Thursday 15 February 2024

What is the difference between constructor and ng On In it Angular?045

 What is the difference between constructor and ng On In it Angular?

In Angular, both the constructor and ngOnInit lifecycle hook play crucial roles in component initialization, but they serve distinct purposes and are called at different points in the component's lifecycle. Here's a breakdown of their key differences:


  • When Called: Automatically executed immediately after the component is created.

  • Purpose: Primarily used for internal setup and dependency injection.

  • Initialize properties that don't depend on external data.

  • Inject essential services or dependencies using constructor parameters.

  • Perform basic configurations within the component itself.

  • Limitations: Cannot access the component's view or its input properties since they haven't been initialized yet.


  • When Called: Invoked after the component has been fully initialized, including its view and input properties.

  • Purpose: Used for initialization tasks that rely on the component's state or external data.

  • Fetch data from APIs or local storage.

  • Subscribe to events emitted by other components or services.

  • Set up timers or intervals based on input properties.

  • Perform calculations or actions requiring the component to be fully available.

  • Restrictions: Should not be used for heavy data fetching or complex logic, as it can delay the component's initial rendering.

Common Usage Scenarios:

  • Constructor:

  • Injecting services required for internal operations.

  • Initializing simple properties with default values.

  • Setting up basic configurations that don't depend on external data.

  • ngOnInit:

  • Fetching data based on input properties or component state.

  • Subscribing to events that might affect the component's behavior.

  • Performing calculations or actions based on the fetched data.

  • Triggering actions or updates based on the initial state.

In summary:

  • Use the constructor for internal setup and dependency injection when the component is first created.

  • Use ngOnInit for initialization tasks that require the component to be fully initialized and have access to its view and input properties.

By understanding these differences and selecting the appropriate hook for your needs, you can ensure your Angular components are initialized efficiently and function as expected.

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