What could be causing my array to be undefined upon the creation of a Vue component?

I'm struggling to comprehend why my array is showing as undefined in my Vue component. The issue arises in the following scenario:

  1. The SelectCategories.vue component uses the router to navigate to the Quiz.vue component. Here, I utilize props to transmit data to the Quiz component. This is how it's executed:
      else {
        this.$router.push({
          name: 'quiz',
          params: {
            items: selectedCategories
          }
        })
      }

We move forward to the Quiz.vue component, where I execute the following within the created lifecycle hook:

    async created() {
        for (const item of this.items) {
            var questions = await QuestionService.getQuestionsByCategory(item);
            this.questions.push(questions);
            console.log(this.questions);
        }
  }

The QuestionService connects with the database and retrieves certain information, which is then added to the questions array that I have defined here:

export default {
    name: 'quiz',
    props: ['items'],
    data: function() {
      return {
        questions: [],
        error: '',
      };  
    }

Ultimately, I attempt to access this data in the template via an h1 tag:

<h1>{{questions[0][0].description}}</h1>

Unfortunately, I encounter the subsequent error:

TypeError: Cannot read property '0' of undefined

In the console, what I observe is:

{{questions[0][0].description}}

This is occurring prior to populating the questions array in the created lifecycle hook. Referencing this screenshot https://i.sstatic.net/7ItKW.png.

My understanding is that created() is invoked before the template rendering, but there might be a misconception on my end. My goal is to ensure that the operations in created() are completed before the template loads, ensuring the array is populated beforehand. Your assistance is greatly appreciated.

Answer №1

I'm looking to ensure that the functionality within created() runs before the template is rendered

Unfortunately, achieving this is not feasible. It's important to understand JavaScript asynchrony. The await keyword does not halt the entire application execution. Instead, the created() hook simply returns a promise and awaits the completion of any asynchronous calls.

The only workaround is structuring your template in a way that accommodates delayed data loading...

Answer №2

Implementing created asynchronously and utilizing await within it does not hinder the creation and mounting process of the component. It simply delays the execution of code inside created after await, until the promise is fulfilled.

This implies that your component will be displayed without the expected content in questions (until questions are updated, post return call).

To address this issue, you can prevent any current disturbances within your component by enclosing those sections in if conditions (or using v-if in <template>).


An effective solution for this scenario is to initiate the call from external sources, and base its rendering (creation) on the response. Typically, data shared across multiple components should be stored in a centralized location (single source of truth) which can then be accessed wherever required.

For more intricate storage requirements, Vuex can be utilized while for simpler data sharing needs, Vue.observable() can suffice.

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