VueJS $set not making new property in array of objects reactive - vue.js

Question

In my VueJS 2 component below, I can add the imgdata property to each question in the area.questions array. It works - I can see from the console.log that there are questions where imgdata has a value. But despite using $set it still isn't reactive, and the imgdata isn't there in the view! How can I make this reactive?
var componentOptions = {
props: ['area'],
data: function() {
return {
qIndex: 0,
};
},
mounted: function() {
var that = this;
that.init();
},
methods: {
init: function() {
var that = this;
if (that.area.questions.length > 0) {
that.area.questions.forEach(function(q) {
Util.HTTP('GET', '/api/v1/photos/' + q.id + '/qimage').then(function(response) {
var thisIndex = (that.area.questions.findIndex(entry => entry.id === q.id));
var thisQuestion = (that.area.questions.find(entry => entry.id === q.id));
thisQuestion.imgdata = response.data;
that.$set(that.area.questions, thisIndex, thisQuestion);
})
});
}
console.log("area.questions", that.area.questions);
},

Answer 1

Since area is a prop, you should not be attempting to make changes to it within this component.
The general idea is to emit an event for the parent component to listen to in order to update the data passed in.
For example
export default {
name: "ImageLoader",
props: {
area: Object
},
data: () => ({ qIndex: 0 }), // are you actually using this?
mounted () {
this.init()
},
methods: {
async init () {
const questions = await Promise.all(this.area.questions.map(async q => {
const res = await Util.HTTP("GET", `/api/v1/photos/${encodeURIComponent(q.id)}/qimage`)
return {
...q,
imgdata: res.data
}
}))
this.$emit("loaded", questions)
}
}
}
And in the parent
<image-loader :area="area" #loaded="updateAreaQuestions"/>
export default {
data: () => ({
area: {
questions: [/* questions go here */]
}
}),
methods: {
updateAreaQuestions(questions) {
this.area.questions = questions
}
}
}

Answer 2

Here that variable has a value of this but it's bound under the scope of function. So, you can create reactive property in data as below :
data: function() {
return {
qIndex: 0,
questions: []
};
}
Props can't be reactive so use :
that.$set(this.questions, thisIndex, thisQuestion);
And assign your API output to directly questions using this.questions.