performance.now() polyfill (aka perf.now())

performance.now()-polyfill.js

// @license http://opensource.org/licenses/MIT
// copyright Paul Irish 2015


// Date.now() is supported everywhere except IE8. For IE8 we use the Date.now polyfill
//   github.com/Financial-Times/polyfill-service/blob/master/polyfills/Date.now/polyfill.js
// as Safari 6 doesn't have support for NavigationTiming, we use a Date.now() timestamp for relative values

// if you want values similar to what you'd get with real perf.now, place this towards the head of the page
// but in reality, you're just getting the delta between now() calls, so it's not terribly important where it's placed


(function(){

  if ("performance" in window == false) {
      window.performance = {};
  }
  
  Date.now = (Date.now || function () {  // thanks IE8
	  return new Date().getTime();
  });

  if ("now" in window.performance == false){
    
    var nowOffset = Date.now();
    
    if (performance.timing && performance.timing.navigationStart){
      nowOffset = performance.timing.navigationStart
    }

    window.performance.now = function now(){
      return Date.now() - nowOffset;
    }
  }

})();

Hello,

I have mixed this polyfill with Aaron Levine's (https://gist.github.com/Aldlevine/3f716f447322edbb3671) and modifier some code:

'use strict';

// @license http://opensource.org/licenses/MIT
// copyright Paul Irish 2015
// Added code by Aaron Levine from: https://gist.github.com/Aldlevine/3f716f447322edbb3671
// Some modifications by Joan Alba Maldonado.
// as Safari 6 doesn't have support for NavigationTiming, we use a Date.now() timestamp for relative values
// if you want values similar to what you'd get with real perf.now, place this towards the head of the page
// but in reality, you're just getting the delta between now() calls, so it's not terribly important where it's placed
// Gist: https://gist.github.com/jalbam/cc805ac3cfe14004ecdf323159ecf40e
// TODO: Think about adding vendor prefixes.
if (!Date.now) { Date.now = function() { return new Date().getTime(); } }
(function()
{
	if (window.performance && window.performance.now) { return; }

	window.performance = window.performance || {};

	if
	(
		window.performance.timing && window.performance.timing.navigationStart &&
		window.performance.mark &&
		window.performance.clearMarks &&
		window.performance.getEntriesByName
	)
	{
		window.performance.now = function()
		{
			window.performance.clearMarks('__PERFORMANCE_NOW__');
			window.performance.mark('__PERFORMANCE_NOW__');
			return window.performance.getEntriesByName('__PERFORMANCE_NOW__')[0].startTime;
		};
	}
	else if ("now" in window.performance === false)
	{
		var nowOffset = Date.now();

		if (window.performance.timing && window.performance.timing.navigationStart)
		{
			nowOffset = window.performance.timing.navigationStart
		}

		window.performance.now = function now()
		{
			return Date.now() - nowOffset;
		}
	}
})();

I have placed it in this gist: https://gist.github.com/jalbam/cc805ac3cfe14004ecdf323159ecf40e

Any comments will be welcome. Thank you very much.