JavaScript file to allow injecting scripts into a page using GreaseMonkey/TamperMonkey and running a callback when loading is complete. Based on http://stackoverflow.com/questions/6725272/dynamic-cross-browser-script-loading .

inject.js

function inject(src, callback) {
    if (typeof callback != 'function') callback = function() { };
    
    var el;
    
    if (typeof src != 'function' && /\.css[^\.]*$/.test(src)) {
        el      = document.createElement('link');
        el.type = 'text/css';
        el.rel  = 'stylesheet';
        el.href = src;
    } else {
        el      = document.createElement('script');
        el.type = 'text/javascript';
    }
    
    el.class = 'injected';
    
    if (typeof src == 'function') {
        el.appendChild(document.createTextNode('(' + src + ')();'));
        callback();
    } else {
        el.src   = src;
        el.async = false;
        el.onreadystatechange = el.onload = function() {
            var state = el.readyState;
            if (!callback.done && (!state || /loaded|complete/.test(state))) {
                callback.done = true;
                callback();
            }
        };
    }

    var head = document.head || document.getElementsByTagName('head')[0];
    head.insertBefore(el, head.lastChild);
}

How to call that function and where do I specify what to inject?

@PBonski commented on May 13, 2015, 1:39 PM GMT+2:

How to call that function and where do I specify what to inject?

AFAIK just write a function for ex:

function pbonski(){
    doYourStuff();
    var here = 1;
}

and then:

inject(pbonski, callback);

Callback function is not mandatory.