Load images from firebase storage with database in a list

I am using ionic 2 and Firebase 3, I want to get the data from firebase database and firebase storage, to make a list of news, each with thumbnail picture and news title and brief. I can achieve the news info from firebase database by news.ts as below :

this.newsData.getNewsList().on('value', snapshot => {
       let rawList = [];
       snapshot.forEach( snap => {
       id: snap.key,
       brief: snap.val().brief,
       title: snap.val().title,
       details : snap.val().details,
       date : snap.val().news_date,
       thumbnail :  snap.val().thumbnail
  this.newsList = rawList;

And achieve it with news.html

<ion-card class="cardstyle" *ngFor="let news of newsList" >
<img [src]=“news.thumbnailurl”>

However, the thumbnails are file names of firebase storage (image/filename.jpg),I need to convert them to downloadable URL first, it should perform getDownloadURL() first. May I know how I can achieve it in ionic2 ?

Many thanks for the help !

see answer i posted in StackOverflow and also checkout my video and project on this topic