Lightning Web Components – LWC Lifecycle Hooks Questions

Understanding the component lifecycle is crucial for building efficient and well-structured Lightning Web Components. These questions cover the key lifecycle hooks including constructor, connectedCallback, renderedCallback, disconnectedCallback, and errorCallback, along with their proper usage, limitations, and best practices. Mastering lifecycle hooks will help you create components that initialize correctly, respond to changes, and clean up resources appropriately.

LWC Lifecycle Hooks - Q&A

Q1. What are the key lifecycle hooks in LWC?
Ans: Core lifecycle methods: - constructor(): Initial setup (avoid DOM operations) - connectedCallback(): Component inserted in DOM - renderedCallback(): After every render (careful with loops) - disconnectedCallback(): Cleanup when removed - errorCallback(): Catches errors in child components
Q2. When should you use connectedCallback()?
Ans: Use for: - Initial data loading - Event listener setup - Initializing third-party libraries Example:
```
connectedCallback() {
  this.loadData();
  window.addEventListener('resize', this.handleResize);
}
```
Q3. What are the limitations of renderedCallback()?
Ans: Key considerations: - Runs after every render (potential performance hit) - Avoid changing state that would trigger re-render - Use guards to prevent infinite loops:
```
renderedCallback() {
  if(!this._initialized) {
    this._initialized = true;
    // One-time initialization
  }
}
```

Q4. How do you prevent memory leaks in disconnectedCallback()?
Ans: Cleanup patterns:

disconnectedCallback() {
  // Remove event listeners
  window.removeEventListener('resize', this.handleResize);
  
  // Cancel pending promises
  if(this.pendingPromise) {
    this.pendingPromise.cancel();
  }
  
  // Clear timeouts/intervals
  clearTimeout(this.timeoutId);
}

Q5. What is the execution order of lifecycle hooks?
Ans: Sequence: 1. constructor() 2. connectedCallback() 3. render() (template) 4. renderedCallback() 5. disconnectedCallback() (when removed) errorCallback() triggers on errors

Q6. How does errorCallback() work?
Ans: Error boundary pattern:

errorCallback(error, stack) {
  this.error = error;
  logErrorToService(error, stack);
}

render() {
  return this.error 
    ? html`` 
    : html``;
}

Q7. When should you avoid using lifecycle hooks?
Ans: Avoid when: - Logic can be handled declaratively in templates - @wire services can manage data loading - Lightning Data Service handles record operations - Overusing renderedCallback() for state changes

Q8. How do you test lifecycle hooks?
Ans: Testing approach:

// Test connectedCallback
element = createElement('c-component', { is: MyComponent });
document.body.appendChild(element); // Triggers connected

// Test disconnected
document.body.removeChild(element); // Triggers disconnected

// Verify behavior
assert(spy.calledOnce);

Q9. What are anti-patterns in lifecycle usage?
Ans: Common mistakes: - Loading data in constructor() - Modifying state in renderedCallback() causing loops - Not cleaning up event listeners - Performing DML in connectedCallback() - Overusing setTimeout() to "wait" for rendering

Q10. How do you implement lazy loading with hooks?
Ans: Performance pattern:

connectedCallback() {
  if(this.shouldLoad) {
    // Load only when visible
    IntersectionObserver(() => this.loadData());
  }
}

Q11. How do you handle asynchronous operations in lifecycle hooks?
Ans: Async patterns:

async connectedCallback() {
  try {
    // Async initialization
    this.data = await fetchData();
    this.isLoading = false;
  } catch (error) {
    this.error = error.message;
  }
}

// Or with promises
connectedCallback() {
  fetchData()
    .then(data => {
      this.data = data;
      this.isLoading = false;
    })
    .catch(error => {
      this.error = error.message;
    });
}

Q12. What is the difference between constructor and connectedCallback?
Ans: Key differences: - constructor(): * Runs first, before DOM connection * Cannot access DOM elements * Cannot access @api properties * Used for class initialization - connectedCallback(): * Runs after component is inserted in DOM * Can access DOM elements * @api properties are available * Used for initialization requiring DOM

Q13. How do you handle component re-rendering efficiently?
Ans: Optimization techniques:

// Use renderedCallback sparingly
renderedCallback() {
  // Only run when specific conditions change
  if (this.dataChanged) {
    this.updateChart();
    this.dataChanged = false;
  }
}

// Use getters for computed properties
get formattedValue() {
  return this.value ? format(this.value) : '';
}

Q14. How do you implement cleanup for third-party libraries?
Ans: Library cleanup:

connectedCallback() {
  // Initialize third-party library
  this.chart = new Chart(this.template.querySelector('canvas'));
}

disconnectedCallback() {
  // Cleanup third-party resources
  if (this.chart) {
    this.chart.destroy();
  }
  
  // Remove event listeners
  this.template.removeEventListener('click', this.handleClick);
}

Q15. How do you handle conditional rendering in lifecycle hooks?
Ans: Conditional patterns:

connectedCallback() {
  // Check conditions before initialization
  if (this.recordId && this.isEditMode) {
    this.loadRecordData();
  }
  
  // Add event listeners conditionally
  if (this.enableTracking) {
    this.template.addEventListener('mousemove', this.trackMouse);
  }
}

renderedCallback() {
  // Only run expensive operations when needed
  if (this.needsChartUpdate && this.template.querySelector('canvas')) {
    this.updateChart();
    this.needsChartUpdate = false;
  }
}

Q16. How do you debug lifecycle hook issues?
Ans: Debugging strategies:

// Add console logs for tracing
constructor() {
  super();
  console.log('Constructor called');
}

connectedCallback() {
  console.log('Connected to DOM');
  console.log('Record ID:', this.recordId);
}

renderedCallback() {
  console.log('Rendered - iteration:', this.renderCount++);
}

disconnectedCallback() {
  console.log('Disconnected from DOM');
}

// Use browser dev tools to inspect component state
// Check for infinite loops in renderedCallback
// Monitor memory usage for leaks

Q17. How do you handle errors in lifecycle hooks?
Ans: Error handling:

connectedCallback() {
  try {
    // Risky initialization
    this.initializeComponent();
  } catch (error) {
    // Log error
    console.error('Initialization error:', error);
    
    // Set error state
    this.error = error.message;
    
    // Notify parent or service
    this.dispatchEvent(new CustomEvent('error', { detail: error }));
  }
}

// Use errorCallback for child component errors
errorCallback(error, stack) {
  console.error('Child component error:', error);
  this.childError = error.message;
}

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