riot js this.update strange behavior

Question

I have a simple code

<test>
<ul>
  <li each={ books }> { title }</li>
</ul>

<button type="button" name="button" onclick={ loadBooks }>Load books</button>

<script>
  'use strict';

  this.books = [];
  let offset = 0;

  this.on('mount', () => {
    console.log('mounted');
  });

  this.on('update', () => {
    console.log('updated');
  });

  loadBooks ()  {
    fetch('/api/books?limit=10&offset='+offset)
    .then(response => response.json())
    .then(books => {
      offset+=10;
      this.books = books;
      // this.update();
    })
    .catch(err => console.log(err));
  }
</script>

in my app.js i just requrie riot, test.tag and do riot.mount('#root', 'test'); this is init screen, why update fired before mount ? when i click first time, it log that update fired but view do not re-rendered and after second click, event update fired and view re-rendered

if i uncomment this.update(); inside promise, event update will fired twice

upd: if remove this.books = books and write this.update({books}); event update will fire twice, and render will work, but why 2 times???

Answer 1

1) update before mount : this is fixed in @next / 3.0.0, as per: https://github.com/riot/riot/issues/1661

2) Why 2x update : possibly, one from the tag and one regarding the child tag. This was asked here: https://muut.com/i/riot-js/using:why-update-event-is-trigg