loop.js

const arraySize = 1e3;
const trialSize = 1e6;
const a = [];
let i = arraySize;
while (i--) {
	a[i] = i;
}
const t = [];

const x = a.slice();
t[0] = Date.now();
i = trialSize;
while (i--) {
	// Test case:
	for (let n = 0; n < x.length; n++) {
		x[n]++;
	}
}
t[0] = Date.now() - t[0];
print("Forward for loop: " + t[0]);

const y = a.slice();
t[1] = Date.now();
i = trialSize;
while (i--) {
	// Test case:
	let n = 0;
	while (n++ < y.length) {
		y[n - 1]++;
	}
}
t[1] = Date.now() - t[1];
print("Forward while loop:", t[1]);

const z = a.slice();
t[2] = Date.now();
i = trialSize;
while (i--) {
	// Test case:
	for (let n = z.length - 1; n >= 0; n--) {
		z[n]++;
	}
}
t[2] = Date.now() - t[2];
print("Backward for loop: " + t[2]);

const w = a.slice();
t[3] = Date.now();
i = trialSize;
while (i--) {
	// Test case:
	let n = w.length;
	while (n--) {
		w[n]++;
	}
}
t[3] = Date.now() - t[3];
print("Backward while loop:", t[3]);

Results when run with v8 standalone:

Forward for loop: 2258
Forward while loop: 1300
Backward for loop: 2272
Backward while loop: 1149