Javascript Proxy Madness

loop-u-s-v.js

loop[0][2][20] = function(i) {
  console.log(i);
};

// Output:
// 0
// 2
// 4
// 6
// 8
// 10
// 12
// 14
// 16
// 18

loop-u-v.js

loop[5][10] = function(i) {
  console.log(i);
};

// Output:
// 5
// 6
// 7
// 8
// 9

loop-v.js

loop[10] = function(i) {
  console.log(i);
};

// Output:
// 0
// 1
// 2
// 3
// 4
// 5
// 6
// 7
// 8
// 9

loop.js

var loop = new Proxy({
	u: null, // Initial condition
	v: null, // Length of loop
	step: null // Step
}, {
	get: function(target, property) {
		if(target.v === null) { // Set the length of the loop initially
			target.v = parseFloat(property);
			return loop;
		} else if(target.u === null) { // Add a start condition
			target.u = target.v;
			target.v = parseFloat(property);
			return loop;
		} else throw new Error("Too many arguments for loop."); // The lat condition is on the set
	},

	set: function(target, property, callback) {
		// Parse the arguments
		if(target.v === null) target.v = parseFloat(property);
		else if(target.u === null) target.u = target.v, target.v = parseFloat(property);
		else if(target.step === null) target.step = target.v, target.v = parseFloat(property);

		// Make sure the loop conditions are set
		if(target.v !== null) {
			// Make sure the iterator is a callback
			if(typeof callback !== "function") throw new Error("Loop iterator is not a function.");

			// Create the loop conditions
			var u = target.u !== null ? target.u : 0,
				v = target.v,
				step = target.step !== null ? target.step : 1;

			// Run the loop
			for(var i = u; i < v; i += step) callback(i, v - u, u, v);

			// Reset the options
			target.u = target.v = target.step = null;
		} else throw new Error("Loop conditions not set.");
	}
});