NCA fallback library

nca.js

var NCA = function(type, version, compiler){
    this.type = type;
    this.version = version;
    // compiler must have interface method "compile",
    // which accepts a string as single argument
    this.compiler = compiler;
};
NCA.prototype = {
    queue: [],
    fetching: false,

    run: function(callback){
        this.callback = callback;
        var scripts = Array.prototype.slice.call(document.querySelectorAll('script'), 0);
        // find all scripts with type="jsq" and version="1.0"
        scripts.filter(function(s){
            return s.getAttribute('type') == 'jsq' && s.getAttribute('version') == '1.0';
        }).forEach(function(s){
            this.queue.push(s);
            this.next();
        },this);
    },
    next: function(){
        if (!this.queue.length) {
            if (this.callback) this.callback();
        } else if (!this.fetching) {
            this.fetching = true;
            var next = this.queue.shift();

            if (next.src) {
                this.GET(next.src+'?'+Math.random(), function(err, src){
                    if (err) {
                        this.fetching = false;
                        throw new Error(err);
                    } else {
                        this.process(src, next.src);
                    }
                }.bind(this));
            } else {
                this.process(next.textContent);
            }
        }
    },
    process: function(src, url){
        var compiled = this.compiler.compile(src);
        var script = document.createElement('SCRIPT');
        script.textContent = compiled;
        script.className = 'compiled-script';
        script.setAttribute('data-src-url', url || 'inline');
        document.body.appendChild(script);
        // prev appendChild is blocking...

        this.fetching = false;
        this.next();
    },
    GET: function(url, callback){
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4) {
                try { xhr.status; // status is a getter, this checks for exception
                } catch (errrr) {
                    callback(new Error("Warning: Unknown error with server request (timeout?)."));
                }

                if (xhr.status == 200) callback(null, xhr.responseText);
                else callback(new Error("File request problem (code: "+xhr.status+")!"));
            }
        };
        xhr.open("GET", url);
        xhr.send(null);
    },
};