On Sep 11, 2014, at 11:39 PM, Jeff Morrison wrote:
> " If a constructor body contains an assignment of the form this = then
> automatic allocation is not performed and the constructor is expected to
> perform manual allocation."
> If I'm understanding this correctly, this means that snippet (A) would never
> have access to `this`, but snippet (B) would have an implicit `this` binding
> -- is that correct?
>
> (A)
> ```
> class Foo extends Bar {
> constructor() {
> if (false) {
> this = super();
> }
> this; // undefined
> }
> }
> ```
>
> (B)
> ```
> class Foo extends Bar {
> constructor() {
> // No call to `this =` present in the constructor
> this; // the automatically allocated object -- i.e. this !== undefined
> }
> }
> ```
>
> If this is the case, it occurs to me that it would raise issues for things
> like automated refactoring and/or dead code elimination (as a minifier might
> do).
> Normally a minifier (or even a human) would expect to be able to eliminate
> the entire conditional altogether if they were confident the condition never
> evaluated to true. But with this static pre-check acting as the indicator for
> whether automatic allocation/binding should happen, doing so would cause the
> constructor to act very unexpectedly differently in the two cases.
>
> I wish I could suggest an alternative, but nothing comes to mind right now.
First a minor but technically important point: `this` in (A) above is/isn't
uninitialized rather than not having/having the value undefined.
Good overall point , tools that eliminate dead code will need to be aware of
the significant of `this =`.
Arguably, this is a point in favor of alternative two (no auto allocation).
Another possibility that we've considered is to tag non-auto-allocating
constructors in their declaration header For example:
constructor() new {...}
function foo() new {...}
But, overall, we were trying to minimize syntactic embellishments and
bikesheding opportunities.
Allen
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