or runtime errors.

---

⚡ Best Practices to Remember

✅ Keep constructor() light → No DOM, no heavy logic.
✅ Use connectedCallback() → For API calls & subscriptions.
✅ renderedCallback() with caution → Guard with flags to avoid loops.
✅ Always cleanup in disconnectedCallback() → Prevent leaks.
✅ errorCallback() = friend → For user-friendly debugging.

---

🎯 Final Words

Lifecycle hooks aren’t just technical jargon — they’re the rhythm of your LWC’s life. Mastering them will help you:
✔ Build performant components
✔ Reduce bugs & crashes
✔ Deliver enterprise-grade Salesforce apps

👉 Next time you write an LWC, think: Which lifecycle hook is best for this job?

🔟 Frequently Asked Interview Questions on LWC Lifecycle Hooks

1. What are Lifecycle Hooks in LWC? Why are they important?
   👉 Interviewers expect you to explain the concept and how they control component creation, rendering, and cleanup.
2. When should you use constructor() in LWC? Can you call Apex methods inside it?
   👉 Highlight that it’s only for initializing state, not for DOM or Apex calls.
3. Difference between constructor() and connectedCallback(). Which one is better for fetching data?
   👉 Stress that connectedCallback() is the right place for Apex calls and subscriptions.
4. What happens if you perform DOM manipulation inside constructor()?
   👉 Expected answer: DOM is not yet available → it leads to runtime errors.
5. How does renderedCallback() differ from connectedCallback()?
   👉 Explain that renderedCallback() executes after rendering and can run multiple times, while connectedCallback() runs only once when added to the DOM.
6. How do you avoid infinite loops in renderedCallback()?
   👉 Use a boolean flag (e.g., this.isRendered) to ensure DOM logic executes only once.
7. What is the purpose of disconnectedCallback()? Give a real-life scenario.
   👉 Example: unsubscribing from a message channel or clearing setInterval when user navigates away.
8. Explain errorCallback(). How does it help in debugging and user experience?
   👉 Expected: catches errors from child components, allows logging, or showing fallback UI.
9. Can you use all lifecycle hooks in a single LWC component? Would that be a good practice?
   👉 Yes, but only if necessary. Emphasize best practices like keeping hooks lean and focused.
10. Real-world scenario: You’re building a Student Dashboard. Which lifecycle hooks would you use and why?
    👉 This tests applied knowledge. You should explain constructor() for defaults, connectedCallback() for fetching records, renderedCallback() for scroll handling, disconnectedCallback() for cleanup, and errorCallback() for handling chart errors.

📚 More Resources for You

🚀 Trusted by 1000+ learners to crack interviews at TCS, Infosys, Wipro, EY, and more.

Here are some resources to go deeper:

🔗 100 Scenarios (1–4 YOE)
🔗 100 Scenarios (4–8 YOE)
🔗 LWC Q&A (Real Answers Explained)
🔗 600+ Qs from Recruiter Calls
🔗 TCS, Infosys, EY Interview Qs
🔗 Salesforce Project – Hindi + English