fix(__extends): Use correct behaviour with null prototype

[ExE Boss (ExE-Boss)]

If you were to call __extends(Foo, null), it would cause incorrect behaviour for extendStatics, as Foo is supposed to inherit from %Function.prototype% in that case, instead of also inheriting from null.

[Holger Jeromin (HolgerJeromin)]

Does this change relate to a change in Typescript? Or should there be a change there?

[ExE Boss (ExE-Boss)]

There should also be a change in TypeScript.

[Ron Buckton (rbuckton)]

Why not just do this instead?

Suggested change

	extendStatics(d,b);
	if(b!==null)extendStatics(d,b);

[ExE Boss (ExE-Boss)]

Because it’s necessary to also add the d.prototype.constructor property in the b === null case.

[Ron Buckton (rbuckton)]

Your proposed change at the time I wrote that comment also did not set the constructor when b === null. Given your point about constructor, I'd probably recommend something simpler (and with fewer characters overall):

__extends=function(d,b){if(typeofb!=="function"&&b!==null)thrownewTypeError("Class extends value "+String(b)+" is not a constructor or null");if(b)extendStatics(d,b);function__(){}(d.prototype=b ? (__.prototype=b.prototype,new__()) : Object.create(b)).constructor=d;};

A few notes:

• I'm using if (b) rather than if (b !== null) since the null check happens on the first line and a function's "truthiness" can't be overridden through valueOf, Symbol.toPrimitive, or a Proxy
• I swapped the condition on the last line from b === null to b, and therefore swapped the order of expressions in the conditional.
• I moved the constructor assignment out of the __ prototype helper and onto the last line.

[ExE Boss (ExE-Boss)]

It might be better to fold the extendStatics call inside the b ? conditional to lower the amount of ToBoolean(b) checks necessary:

function__extends(d,b){// This should also check that `b.prototype` is an object:if(typeofb!=="function"&&b!==null)thrownewTypeError("Class extends value "+String(b)+" is not a constructor or null");function__(){}(d.prototype=b
? (extendStatics(d,b),__.prototype=b.prototype,new__())
: Object.create(b)).constructor=d;}

[Ron Buckton (rbuckton)]

Apparently "correct" behavior when extending null is up for debate. The algorithms in the ECMA-262 specification related to extends null actually prevent it from working, and no engine currently supports extends null. The last time the behavior was discussed in plenary was last October and the conclusion was that more time is needed to explore solutions.

Until this has been resolved in the specification, I'd rather not make any more changes to extends null semantics in TypeScript. Until then, I plan to leave this PR open so that we can come back to it once the specification issues have been resolved.