js> obj=Object("0") (new String("0")) js> Object.assign(obj, {0:1, 1: 1}) typein:2:0 TypeError: 0 is read-only js> obj[1] 1 // this should be undefined TC39 decided to remove this for performance reasons: https://twitter.com/bterlson/status/535921786525736960

Comment on attachment 8580752 [details] [diff] [review] Object.assign should stop when an exception is thrown Review of attachment 8580752 [details] [diff] [review]: ----------------------------------------------------------------- Mmmmmh, so much nicer! ::: js/src/builtin/Object.js @@ +30,4 @@ > > + // Steps 5.c.i-iii. We abbreviate this by calling propertyIsEnumerable > + // which is faster and returns false for not defined properties. > + if (callFunction(std_Object_propertyIsEnumerable, from, nextKey)) { It'd be really nice to inline std_Object_propertyIsEnumerable to substantially speed this up. Perhaps file a bug so it's at least visible?

https://treeherder.mozilla.org/#/jobs?repo=try&revision=3e4f8caef1dc https://hg.mozilla.org/integration/mozilla-inbound/rev/c279eefced07 Filed bug 1147154 for inlining propertyIsEnumerable. That was quick, even before coming back from your vacation! :)

https://developer.mozilla.org/en-US/Firefox/Releases/39#JavaScript https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign