Wrapping a function to hack a global temporarily

hackGlobal.js

var g = 3;

function hackGlobal(fn) {
	return function() {
		var saveG = g;
		try {
			g = 5;
			fn.apply(this, arguments);
		} finally {
			g = saveG;
		}
	}
}

function foo(msg) {
	console.log(msg + ": " + g);
}

var bar = hackGlobal(foo);

foo("not hacked");
bar("hacked");
foo("not hacked again");

Output:

not hacked: 3
hacked: 5
not hacked again: 3

This isn't guaranteed to work. It depends on what the foo function is doing. For example, replacing foo with:

function foo(msg) {
    setTimeout(function () {
        console.log(msg + ": " + g);
    }, 0);
}

gives me:

not hacked: 3
not hacked again: 3
hacked: 3

because g has been reset before the console.log statement accesses it.

https://gist.github.com/2862221

That's the only solution I could think of now.