Easiest, proper custom error in JavaScript

easyError.js

//Ensure report
console = typeof(console)=="undefined"?{log:alert}:console;

//Quick diff function
function diffContents(c){
	return "(([\\s\\S]*?)("+(c
		.replace(/\\/,"\\\\")
		.replace(/([\[\]{}()^$+*.?|])/,"\\$1")
	)+"?))?";
}
function diff(a,b){
	a = a || ""; b = b || "";
	var df = b.match(new RegExp("^"+a.replace(/([\s\S])/g,diffContents)+"(([\\s\\S]*)($))")),
	    ret = [], lastWasDiff=false;
	for(var i=1, j=0, len=df.length; i < len; i+=3, ++j){
		//i: whole, i+1: difference, i+2: similarity
		if(df[i+1]){
			var atxt = df[i+2]?"":a.charAt(j);
			if(lastWasDiff){
				ret[ret.length-1].atxt += atxt;
				ret[ret.length-1].btxt += df[i+1];
			} else {
				ret.push({
					"pos": j, "atxt": atxt, "btxt": df[i+1]
				});
				lastWasDiff=true;
			}
		} else lastWasDiff=false;
	}
	return ret;
}

//Define error class
function TestError(x){
	this.__proto__.__proto__ = (new Error(this.message = x));
	this.name = this.constructor.name;
	if("stack" in this)this.stack = this.stack.replace("Error",this.name);
}

//Perform tests
var e = new TestError("<MESSAGE>"),stackDiff;
console.log(
	(e instanceof TestError?"Passed":"Failed")+": instanceof TestError",
	(e instanceof Error?"\nPassed":"\nFailed")+": instanceof Error"
);
if("stack" in e){
	console.log((e.stack.indexOf("<MESSAGE>")!=-1?"Passed":"Failed")+": finding message");
	console.log((e.stack.indexOf("TestError",e.stack.indexOf("TestError")+1)!=-1?
		"Passed":"Failed")+": finding own name"
	);
	if((stackDiff=diff(e.stack,(new TestError("<MESSAGE>")).stack)).length){
		console.log(stackDiff);
	} else {
		console.log("Failed: stack trace is the same");
	}
} else {
	console.log("No stack traces in this JS implementation.");
}