Interpolation

Interpolation is used for one-way data binding in Angular. It embeds an expression into the HTML template. By default, expression should be surrounded by {{ and }}. This expression is also known as template expression.

{{ expression }}

Angular evaluates an expression surrounded by {{ and }} and then converts a result to a string and assigns it to an element or directive property.

{{ expression }} output=> string

The following example evaluates an arithmetic expression, converts the result into a string.

Example: Interpolation
<p> 2 + 2 = {{ 2 + 2 }} </p> <!-- output: "2 + 2 = 4" -->
<p> 2 * 3 = {{ 2 * 3 }} </p> <!-- output:`"2 * 3 = 6" -->
<p> {{ “2 + 2 != ”+  2 + 2 }} </p> <!-- output:"2 + 2 != 22" -->
<p> {{ “2 + 2 = ”+  (2 + 2) }} </p> <!-- output:"2 + 2 = 4" -->

The context of the interpolation expression is a component class. It means it can access public properties or methods of a component class. However, it cannot access private or protected members.

Consider the following component class.

Example: Angular Component
export class InterpolationDemo implements OnInit {

  public text: string = "Hello";
  public caption: string = "Click Me!";
  num: number = 0;
  randomNums: number[] = [3, 6, 7, 8, 1, 122, 44, 67, 790];

  private count:number = 0; 

  ngOnInit(): void {
  }

  getCurrentTime(): any {
    return Date.now();
  }
}

The HTML template of the above component can use members as an expression in interpolation, as shown below.

Example: Interpolation
<p>{{ text }} </p>
<p>{{ num }} </p>
<p>{{ getCurrentTime() }} </p>
<button innerText={{caption}}></button>

It can also use template input variable, as shown below.

Example: Input Variable
<ul>
  <li *ngFor="let rndNum of randomNums">{{rndNum}}</li>
</ul>

Most JavaScript expressions are valid template expression except the following: - Assignment, bitwise, increment and decrement perators (=,+=, -=, |, &, ++, –,!, ?. etc.) - new, typeof, instanceof, etc. - chaining expression with ; or , - some ES2015+ operators

The following will give a compile-time error.

Example: Invalid Interpolation
<p>{{ num++ }} </p>
<p>{{ num += 1}} </p>
<p>{{ count}} </p>
<p>{{ typeof(num) }} </p>
<p>{{ Date.now()}} </p>
<p>{{ window.name}} </p>
<p>{{ console.log(‘This is an error’)}} </p>

Configure Interpolation Delimiter

Note:
By default, Angular use {{ and }} delimiter. However, you can configure it and specify different delimiter using the interpolation option in the @Component decorator.

For example, the following configures ? and ? as starting and ending interpolation delimiter.

Example: Custom Delimiter
@Component({
  selector: 'app-event-binding-demo',
  interpolation:['?','?'],

})

Thus, interpolation is an easy way of one-way data binding using template expression.