Hi, there is an issue in the example code. You should replace for(let weakStopwatch in allStopWatches){ with something like for(let weakStopwatch of allStopWatches.values()){ as allStopWatches is a map. Sorry for the confusion :).
Sihui
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Hi, there should be no difference between WKWebView and Safari in support for new JavaScript features.
If you notice anything does not work as expected in WKWebView, you can file a bug with WebKit Bugzilla.