Greasemonkey user variable definition example

userval.user.js

function makeUserModifiable(varName, defaultVal, parseFunc) {
	if(typeof parseFunc !== "function")
		parseFunc = function(a) { return a; };

	var currentVal = GM_getValue(varName, defaultVal);
	GM_registerMenuCommand("Set variable " + varName, function() {
		var val = prompt("Value for " + varName, currentVal);
		GM_setValue(varName, val);
		// if you need live modification, uncomment this (requires varName to be a
		// global variable)
		// window[varName] = parseFunc(val);
	});

	return parseFunc(currentVal);
}

var var1 = makeUserModifiable("var1", "foo");
var var2 = makeUserModifiable("var2", "foo1,foo2,foo3", function(a){return a.split(/,\s*/);});
console.log(var1);
console.log(var2.join(" - "));