Passing data to props after asynchronous call in Vue - asynchronous

Question

I have set up a bare bones vue project to show the problem. The only thing I added was the axios package. The problem is when I try to set the property of child component after an asynchronous call I cant read that property in the component. If you look at the code you can see I console log several times to show when I can get the data and when I cant. Please help me figure out what im missing here.
Parent
<template>
<div id="app">
<HelloWorld :test_prop="testData" :test_prop2="testData2" :test_prop3="testData3" test_prop4="I work also"/>
<div>{{testData5}}</div>
</div>
</template>
<script>
import HelloWorld from './components/HelloWorld.vue'
import axios from 'axios';
export default {
name: 'app',
components: {
HelloWorld
},
data() {
return {
testData: '',
testData2: 'I work just fine',
testData3: '',
testData5: ''
}
},
created: function(){
var self = this;
this.testDate3 = 'I dont work';
axios.get('https://jsonplaceholder.typicode.com/posts/42').then(function(response){
//I need this one to work
self.testData = 'I dont work either';
self.testData5 = 'I work also';
});
}
}
</script>
Child
<template>
</template>
<script>
export default {
name: 'HelloWorld',
props: ['test_prop', 'test_prop2', 'test_prop3', 'test_prop4'],
data() {
return {
comp_data: this.test_prop,
comp_data2: this.test_prop2,
comp_data3: this.test_prop3,
comp_data4: this.test_prop4
}
},
created: function(){
console.log(this.test_prop);
console.log(this.test_prop2);
console.log(this.test_prop3);
console.log(this.test_prop4);
}
}
</script>

Answer 1

Your console.log inside created hook will show you the initial state of this variables in Parent component. That's because Parent's created hook and Child's created hook will run at the same time.
So, when you solve your promise, Child component was already created. To understand this behavior, put your props in your template using {{ this.test_prop }}.
To solve it, depending on what you want, you can either define some default value to your props (see) or render your child component with a v-if condition. That's it, hope it helps!

Answer 2

On Vue created hook only the initial values of properties passed from main component. Therefore later updates (like your example "after ajax call") in main component will not effect to child component data variables because of that already child created hook take place.
If you want to update data later one way you can do like this:
watch: {
test_prop: function(newOne){
this.comp_data = newOne;
}
}
Adding watcher to property changes will update the last value of property from main component.
And also edit the typo this.testDate3. I guess it must be this.testData3