Hi,
I have read the recent "Subclassing" Built-in Constructors" page [1] and
I have some feedback. This page exhibits a distinction that wasn't clear
in my mind before reading it: some internal properties can be added
lazily like the ones for dates while some (essential internal methods)
cannot.
class MyDate extends Date {
constructor(...args) {
// |this| has no internal date property
super();
// now it does have them and can be considered as a date
object, [[NativeBrand]] aside
}
}
For constructor of objects that only add internal data properties and
additional internal method (like RegExps [[Match]]), this can work fine.
However, for objects with different essential internal methods, it
cannot work.
class MyArray extends Array {
constructor(...args){
// |this| not an array until i've called super(), right?
'length' in this; // false
this[2] = "yo";
this.length = 1; // ? I guess nothing happens except that
|this| has a new 'length' property?
super(); // If nothing serious happens, the array invariant
is broken.
this[2] // ?
this.length // ?
}
}
I don't think there is a way out unless |this| is already an array in
which case 'super' is useless.
David
[1] http://wiki.ecmascript.org/doku.php?id=strawman:subclassable-builtins
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