Thursday 15 February 2024

What is ng On Changes life cycle hook in Angular? 042

 What is ng On Changes life cycle hook in Angular? 

In Angular, the ngOnChanges lifecycle hook is a powerful tool for detecting and responding to changes in input properties bound to your component. It's called whenever one or more of these input properties change, offering you a chance to react to the updated data and adjust your component's behavior accordingly.

Key Characteristics:

  • Executed: After initial setup (in ngOnInit) and whenever an input property bound to the component changes.

  • Receives: A SimpleChanges object as an argument, providing information about the changed properties:

  • previousValue: The previous value of the changed property.

  • currentValue: The current value of the changed property.

  • firstChange: A boolean indicating whether this is the first change (true) or a subsequent change (false).

  • Purpose: React to data changes, update the component's state, trigger actions, or perform calculations based on the new input values.

When to Use:

  • When your component's behavior depends heavily on external data coming through input properties.

  • To update internal state, trigger actions, or re-render parts of the component based on data changes.

  • For calculations or logic that require access to both the previous and current values of the changed properties.


Here's an example of how you might use ngOnChanges to update a title based on a changing name input property:


import { SimpleChanges, OnChanges } from '@angular/core';

export class MyComponent implements OnChanges {
  title = '';
  @Input() name!: string;

  ngOnChanges(changes: SimpleChanges) {
    if (changes['name']) {
      this.title = `Hello, ${changes['name'].currentValue}!`;

Things to Remember:

  • Be mindful of performance implications, as ngOnChanges can run frequently. Optimize your logic and avoid unnecessary calculations.

  • Consider using ngDoCheck for advanced change detection scenarios, but use it cautiously due to potential performance drawbacks.

  • Understand the differences between ngOnChanges and ngOnInit for choosing the appropriate hook for your initialization needs.

By effectively using ngOnChanges, you can build reactive and dynamic components that gracefully adapt to changes in their input data, enriching the user experience in your Angular applications. Feel free to ask if you have any further questions or specific use cases for ngOnChanges!

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