How to expose a function from a Chrome devtools extension "before" page loads?

Question

I am working on a Chrome devtools extension that exposes a function (extensionEntryPoint) to the inspected page. The problem is that extensionEntryPoint is not available at the initial scripts in the inspected page loads and runs. I can use it from window.onload, but that is too late.

Here's my code:

manifest.json:

{
  "name": "Extension",
  "version": "1",
  "manifest_version": 2,
  "permissions": [
    "activeTab"
  ],
  "web_accessible_resources": ["api.js"],
  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["content-script.js"],
      "run_at": "document_start",
      "all_frames": true
    }
  ]
}

content-script.js:

const script = document.createElement("script");
script.src = chrome.extension.getURL("api.js");
document.documentElement.appendChild(script);

api.js:

function extensionEntryPoint(data) {
  console.log("Thanks for calling, I'll debug your data now!")
}

Ideally, I would want for extensionEntryPoint to be available to any script on the page as it's loading (e.g. before DOMContentLoaded fires). Is this possible?

Answer 1

Due to a quirk/bug in Chrome currently your script is queued among other page scripts and hence it's not guaranteed to be the first one to run.

Solution: put the contents of api.js inside a literal string and assign it to script.textContent.
And then you can remove web_accessible_resources from manifest.json.

content-script.js:

const script = document.createElement("script");
script.textContent = `
// the actual contents of api.js
// .......
`;
document.documentElement.appendChild(script);
script.remove();

To preserve syntax highlighting of the code in an IDE, put the code into a function, but only if it's small as the browser will have to do the extraneous work parsing that code twice and stringifying it.

script.textContent = '(' + (() => {
  // the actual contents of api.js
  // .......
}) + ')()';

If you build your code prior to running it (e.g. via node.js), you can write a script that embeds the contents of api.js for example after a special comment placeholder inside your content script.