What are Symbols? Objects? New primitive type?

Claude Pache claude.pache at gmail.com
Sat Apr 13 07:29:03 PDT 2013



Le 13 avr. 2013 à 15:25, Andreas Rossberg <rossberg at google.com> a écrit :

> On 13 April 2013 13:36, Claude Pache <claude.pache at gmail.com> wrote:
>> 
>> 
>> Le 13 avr. 2013 à 09:56, Andreas Rossberg <rossberg at google.com> a écrit :
>> 
>>> On 13 April 2013 00:35, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:
>>>> <snip>
>>>> 
>>>> Regarding #3
>>>> 
>>>> The biggest footgun is if
>>>> new Symbol()
>>>> creates a Symbol wrapper.  However,
>>>> new Symbol()
>>>> returning a primitive values would be unlike anything we currently have in
>>>> the language
>>> 
>>> I agree, it has to be the former. But what footgun are you seeing
>>> there? Any attempt to use a wrapper as a key throws under the
>>> agreed-upon semantics, so I don't see the issue.
>> 
>> As a JS programmer, I will be very tempted to write `new Symbol` to get a new symbol. The issue is that you get an error in a different line of code from where the problem lies, and you have to be fond of the subtle distinction between value and object wrapper (which is a feature we never use in our daily coding) to understand what is happening without external help.
> 
> While that is true, and unfortunate, it also is the case for pretty
> much every other mistake you can make in JavaScript (or any dynamic
> language, for that matter) -- and there are far more subtle ones in
> that class. So I'm not convinced that this particular case justifies
> special casing.
> 
> /Andreas

The idea is simply to avoid adding footguns for new features, even if we can't remove the existing ones.

Regarding the issue discussed here, let me elaborate:

Until now, `new Primitive`, where `Primitive` is `Number`, `String` or `Boolean` is, in practice, not a problem, because you never need to create a new number, string or boolean that way. (You can use `Primitive` as a function for typecasting, but it is not felt as the same thing as creating a new value.) Things are different with symbols, so the `new Symbol` footgun is practically a new type of footgun.

Note also that `new Date`, `new RegExp`, `new Map`, etc., work as intuitively expected, and, if I have correctly followed the last discussions, `function* gen() { /* ... */}; myIterator = new gen` (meaning: "I want to get a *new* iterator from that generator function") would also work.

I do agree that special-casing should be avoided. Therefore, I think we should make a rule: `new Primitive` should throw for primitive types, except for legacy numbers, booleans and strings (Don't Break The Web) for which it is not a real issue.

—Claude





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