how to test execution time many times in javascript code example

Example 1: Javascript measure function run time

var startTime = performance.now();

alert('Run some function here');

var endTime = performance.now();
var totalTime=endTime-startTime;// time took to run in milliseconds

alert('Total time:'+totalTime +'ms');

Example 2: get execution time in javascript

var t0 = performance.now()

doSomething()   // <---- The function you're measuring time for 

var t1 = performance.now()
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.")