Importing javascript file for use within vue component

Include an external JavaScript file

Try including your (external) JavaScript into the mounted hook of your Vue component.

<script>
export default {
  mounted() {
    const plugin = document.createElement("script");
    plugin.setAttribute(
      "src",
      "//api.myplugincom/widget/mykey.js"
    );
    plugin.async = true;
    document.head.appendChild(plugin);
  }
};
</script>

Reference: How to include a tag on a Vue component

Import a local JavaScript file

In the case that you would like to import a local JavaScript in your Vue component, you can import it this way:

MyComponent.vue

<script>
import * as mykey from '../assets/js/mykey.js'

export default {
  data() {
    return {
      message: `Hello ${mykey.MY_CONST}!` // Hello Vue.js!
    }
  }
}
</script>

Suppose your project structure looks like:

src
- assets
    - js
      - mykey.js
- components
    MyComponent.vue

And you can export variables or functions in mykey.js:

export let myVariable = {};
export const MY_CONST = 'Vue.js';
export function myFoo(a, b) {
    return a + b;
}

Note: checked with Vue.js version 2.6.10