Replaceing/swapping JavaScript file in html using Greasemonkey

Question

My goal is to replace an original JavaScript file from a web-page with my home-made file that I have locally. I have found an interesting script here which I repurposed to fit my objective.

// ==UserScript==
// @name        JS_tamper
// @namespace   namespace
// @include     http://192.168.1.1/*
// @version     1
// @grant       none
// @run-at      document-start
// ==/UserScript==

var newJSFile = "http://localhost:8080/tamper.js"; //The JS file to load in replacment of old JS file

var oldJSFile = "http://192.168.1.1/menuBcm.js"; //The old JS file as it is named in inspect element (make sure its spelled EXACTLY the same)

var pattern = new RegExp(oldJSFile, "i"); //Create the RegExp pattern with the /i switch to make it case-insensitive

function injectScript(originalPage) { //Function injectScript replaces the file
    console.log('Replace stage 2: Replace text matching', oldJSFile, 'with', newJSFile);
    var moddedPage = originalPage.replace(pattern, newJSFile); //Modify the HTML code that we got, replacing the old JS file with the new one
    document.open();
    console.log('Replace stage 3: Write new HTML to page...');
    document.write(moddedPage); //Write to the page the new HTML code
    document.close();
}

setTimeout(function() { //Wait a bit for the page's HTML to load...
    console.log('Replace stage 1: target HTML');
    injectScript(document.documentElement.outerHTML); //Run function injectScript with the page's HTML as oldPage in the function
}, 1111);

But the console log never reaches stage 3 presumably due to some kind of error opening ( document.open() ) the file or replacing ( .replace) the script.

Output:

//Replace stage 1: target HTML  JS_tamper.user.js:29:5
//Replace stage 2: Replace text matching "http://192.168.1.1/menuBcm.js" with "http://localhost:8080/tamper.js"

Does anyone know why I am having this issue or perhaps another way of solving this problem using Greasemonkey(or other extension)?

I have created another thread with another (different) question on the same project if you are interested in some of the involved files.

Pelvis

I thought the extension Resource Override proposed by answer may help. – Cerement

Answer 1

The original script was already executed before the injectScript, so even if your tampermonkey works as expected, the result may not be like what you need. Generally,You need a proxy server to achieve this.

If you don't want to build a proxy server, try to find something like if(window.a)return; in the original script, write window.a=true in your tampermoneky script, runs it at document-start, and insert your own script

Plebe

Thank you for your answer! Should I replace a from window.a with something? – Pelvis

@Pelvis It's just an example. It depends on the content of the oldJSFile . You can even override some native api to raise an error in it. – Plebe