Redirector example for Gecko 20+

redirector.js

// https://gist.github.com/Infocatcher/9523286

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

// Redirector example for Gecko 20+
// (c) Infocatcher 2014 <https://github.com/Infocatcher>

Services.obs.addObserver(observer, "http-on-modify-request", false);
setTimeout(function() {
	Services.obs.removeObserver(observer, "http-on-modify-request");
}, 10e3); // Disable after 10 seconds

function observer(subject, topic, data) {
	if(subject instanceof Components.interfaces.nsIHttpChannel) {
		var url = subject.URI.spec;
		// Example for http://www.yandex.ru/
		if(url == "http://yandex.st/jquery/1.8.3/jquery.min.js") {
			// Note: we can't redirect to local files (file:// or resource://) due to security restrictions!
			//var newUri = Services.io.newURI("data:text/javascript;charset=UTF-8,alert(%22Redirected!%22)%3B", null, null);
			var newUri = getDataURI("Z:\\test.js", "text/javascript");
			subject.redirectTo(newUri);
		}
	}
}

function getDataURI(path, type) {
	var cache = { __proto__: null };
	var expire = 60*1000; // 60 seconds
	getDataURI = function(path, type) {
		var t = performance.now();
		if(path in cache) {
			var {uri, timer} = cache[path];
			timer.cancel();
		}
		else {
			var data = getFileContents(path);
			var dataUrl = "data:" + type + ";charset=UTF-8," + encodeURIComponent(data);
			var uri = Services.io.newURI(dataUrl, null, null);
			var timer = Components.classes["@mozilla.org/timer;1"]
				.createInstance(Components.interfaces.nsITimer);
			cache[path] = {
				uri: uri,
				timer: timer
			};
		}
		timer.init(function() {
			delete cache[path];
		}, expire, timer.TYPE_ONE_SHOT);
		Services.console.logStringMessage("xxx getDataURI(): " + (performance.now() - t).toFixed(3) + " ms");
		return uri;
	};
	return getDataURI.apply(this, arguments);
}
function getFile(path) {
	var file = Components.classes["@mozilla.org/file/local;1"]
		.createInstance(Components.interfaces.nsILocalFile || Components.interfaces.nsIFile);
	try {
		file.initWithPath(path);
	}
	catch(e) {
		Components.utils.reportError(e);
		return null;
	}
	return file;
}
function getFileContents(path) {
	var file = getFile(path);
	if(!file)
		return "";
	return readFromFile(file);
}
function readFromFile(file) {
	var fis = Components.classes["@mozilla.org/network/file-input-stream;1"]
		.createInstance(Components.interfaces.nsIFileInputStream);
	try {
		fis.init(file, 0x01, parseInt("0444", 8), 0);
	}
	catch(e) {
		Components.utils.reportError(e);
		return "";
	}
	var sis = Components.classes["@mozilla.org/scriptableinputstream;1"]
		.createInstance(Components.interfaces.nsIScriptableInputStream);
	sis.init(fis);
	var str = sis.read(fis.available());
	sis.close();
	fis.close();
	return convertToUnicode(str);
}
function convertToUnicode(str) {
	var suc = Components.classes["@mozilla.org/intl/scriptableunicodeconverter"]
		.createInstance(Components.interfaces.nsIScriptableUnicodeConverter);
	suc.charset = "utf8";
	try {
		return suc.ConvertToUnicode(str);
	}
	catch(e) {
		Components.utils.reportError(e);
	}
	return str;
}