Disclaimer

First of all, it’s important to say that content scripts don’t support modules as of January 2018. This workaround sidesteps the limitation by embedding module script tag into the page that leads back to your extension.

THIS IS AN UNSAFE WORKAROUND!

A web page script (or another extension) can exploit your code and extract/spoof the data by using setters/getters on Object.prototype and other prototypes, proxying functions and/or global objects, because the code inside a script element runs in the JS context of the page, not in the safe isolated JS environment where content scripts run by default.

A safe workaround is the dynamic import() shown in another answer here.

Workaround

This is my manifest.json:

    "content_scripts": [ {

       "js": [

         "content.js"

       ]

    }],

    "web_accessible_resources": [

       "main.js",

       "my-script.js"

    ]

Note that I have two scripts in web_accessible_resources.

This is my content.js:

    'use strict';

    

    const script = document.createElement('script');

    script.setAttribute("type", "module");

    script.setAttribute("src", chrome.extension.getURL('main.js'));

    const head = document.head || document.getElementsByTagName("head")[0] || document.documentElement;

    head.insertBefore(script, head.lastChild);

This will insert main.js into the webpage as a module script.

All my business logic is now in main.js.

For this method to work, main.js (as well as all scripts that I will import) must be in web_accessible_resources in the manifest.

Example Usage: my-script.js

    'use strict';

    

    const injectFunction = () => window.alert('hello world');

    

    export {injectFunction};

And in main.js this is an example of importing the script:

    'use strict';

    

    import {injectFunction} from './my-script.js';

    injectFunction();

This works! No errors are thrown, and I am happy. :)