curry.js

module.exports = curry;

function curry (f) {
	var arity = f.length;
	var params = [];
	var end = createEnd(f, arity);
	return createCurried(params, arity, end);
}

function createEnd (f, arity) {
	var src = 'return function ' + f.name + '_curried (args) { return f(';
	for (var i = 0; i < arity; i++) {
		src += (i ? ', args[' : 'args[') + i + ']';
	}
	src += '); };';
	return (new Function ('f', src)(f));
}

function createCurried (collected, arity, end) {
	return function () {
		var params = concat(collected, arguments);
		return params.length < arity
			? createCurried(params, arity, end)
			: end(params);
	};
}

function concat(a, b) {
	var al = a.length;
	var bl = b.length;
	var c = new Array(al + bl);
	var i;

	for(i=0; i<al; ++i) {
		c[i] = a[i];
	}

	for(i=0; i<bl; ++i) {
		c[i+al] = b[i];
	}

	return c;
}

function add(x, y) {
	return x + y;
}

var addc = curry(add);

var z = addc(1)(2);
console.log(z);

Update: I added a hand-coded concat() for appending arguments. It's significantly faster than using Array.prototype.concat + slice.