https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap
https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
The Map and WeakMap primordial has been part of ECMAScript (and thus JavaScript) for many years now. I'm reasonably certain that, among the browsers listed on the MutationObserver page, most have reached end-of-life for the versions that don't support Map or WeakMap a long time ago.
WeakMap would be preferable. The only place NodeMap.prototype.keys is used is line 670 of the current code (change.maybeMoved.keys().forEach(isMoved);).
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WeakMap
https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
The Map and WeakMap primordial has been part of ECMAScript (and thus JavaScript) for many years now. I'm reasonably certain that, among the browsers listed on the MutationObserver page, most have reached end-of-life for the versions that don't support Map or WeakMap a long time ago.
WeakMap would be preferable. The only place
NodeMap.prototype.keysis used is line 670 of the current code (change.maybeMoved.keys().forEach(isMoved);).