Vue v-model/v-for doesn't update on mount, but after first manual change - vue.js

Question

I have a dropdown list "functions" that is filled with database entries and a dropdown list with 2 hardcoded entries. When the vue website is opened the dropdown list remains empty but as soon as I change the value of the other dropdown field the desired data from the database is available.
I'm a bit confused because I expected that adding "retrieveFunctions()" into the mounted() function would trigger the v-for, and even more confused that changing something in another select field suddenly triggers it.
The HTML code:
<template>
<div class="submit-form">
<div v-if="!submitted">
<div class="row">
<div class="col-sm-12">
<p><a style="width:500px" class="btn btn-info" data-toggle="collapse" href="#generalInformation" role="button" aria-expanded="true" >
General Information</a></p>
<div class="collaps show" id="generalInformation">
<!-- NAME -->
<div class="form-group">
<input placeholder="Name" type="text" class="form-control"
id="name" required v-model="component.name" name="name">
</div>
<!-- DOMAIN -->
<div class="input-group mb-3">
<div class="input-group-prepend">
<label style="width:100px" class="input-group-text" for="inputGroupDomain">Domain</label>
</div>
<select v-model="component.domain"
class="custom-select"
id="inputGroupDomain"
>
<option value="Power System">Power System</option>
<option value="ICT">ICT</option>
</select>
</div>
<!-- FUNCTION -->
<div class="input-group mb-3">
<div class="input-group-prepend">
<label style="width:100px" class="input-group-text" for="inputGroupFunction">Functions</label>
</div>
<select v-model="_function" class="custom-select" id="inputGroupFunction">
<option :class="{ active: index == currentIndex }"
v-for="(_function, index) in functions"
:key="index"
value= _function.name>
{{ _function.name }}
</option>
</select>
</div>
</div>
<p>
<button #click="saveComponent" class="btn btn-success">Add Component</button>
</p>
</div>
</div>
</div>
<div v-else>
<h4>Component was added succesfully!</h4>
<button class="btn btn-success" #click="newComponent">Proceed</button>
</div>
The script part:
<script>
import FunctionDataService from "../services/FunctionDataService";
export default {
name: "add-component",
data() {
return {
component: {
id: null,
name: "",
type: "",
domain: "",
},
submitted: false
};
},
methods: {
retrieveFunctions() {
FunctionDataService.getAll()
.then(response => {
this.functions = response.data;
console.log(response.data);
})
.catch(e => {
console.log(e);
});
},
refreshList() {
this.retrieveFunctions();
},
},
mounted() {
this.retrieveFunctions();
}
};
</script>
refreshList() {
this.retrieveFunctions();
},
},
mounted() {
this.retrieveFunctions();
}
};
</script>
State in the beginning: Dropdown list empty
State after selecting something in the upper dropdown list: Database entries are visible and correct

Answer 1

You need to initiate all responsive properties on the data return object with either a value (empty string, array, object, etc) or null. Currently it's missing the _function attribute used in the select v-model and the functions array used in the v-for. You can try to change the data to the following:
data() {
return {
_function: "",
functions: [],
component: {
id: null,
name: "",
type: "",
domain: "",
},
submitted: false
};
},