What triggers a recalculation of a computed property in an async context? - vue.js

Question

I fetch some data from a JSON file to be processed. This asynchronous operation (the fetch) seems to disturb the functionality of computed() properties.
The code below works as expected -- data are not retrieved but hardcoded in mounted(). When you run the code and look at the console, you see a first computation for filteredTags (empty), then allTags are modified, which triggers a calculation of filteredTags.
→ everything is as expected
new Vue({
el: "#app",
data: {
posts: [],
tags: {},
allTags: {},
},
computed: {
// provides an Array of tags which are selected
filteredTags: function() {
let t = Object.keys(this.allTags).filter(x => this.allTags[x])
console.log("computed filteredtags: " + JSON.stringify(t))
return t
}
},
watch: {
},
mounted() {
const r = '{"tag1":["/posts/premier/"],"post":["/posts/premier/","/posts/second/"]}'
this.tags = JSON.parse(r)
// bootstrap a tags reference where all the tags are selected
Object.keys(this.tags).forEach(t => {
this.$set(this.allTags, t, true)
// this.allTags[t] = true
});
console.log("allTags in mounted: " + JSON.stringify(this.allTags))
}
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="app">
<div v-for="tag in filteredTags">{{tag}}</div>
</div>
Now the same code, but with the case where {"tag1":["/posts/premier/"],"post":["/posts/premier/","/posts/second/"]} is retrieved via a HTTP call:
<div id="app">
<div :click="allTags[tag]=!allTags[tag]" v-for="tag in filteredTags">{{tag}}</div>
</div>
<script>
new Vue({
el: "#app",
data: {
posts: [],
tags: {},
allTags: {},
},
computed: {
// provides an Array of tags which are selected
filteredTags: function () {
let t = Object.keys(this.allTags).filter(x => this.allTags[x])
console.log("computed filteredtags: " + JSON.stringify(t))
return t
}
},
watch: {
},
mounted() {
// get source tags
fetch("tags.json")
.then(r => r.json())
.then(r => {
this.tags = r
// bootstrap a tags reference where all the tags are selected
Object.keys(this.tags).forEach(t => {
this.$set(this.allTags, t, true)
// this.allTags[t] = true
});
console.log("allTags in mounted: "+ JSON.stringify(this.allTags))
})
}
})
</script>
The output of the console is:
computed filteredtags: []
allTags in mounted: {"tag1":true,"post":true}
computed filteredtags: ["tag1","post"]
computed filteredtags: []
The first three lines are OK. Why is there an extra recomputation of filteredtags at the end? And more importantly -- why is the result empty?