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See console.log()
<script>
function doSomething(){
	var x = 1;
	for(i=0; i<1000000; i++){
		var x = x+5;
	}
}

var start = Date.now();
var start2 = start;

var period = 1; 					//period time seconds
var periods = 5;				//number of periods for all test

var iterations = 0;
var min_iterations = 0;
var max_iterations = 0;

while(Date.now()<=start+periods*period*1000){		//up to all periods time
	while(Date.now()<=start2+period*1000){			//up to period time
		doSomething();								//run this function
		iterations++;								//count iterations
	}
	
	if(min_iterations==0){min_iterations = iterations;}				//set value of minimum iterations for one period
	if(min_iterations>iterations){min_iterations = iterations;}		//change this if not minimum
	if(max_iterations<iterations){max_iterations = iterations;}		//set value of maximum iterations for one period

	start2=Date.now();												//update time
	
	console.log(
		'period', Math.floor((start2-start)/(period*1000)),'\n',
		'iterations', iterations,'\n',
		'minimum iterations per',period,'seconds:', min_iterations,'\n',
		'maximum iterations per',period,'seconds:', max_iterations
	);
	iterations=0;													//reset iterations
}

console.log(
	'result\n',
	'minimum iterations per',period,'seconds:', min_iterations,'\n',
	'maximum iterations per',period,'seconds:', max_iterations
);
</script>