Angular OnChanges: Building Robust Applications
Tags: angular, onchanges, lifecycle, simplechanges
Published: 2023-01-24 21:23:12


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[Angular](https://angular.io) is a widely-used JavaScript framework for building web applications. One of the key features of Angular is its use of **lifecycle hooks**, which allow developers to tap into different stages of a component’s lifecycle and perform specific actions at each stage. One of the most useful of these hooks is the **OnChanges lifecycle hook**, which is called whenever a bound input property of a component changes.

The OnChanges lifecycle hook is a method that is defined on a component class and decorated with the _@OnChanges_ decorator. This method takes a _single parameter_, an object that contains information about the changes that have occurred to the input properties of the component. The object is of type **SimpleChanges**, which has a property for each input property of the component, and each property contains the current and previous values of the input property.

The OnChanges lifecycle hook is **called after** the first _ngOnInit_ lifecycle hook **and before** the _ngDoCheck_ and _ngAfterContentChecked_ hooks. It’s also called whenever an input property of the component changes. This makes it a useful tool for performing actions in response to changes to input properties, such as updating the component’s internal state or triggering an API call.

An example of a simple component that uses the OnChanges lifecycle hook is:

```
import { Component, OnChanges, Input } from '@angular/core';

@Component({
  selector: 'app-test',
  template: `
    <p>{{message}}</p>
  `
})
export class TestComponent implements OnChanges {
  @Input() name: string;
  message: string;

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

In this example, the component has an input property called “name” that is bound to a value in the parent component. The ngOnChanges method is called whenever the value of the “name” input property changes. Inside the ngOnChanges method, the changes object is used to check if the “name” property has changed, and if so, the component’s “message” property is updated to display a greeting with the current value of the “name” property.

It’s important to note that if you want to update the value of an input property inside the ngOnChanges method, it’s recommended to use the ngDoCheck lifecycle hook, since ngOnChanges only tracks the first change of input properties and not subsequent changes.

In conclusion, the OnChanges lifecycle hook is an essential tool for responding to changes in input properties in Angular components. It enables developers to perform specific actions in response to changes and keep the component’s internal state in sync with the bound input properties, which is crucial for building robust and maintainable Angular applications. With the Angular OnChanges lifecycle hook, developers can ensure that their Angular applications are responsive, efficient and adaptable to changes in input properties.

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