'this' return type

[jamiewinder]

Admittedly I haven't given this as much thought as it may need, but I thought it worth suggesting.

In JavaScript, it's common to return 'this' so the function can be chained. In most cases this can be easily defined:

class Obj
{
    public chainable(): Obj { ...; return this; }
}

However, if I extend the class:

class MegaObj extends Obj
{
    public doThing(): MegaObj { ...; return this; }
}

Then I can do:

var mo: MegaObj = ...
mo.doThing().chainable();

But I can't do...

mo.chainable().doThing();

Because 'chainable' returns 'Obj'. I think it'd be useful to be able to specify that the return type is the same as the object that the method was called on:

class Obj
{
    public chainable(): this  { ...; return this; }
}

class MegaObj extends Obj
{
    public doThing(): this  { ...; return this; }
}

var mo: MegaObj = ...
var mo2 = mo.chainable(); // mo2 is a MegaObj, because mo is a MegaObj
mo2.doThing();  // ok

Any feedback welcome, if only to inform me why this is a bad idea! Thanks.

[danquirk]

Yep, would be nice, already covered here #513