forLoopTest.js

var x,
	y,
	i,
	j,
	k,
	begin,
	end,
	len,
	sum,
	COUNT = 10;

function reset(times) {
	x = [], y = []
	for (var i = 0; i < times; i++) {
		x.push(i)
		y.push(2*i)
	}
}

var testcases = [1e5, 1e6, 5e6, 1e7, 2e7];

for (i = 0; i < testcases.length; i++) {

	sum = 0;

	for (k = 0; k < COUNT; k++) {
		reset(testcases[i]);

		begin = Date.now();
		len = x.length;
		for (j = 0; j < len; j++) {
			y[j] = x[j]
		}
		end = Date.now();
		console.log(end - begin);
		sum += end - begin;
	}

	console.log('test case %d of predefining len: %d ms', testcases[i], sum / COUNT)

	sum = 0;
	
	for (k = 0; k < COUNT; k++) {
		reset(testcases[i]);
		
		begin = Date.now();
		for (j = 0; j < x.length; j++) {
			y[j] = x[j]
		}
		end = Date.now();
		console.log(end - begin);
		sum += end - begin;
	}

	console.log('test case %d of not predefining len: %d ms', testcases[i], sum / COUNT)
}