How to implement iron-list lazy-loading with typescript?

Question

I want to implement lazy-loading with iron-list. I am using typescript for my application. I found theshold example. But it is not working with typescript. The event is not triggered when scroll down. It is triggered when click on list item. How to implement when scrolling down?. My iron-list is some part of the page so when scroll down to end of the list I want to add more result from server

<iron-list .items=${this.items} as="item" scroll-target="scrollTheshold">
     <template>
           <div>content here.....</div>
      </template>
  </iron-list>
<iron-scroll-threshold id="scrollTheshold"  lower-threshold="200" on-lower-threshold="${this._loadData()}">
  </iron-scroll-threshold>