[1]

Safari 9.0 (2015-11-04 23:30:48 +09:00 版) https://developer.apple.com/library/safari/releasenotes/General/WhatsNewInSafari/Articles/Safari_9.html#//apple_ref/doc/uid/TP40014305-CH9-SW27

The following ECMAScript 6 content is now supported by Safari:
Weak Set

[2] (2017-12-12 20:45:20 +09:00) about:blank

[3] WeakSet - JavaScript | MDN (2017-12-07 20:18:32 +09:00) https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakSet