Lifecycle Hooks in LWC | Lifecycle hooks in LWC Interview Questions

Lifecycle Hooks in LWC | Lifecycle hooks in LWC Interview Questions: Lightning Web Components (LWC) leverage lifecycle hooks to give developers control over the component’s behavior at various stages. Each lifecycle hook is a callback method that Salesforce automatically invokes as the component transitions through phases of creation, update, and removal. This makes lifecycle hooks essential for managing a component’s state, performance, and behavior.

In this blog, we’ll cover the key lifecycle hooks in LWC and some common interview questions to help you prepare for an interview on this topic.

What are Lifecycle Hooks in Lightning Web Component?

Lifecycle hooks are used to handle the lifecycle of components. Here is list of all method:

1. Constructor: The constructor() fires when a component instance is created.
2. ConnectedCallback: Fires when a component is inserted into the DOM.
3. Render
4. RenderedCallback: Fires when a component is rendered on the DOM.
5. DisconnectedCallback: Fires when a component is removed from the DOM.
6. ErrorCallback: Fires in case of any error during a lifecycle hook or event

1) Constructor:

• Purpose: Initializes the component.
• Execution: Called once when the component is created.
  • Common Use Cases: 1) Setting initial properties. 2) Establishing initial state or default values. 3) Setting up event listeners.

constructor() {
  super();
  console.log('Component is initialized');
  this.someProperty = 'Initial Value';
}

2. ConnectedCallback()

• Purpose: Executes when the component is inserted into the DOM.
• Execution: Called once after component initialization and after the component is added to the DOM.
• Common Use Cases: 1) Fetching data or making server calls. 2) Subscribing to pub-sub events. 3) Setting up any global listeners.

Other Points:

• Child elements can’t be accessed because they don’t exist yet
• This hook flows from parent component to child component
• Eventually this method is invoked, all the public properties (decorated with @api) would have been received from the parent component by which we can call an apex method which requires these public properties as input parameters.
• In order to verify if a component is connected to DOM, the isConnected property can be used
• Parent elements can be accessed and modified in this lifecycle hook

connectedCallback() {
  console.log('Component connected to the DOM');
  // Fetch data or initiate service calls here
}

3. Render()

Render() Invoked after the execution of connectedCallback() method. This hook is used to override the standard rendering functionality in Lightning web components & to update the UI.

• Flows from parent component to child component
• Rendering process can be controlled by conditionally rendering the template on the basis of certain conditions or criteria
• This hook is not technically a lifecycle hook. It is protected method on the LightningElement class

4. RenderedCallback()

• Purpose: RenderedCallback() Invoked when a component is completely rendered on UI. or Invoked after every render of the component.
• Execution: Called initially after the component is rendered and after every re-render due to data changes.
• Common Use Cases:
• Accessing or modifying DOM elements after rendering.
• Applying third-party libraries or custom styles.
• Running one-time DOM setup tasks (guarded with flags).

Other Points:

• Flows from child component to parent component
• This hook should be used cautiously so that an infinite rendering loop is not triggered since this hook is called after the component gets rendered every time
• Make sure to use a private boolean property like isRendered to track whether renderedCallback() has been executed
• Not recommended to use renderedCallback() to change the state of the component instead use getter and setter
• Reactive property in renderedCallback() leads to infinite loop.

renderedCallback() {
  if (!this.isRendered) {
    console.log('Component is rendered');
    this.isRendered = true; // Prevent repetitive actions
  }
}

5. DisconnectedCallback()

• Purpose: Executes when the component is removed from the DOM.
• Execution: Called once just before the component is removed.
• Common Use Cases:
• Cleaning up resources (timers, intervals, etc.).
• Unsubscribing from pub-sub events or external listeners.

disconnectedCallback() {
  console.log('Component disconnected from the DOM');
  // Unsubscribe or clear resources here
}

6. ErrorCallback(error, stack)

• Purpose: Handles errors thrown by descendant components.
• Execution: Called when any error occurs in a child component.
• Common Use Cases:
• Error handling and logging.
• Displaying fallback UI for user-friendly error messages.

Other Points:

• Similar to JavaScript catch{} block, error and stack are the two arguments.error is javascript native error object whereas stack is a string
• To capture the stack information and render a different template when error is occurred

errorCallback(error, stack) {
  console.error('Error:', error);
  console.log('Stack trace:', stack);
  // Optionally display error message to the user
}