Setting intial local data from Vuex store giving "do not mutate" error - vue.js

Question

I thought I understood the correct way to load inital state data from Vuex into the local data of a component, but why is this giving me “[vuex] do not mutate vuex store state outside mutation handlers.” errors! I am using a mutation handler!
I want my component data to start empty, unless coming back from a certain page (then it should pull some values from Vuex).
The component is using v-model=“selected” on a bunch of checkboxes. Then I have the following:
// Template
<grid-leaders
v-if="selected.regions.length"
v-model="selected"
/>
// Script
export default {
data() {
return {
selectedProxy: {
regions: [],
parties: [],
},
}
},
computed: {
selected: {
get() {
return this.selectedProxy
},
set(newVal) {
this.selectedProxy = newVal
// If I remove this next line, it works fine.
this.$store.commit("SET_LEADER_REGIONS", newVal)
},
},
},
mounted() {
// Restore saved selections if coming back from a specific page
if (this.$store.state.referrer.name == "leaders-detail") {
this.selectedProxy = {...this.$store.state.leaderRegions }
}
}
}
// Store mutation
SET_LEADER_REGIONS(state, object) {
state.leaderRegions = object
}

Answer 1

OK I figured it out! The checkbox component (which I didn't write) was doing this:
updateRegion(region) {
const index = this.value.regions.indexOf(region)
if (index == -1) {
this.value.regions.push(region)
} else {
this.value.regions.splice(index, 1)
}
this.$emit("input", this.value)
},
The line this.value.regions.push(region) is the problem. You can't edit the this.value prop directly. I made it this:
updateRegion(region) {
const index = this.value.regions.indexOf(region)
let regions = [...this.value.regions]
if (index == -1) {
regions.push(region)
} else {
regions.splice(index, 1)
}
this.$emit("input", {
...this.value,
regions,
})
},
And then I needed this for my computed selected:
selected: {
get() {
return this.selectedProxy
},
set(newVal) {
// Important to spread here to avoid Vuex mutation errors
this.selectedProxy = { ...newVal }
this.$store.commit("SET_LEADER_REGIONS", { ...newVal })
},
},
And it works great now!
I think the issue is that you can't edit a v-model value directly, and also you also have to be aware of passing references to objects, and so the object spread operator is a real help.