How to add image download functionality in images which are displayed in an angular 4 app?

I have created an image gallery app in Angular 4.

This is my code for displaying an image from Firebase, now I basically want a button, and on clicking it, I want this image to be downloaded.

<div class=col-md-12>

    <div [ngStyle]="{'background-image':'url('+ imageUrl +')'}" 
  class="img-container">

    </div>

 </div>