All of the new constructors, Map, Set, WeakMap and WeakSet should fail
when called as a function unless `this` already has a certain internal
property, ie [[MapData]]. This is so that sub classing works as
expected.
However, all JS engines that supports Map, Set and WeakMap are future
hostile and they all return a new instance when called as a function.
For example, here is the V8 code:
function MapConstructor() {
if (%_IsConstructCall()) {
%MapInitialize(this);
} else {
return new $Map();
}
}
I understand that fully supporting @@create requires a lot of work but
until then we should at least throw when called as function to allow
us to get out of this mess eventually.
--
erik
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