Comments on Meeting Notes

Brendan Eich brendan at
Wed Dec 5 06:28:32 PST 2012

Claude Pache wrote:
> Le 4 déc. 2012 à 22:28, Brendan Eich<brendan at>  a écrit :
>> Mark S. Miller wrote:
>>> On Tue, Dec 4, 2012 at 10:48 AM, Brendan Eich<brendan at>   wrote:
>>>> Kevin Smith wrote:
>>>>> I recommend allowing let declarations only in strict mode.  This is the
>>>>> simple, backwards-compatible path.  Strict mode only has a bad reputation
>>>>> because, in ES5, it is restrictive-only.  There are (almost) no carrots
>>>>> leading users there.
>>>> Strict mode has a bad rep for two other important causes:
>>>> * It forks runtime semantics, which requires careful testing in
>>>> pre-ES5-strict implementations. This has been a real-world problem, multiple
>>>> times.
>>> I buy this for old code that needs to just keep working, bugs and all,
>>> without human attention. But strict mode can be opted into
>>> incrementally, per program or even per function.
>> Real world problem: concatenation.
> If we allow "let" and destructuring assignment in non-strict mode,

(I'm not sure why anything I wrote about strict mode problems was cited, 
since you did not repond to the "strict mode has a bad rep for these 
reasons" points...)

>   a
> developer will happily use "let" as variable name, then one day they
> will write "let[i] = j" (Murphy's law), and get an incomprehensible
> failure.

This could happen, but did the developer forget to declare "let" (assign 
an implicit global)? The odds shrink. All uses of 'let' as an identifier 
have abbreviated "letter" and stored a unit-length string value, so have 
not indexed into the value.

We talked about making 'var let' in combination with 'let[i] = j' an 
error, but we did not want parsing to depend on binding. OTOH we did 
talk about warnings.

Murphy's Law does govern us, I agree. It says that anything that can go 
wrong, will go wrong. But this applies to let underutilization if yoked 
to strict mode. Deciding between the alternative paths is not a matter 
of simple logic, since it depends on human factors.


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