ES6 doesn't need opt-in

Brendan Eich brendan at
Fri Jan 6 11:52:52 PST 2012

(This grew out of a conversation Allen and I had yesterday -- great to see it developed.)

One thing to make clear:

> ES5~EAS6:  The construct has identical syntax and static semantics in both ES5 and ES6, but differing semantics.  For example, accessing a formal parameter after an assignment to the corresponding element of the function's arguments object.

This is something we propose to do with completion reform, and also until this year for typeof null.

It's clear we can't get away with changing typeof null == "null". Even with full opt-in, it's a runtime migration hazard (one of the five fingers of fate). I withdraw it -- I'm the one who proposed it in lieu of an Object.isObject predicate -- but I do not think we should add Object.isObject either.

Rather, we need to rethink reflection on types in light of not only null vs. object, but value types/proxies. I'd rather not rush that. In the mean time, and for lo these 16 years (heading toward 17!), developers have coped and can continue to do so with typeof x == "object" && x === null or simpler (!x, x == null, other context-specialized combinations).

In general, the latest "new syntax is its own opt-in" thinking, with Allen's state machine approach, means our five fingers of fate have to be small enough that we can get away with them. At least Mark and I believe completion reform (making the completion value depend on a statically decidable expression-statement) is the only such finger we can get away with folding right now.

> If you want to explicitly force ES6 processing put a:
>   let ES6;

Or (no quotes)

  use strict;

I think we want this pragma supported, not only the string-literal expression-statement "directive".

> at the top of the source file. 
> If you want to explicitly force ES5 processing put a:
>   with (5);
> at the top of the source file

That will potentially deoptimize the top level for some engines, but maybe it doesn't matter. I don't expect it to catch on ;-).


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