Javascript Engines DO optimize integer operations.

explain.js

function bench1(n){
	for(var i = 0; i < n; i++) ;
}
function bench2(n){
	for(var i = 0.5; i < n; i++) ;
}
function bench3(n){
	for(var i = Math.random(); i < n; i++) ;
}
function bench4(n){
	for(var i = Math.random(), top = n + Math.random(); i < top; i++) ;
}
function bench5(n){
	for(var i = 5000000000 + Math.floor(1 + Math.random() * 10), top = 5000000000 + n; i < top; i++)
}
function bench6(n){
	for(var i = 5000000000 + Math.floor(1 + Math.random() * 10), top = 5000000000 + n + Math.floor(1 + Math.random() * 10); i < top; i++)
}
function bench7(n){
	for(var i = 5000000000 + Math.random(), top = 5000000000 + n; i < top; i++)
}
function bench8(n){
	for(var i = 5000000000 + Math.random(), top = 5000000000 + n + Math.random(); i < top; i++)
}

test.moe

def measureTime(args, f) =
	var multiplier = 1
	var estimatedTime = time args, f
	while(estimatedTime < 100)
		if(estimatedTime < 1)
			multiplier *= 110
		else
			multiplier *= (100 / estimatedTime + 10)
		estimatedTime = time args, { for(i <- 1...multiplier) f.apply null, arguments }
	estimatedTime / multiplier
	
def bench1(n) = 
	var i = 0; var top = n; while(i < top) i += 1
def bench2(n) =
	var i = 0.5; var top = n; while(i < top) i += 1
def bench3(n) =
	var i = math.random(); var top = n
	while(i < top) i += 1
def bench4(n) =
	var i = math.random(); var top = n + math.random()
	while(i < top) i += 1
def bench5(n) =
	var i = 5000000000 + math.randInt(1, 10)
	var top = 5000000000 + n
	while(i < top) i += 1
def bench6(n) =
	var i = 5000000000 + math.randInt(1, 10)
	var top = 5000000000 + n + math.randInt(1, 10)
	while(i < top) i += 1
def bench7(n) =
	var i = 5000000000 + math.random()
	var top = 5000000000 + n
	while(i < top) i += 1
def bench8(n) =
	var i = 5000000000 + math.random()
	var top = 5000000000 + n + math.random()
	while(i < top) i += 1
def benchmarks = [bench1, bench2, bench3, bench4, bench5, bench6, bench7, bench8]
start async :>
	for(() <- 1...1000)
		var n = math.randInt 1000000, 10000000
		[n].concat benchmarks.map ((m) => math.round((measureTime [n], m) * 1000)) |.join "\t" |trace
		sleep ! 0