Add class to an element in Angular 4

Use [ngClass] and conditionally apply class based on the id.

In your HTML file:

<li>
    <img [ngClass]="{'this-is-a-class': id === 1 }" id="1"  
         src="https://stackoverflow.com/assets/images/1.jpg" (click)="addClass(id=1)"/>
</li>
<li>
    <img [ngClass]="{'this-is-a-class': id === 2 }" id="2"  
         src="../../assets/images/2.png" (click)="addClass(id=2)"/>
</li>

In your TypeScript file:

addClass(id: any) {
    this.id = id;
}