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Lorenzo Zarantonello for This is Angular

Posted on • Updated on • Originally published at vitainbeta.org

Passing a custom Angular property to child components

While there are several ways to make properties available throughout your Angular app, I found this 3-step process pretty easy to follow and remember.

For the sake of simplicity, we call the parent component Parent and the child component Child. If you prefer a more practical approach, check out this practical example to get a better understanding.

Here are the three steps to pass a property to a child component:

1. Prepare Child.ts for external Input

Prepare the child component class (Child.ts) to receive external inputs.

  • External refers to data that is not available inside the component itself (In this case, Child).
  • Input refers to the property that the parent component (Parent) passes to the child component.
// Child.ts

export class Child implements OnInit {
  @Input() expectedProp: { title: string };
  constructor() {}

  ngOnInit(): void {}
}

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The @Input() decorator expects a property that we named expectedProp. You can name this property as you wish but remember to be consistent in the next steps.
This custom property is of type object with a key named title of type string.

2. Bind Property in Parent.html

First of all, we have some data in our parent component class (Parent.ts)

// Parent.ts

export class AppComponent {

  book = { title: 'Principles' }

}
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We want to pass book to Child.
To do so, we do two things in Parent.html:

  1. We use the selector <child></child> to use the child component.
  2. We need to bind the data coming from the parent component class (Parent.ts) to the custom property, expectedProp, that we declared in Child.ts.

Quick recap:

  • Data in Parent.ts: book
  • Custom property in Child.ts: expectedProp
  • Property binding syntax: [property]="data"

The binding happens in Parent.html which becomes the point of contact between Parent.ts and Child.ts

// Parent.html 

<ul>
  <child 
    [expectedProp] = "book"
    >
  </child>
</ul>

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3. Use Property in Child.html

The custom property is now available in Child and it can be used in Child.html.
As a result, we can use interpolation binding syntax (double curly braces) to presents the component's property value inside the template.

// Child.html

<li>
  {{expectedProp.title}} 
</li>

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Conclusions

If this seems too abstract, check out this practical example.

Otherwise, remember the three steps:

  1. Prepare Child.ts for external Input
  2. Bind Property in Parent.html
  3. Use Property in Child.html

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