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- // relies on Date.now() which has been supported everywhere modern for years.
- // 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(){
- // prepare base perf object
- if (typeof window.performance === 'undefined') {
- window.performance = {};
- }
- if (!window.performance.now){
- const nowOffset = Date.now();
- if (performance.timing && performance.timing.navigationStart){
- nowOffset = performance.timing.navigationStart
- }
- window.performance.now = function now(){
- return Date.now() - nowOffset;
- }
- }
- })();
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