Thursday 15 February 2024

What is ng On In it life cycle hook in Angular?043

 What is ng On In it life cycle hook in Angular?

In Angular, the ngOnInit lifecycle hook is a cornerstone for component initialization. It's called once after Angular has finished initializing the component, giving you a reliable point to perform essential setup tasks and prepare the component for user interaction.

Key Characteristics:

  • Execution: Called only once after the component has been created, its inputs have been set, and its content has been projected.

  • Purpose: Perform initialization tasks like fetching data, subscribing to events, setting up internal state, or executing any logic that requires the component to be fully available.

  • Common Use Cases:

  • Fetching data from APIs or local storage.

  • Subscribing to events emitted by other components or services.

  • Setting up timers or intervals.

  • Initializing variables or objects based on input properties.


Imagine you have a product list component that needs to fetch product data from an API on initialization. You might use ngOnInit like this:


import { OnInit } from '@angular/core';
import { ProductService } from './product.service';

export class ProductListComponent implements OnInit {
  products: Product[] = [];

  constructor(private productService: ProductService) {}

  ngOnInit() {
    this.productService.getProducts().subscribe(products => {
      this.products = products;

In this example, ngOnInit subscribes to the getProducts method of the ProductService to fetch product data and store it in the products array, making it available for display in the component's template.

Things to Remember:

  • Keep ngOnInit concise and focused on essential initialization tasks.

  • Avoid complex logic or heavy data fetching within ngOnInit, as it can block the component's initial rendering.

  • Consider using dedicated services or asynchronous approaches for complex data fetching or operations.

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

By effectively using ngOnInit, you can create components that initialize predictably, load data efficiently, and respond correctly to initial state. Feel free to ask if you have any further questions or specific use cases for ngOnInit!



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