The instance is referencing property or method "foo" during render, but it has not been defined. To ensure reactivity, please make sure that this property is defined

Just starting out with the Vue framework and experimenting with component composition. However, I'm encountering an issue when trying to run the code:

"Property or method "icons" is not defined on the instance but referenced during render. Make sure that this property is reactive"

My template looks like this:

<template>
  <div>
    <h1>Welcome to the new page</h1>
    <div v-for="icon in icons">{‌{ icon }}</div>
  </div>
</template>

In a file named Footer.ts, I've written the following code:

import {Component, Vue} from "vue-property-decorator";
import Template from './Footer.vue';

@Component({
    mixins: [Template]
})
export default class Footer extends Vue {
    public icons!: string[];

    constructor() {
        super();
        this.icons = [
            'fab fa-facebook',
            'fab fa-twitter',
            'fab fa-google-plus',
            'fab fa-linkedin',
            'fab fa-instagram'
        ];

        console.log('Footer rendered');
        this.icons.map((icon) => console.log(icon));
    }
}

And in App.vue:

<template>
  <div>
    <h1>Main Page</h1>
    <footer></footer>
  </div>
</template>

<script lang="ts">
  import {Component, Vue} from 'vue-property-decorator';
  import Footer from '@/component/Footer';

  @Component({
      components: {
          Footer
      }
  })
  export default class App extends Vue {
      mounted() {
          console.log("assembly completed");
      }
  }
</script>

The constructor in Footer.ts seems to never be called.

Thank you!

Answer №1

At last, I was able to address the problem at hand. It turned out that within the MyComponent directory, I had files named MyFooter.ts and MyFooter.vue. By simply renaming MyFooter.ts to component.ts and MyFooter.vue to template.vue, everything started working seamlessly. It was a revelation for me that components and templates needed to follow this naming convention.

Answer №2

One common issue arises when a variable or method is referenced within a component's template without being properly defined within the instance.

In the code snippet provided, the variable icons is utilized in a for loop, yet it has not been declared in the component's data.

If you consult the documentation of the vue-property-decorators library found at this link, you will notice that using the constructor method inside Vue components is not considered a valid approach to declaring properties for components.

Instead, it is recommended to declare data properties directly within the component instance as demonstrated below:

export default class Footer extends Vue {
  icons: string[] = [
    'fab fa-facebook',
    'fab fa-twitter',
    'fab fa-google-plus',
    'fab fa-linkedin',
    'fab fa-instagram'
    ]
  }
}

Answer №3

After encountering the issue in my own project, I discovered that the reason for the error message was due to neglecting to include the @Component annotation on the component class.

import {Component, Prop, Vue} from 'vue-property-decorator';

@Component // Ensure this line is included
export default class CustomComponent extends Vue {
  
  @Prop({default: ''}) text!: string;

}

Answer №4

One common reason for encountering this error is when Vue attempts to access a variable that has not been declared. It is important to ensure that the necessary variables are both declared and defined before your component is rendered by Vue. The concept of getter/setter only applies to variables explicitly listed in the data section. You cannot simply add a new variable like something to the this context and expect it to function correctly. For further information, consult the official documentation.

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