Using loadFrameScript, how can inject file before the page's script execution?

Question

I'm currently using this in the bootstrap.js file. Is used to edit the client's JavaScript code in my case I'm using recentBrowserWindow.messageManager.loadFrameScript(file, true):

const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr} = Components;
Cu.import('resource://gre/modules/Services.jsm');

var file = 'chrome://testAddon/content/inject.js';

function startup(aData, aReason) {
    var recentBrowserWindow = Services.wm.getMostRecentWindow('navigator:browser');
    if(recentBrowserWindow.document.readyState == 'complete') {
        recentBrowserWindow.messageManager.loadFrameScript(file, true);
    }
}

inject.js:

var window = content;
var pageJS = window.wrappedJSObject;
console.log("jQuery is defined:", "jQuery" in window);
//true, jQuery is already assigned on client page.

But here the script inject.js is running after of the client page. The question is:

How can inject inject.js just before the page's script execution?. What method should I use?

Answer 1

From: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIFrameScriptLoader

It says to use document-element-inserted, this will ensure your script runs before the page scripts run.

I don't have much experience with frame scripts, others will have to verify me on this.

Answer 2

This is a really nice example of simple loadFrameScript: https://github.com/mdn/e10s-example-addons/tree/master/run-script-in-all-pages/ported-message-manager/src/chrome/content

See here how he does DOMContentLoaded: https://github.com/mdn/e10s-example-addons/blob/master/run-script-in-all-pages/ported-message-manager/src/chrome/content/frame-script.js

Change that to document-element-inserted but i think this is not an event listener but an observer service.