Computed property "main_image" was assigned to but it has no setter - vue.js

Question

How can I fix this error "Computed property "main_image" was assigned to but it has no setter"?
I'm trying to switch main_image every 5s (random). This is my code, check created method and setInterval.
<template>
<div class="main-image">
<img v-bind:src="main_image">
</div>
<div class="image-list>
<div v-for="img in images" class="item"><img src="img.image"></div>
</div>
</template>
<script>
import axios from 'axios';
export default {
name: 'Item',
data () {
return {
item: [],
images: [],
}
},
methods: {
fetchImages() {
axios.get(`/api/item/${this.$route.params.id}/${this.$route.params.attribute}/images/`)
.then(response => {
this.images = response.data
})
.catch(e => {
this.images = []
this.errors.push(e)
})
},
},
computed: {
main_image() {
if (typeof this.item[this.$route.params.attribute] !== 'undefined') {
return this.item[this.$route.params.attribute].image_url
}
},
},
watch: {
'$route' (to, from) {
this.fetchImages()
}
},
created () {
axios.get(`/api/item/${this.$route.params.id}/`)
.then(response => {
this.item = response.data
})
.catch(e => {
this.errors.push(e)
})
this.fetchImages();
self = this
setInterval(function(){
self.main_image = self.images[Math.floor(Math.random()*self.images.length)].image;
}, 5000);
},
}
</script>

Answer 1

Looks like you want the following to happen...
main_image is initially null / undefined
After the request to /api/item/${this.$route.params.id}/ completes, it should be this.item[this.$route.params.attribute].image_url (if it exists)
After the request to /api/item/${this.$route.params.id}/${this.$route.params.attribute}/images/ completes, it should randomly pick one of the response images every 5 seconds.
I'd forget about using a computed property as that is clearly not what you want. Instead, try this
data() {
return {
item: [],
images: [],
main_image: '',
intervalId: null
}
},
methods: {
fetchImages() {
return axios.get(...)...
}
},
created () {
axios.get(`/api/item/${this.$route.params.id}/`).then(res => {
this.item = res.data
this.main_image = this.item[this.$route.params.attribute] && this.item[this.$route.params.attribute].image_url
this.fetchImages().then(() => {
this.intervalId = setInterval(() => {
this.main_image = this.images[Math.floor(Math.random()*this.images.length)].image;
})
})
}).catch(...)
},
beforeDestroy () {
clearInterval(this.intervalId) // very important
}

Answer 2

You have to add setter and getter for your computed proterty.
computed: {
main_image: {
get() {
return typeof this.item[this.$route.params.attribute] !== 'undefined' && this.item[this.$route.params.attribute].image_url
},
set(newValue) {
return newValue;
},
},
},