Freezing object after adding to WeakMap in IE11

[smares]

I just stumbled upon a problem that affects only IE 11. It does not happen with IE 9, IE 10, Edge, Firefox or Chrome:

var wm = new WeakMap()
var key = {}

wm.set(key, 'foo')
console.log(wm.get(key)) // yield: 'foo'
Object.freeze(key)
console.log(wm.get(key)) // yield: 'undefined' for ie11 using polyfill | 'foo' for ie11 without polyfill

The latter case should be correct, wm.get(key) after calling Object.freeze(key) should return foo as it does in IE 9 with polyfill, IE 10 with polyfill, IE 11 without polyfill, Edge without polyfill, Chrome without polyfill and Firefox without polyfill.

[zloirock]

You have confused the cause and effect. Without a polyfill, IE11 WeakMap can't work with frozen keys completely. core-js adds a fallback and it includes mutation objects in Object.freeze. But this fix does not cover the case from your example, so yes, it can be marked as a bug.

[zloirock]

Fixed in v3 branch.