Should I be able to apply `new`?

[Graydon Hoare]

P T Withington wrote:

> I thought I could memoize by simply returning a value from my 
> constructor.  Is that no longer true?

Ah, yes. A subtlety in the word "constructor": you can return any novel 
value you like from a constructor *function*, as this is legal ES3. But 
in ES4 we also have classes with constructors, and you cannot return 
novel values from a class constructor.

Class constructors are called as part of a specific sequence of nested 
base/derived actions, including instance initializers declared outside 
the ctor and instance settings declared between the parameter list and 
the ctor body. That sequence of actions assumes a certain continuity in 
the constructee: switching the value you're part-way through 
constructing in the middle of the sequence makes it very hard to 
formulate safety conditions that would preserve type judgments the 
verifier might wish to make, so we've prohibited it.

(In general any valid ES3 code, save a very few ill-specified or 
horrendously dangerous and unused cases, should be valid ES4 code. We 
know we can't break the web :)

-graydon