VueJS dynamic data + v-model - vue.js

Question

In my data object, I need to push objects into an array called editions.
data() {
return {
editions: []
}
}
To do this, I am dynamically creating a form based on some predetermined field names. Here's where the problem comes in. I can't get v-model to cooperate. I was expecting to do something like this:
<div v-for="n in parseInt(total_number_of_editions)">
<div v-for="field in edition_fields">
<input :type="field.type" v-model="editions[n][field.name]" />
</div>
</div>
But that isn't working. I get a TypeError: _vm.editions[n] is undefined. The strange thing is that if I try this: v-model="editions[n]"... it works, but I don't have the property name. So I don't understand how editions[n] could be undefined. This is what I'm trying to end up with in the data object:
editions: [
{
name: "sample name",
status: "good"
},
...
]
Can anyone advise on how to achieve this?

Answer 1

But that isn't working. I get a TypeError: _vm.editions[n] is undefined.
editions is initially an empty array, so editions[n] is undefined for all n. Vue is essentially doing this:
const editions = []
const n = 1
console.log(editions[n]) // => undefined
The strange thing is that if I try this: v-model="editions[n]"... it works
When you use editions[n] in v-model, you're essentially creating the array item at index n with a new value. Vue is doing something similar to this:
const editions = []
const n = 2
editions[n] = 'foo'
console.log(editions) // => [ undefined, undefined, "foo" ]
To fix the root problem, initialize editions with an object array, whose length is equal to total_number_of_editions:
const newObjArray = n => Array(n) // create empty array of `n` items
.fill({}) // fill the empty holes
.map(x => ({...x})) // map the holes into new objects
this.editions = newObjArray(this.total_number_of_editions)
If total_number_of_editions could change dynamically, use a watcher on the variable, and update editions according to the new count.
const newObjArray = n => Array(n).fill({}).map(x => ({...x}))
new Vue({
el: '#app',
data() {
const edition_fields = [
{ type: 'number', name: 'status' },
{ type: 'text', name: 'name' },
];
return {
total_number_of_editions: 5,
editions: [],
edition_fields
}
},
watch: {
total_number_of_editions: {
handler(total_number_of_editions) {
const count = parseInt(total_number_of_editions)
if (count === this.editions.length) {
// ignore
} else if (count < this.editions.length) {
this.editions.splice(count)
} else {
const newCount = count - this.editions.length
this.editions.push(...newObjArray(newCount))
}
},
immediate: true,
}
}
})
<script src="https://unpkg.com/vue#2.6.10/dist/vue.min.js"></script>
<div id="app">
<label>Number of editions
<input type="number" min=0 v-model="total_number_of_editions">
</label>
<div><pre>total_number_of_editions={{total_number_of_editions}}
editions={{editions}}</pre></div>
<fieldset v-for="n in parseInt(total_number_of_editions)" :key="n">
<div v-for="field in edition_fields" :key="field.name+n">
<label>{{field.name}}{{n-1}}
<input :type="field.type" v-if="editions[n-1]" v-model="editions[n-1][field.name]" />
</label>
</div>
</fieldset>
</div>