On 7/16/2013 4:10 PM, Erik Arvidsson wrote:
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.
I wonder if the current spec language is a remnant of before @@create
was added to the spec. It should be possible to detect when the
constructors are called with a receiver that is not an initializing
instance and simply create a new instance. Pseudo code:
function Map(...args){
if (!IsObject(this) || !%HasMapData(this)) {
// got a primitive, the global object, or some random object like
from `Map.call({})`
return new Map(...args);
} else if (!%IsInitializedMap(this)) {
// A legit Map but has uninitialized [[MapData]]
%MapInitialize(this);
} else {
// [[MapData]] is present but is already initialized
throw TypeError("Attempted to reinitialize a Map");
}
}
The main point of introducing @@create (I think) was to allow for
distinguishing between initializing, initialized, and everything else.
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