Earlier quoted context omitted.
Unfortunately it uses ES6 features. As an aside, is that really a problem these days? All modern browsers now offer comprehensive native ES6 support. Unless you still need to support older platforms like IE or some of the legacy mobile browsers, it's mostly a non-issue.
IE11 is holding out. Mobile Safari is hit & miss
What's missing from any recent version of iOS Safari, though? Anything from 10 upwards supports pretty much all of ES6 with very minor exceptions, AFAIK. Even features from more recent versions of ES tend to be supported quite well quite quickly in the era of self-updating "evergreen" browsers.