What do you think about a C# 6 like nameof() expression for

Isiah Meadows isiahmeadows at gmail.com
Sun Jun 16 07:04:18 UTC 2019


Here's my opinion:

- If `y` is directly visible in scope and is neither a parameter or
destructured binding, `nameof y` should just evaluate to `"y"`. This
should be agnostic to whether the binding has been declared yet, so in
your example, `x` should be set to `"y"`.
- If `y` is entirely undeclared, it should be a runtime
`ReferenceError` in the same way it is when accessing undefined
globals. So in your second example, I'd expect it to throw before even
attempting assignment

-----

Isiah Meadows
contact at isiahmeadows.com
www.isiahmeadows.com

On Sun, Jun 16, 2019 at 12:00 AM guest271314 <guest271314 at gmail.com> wrote:
>
> ```
> const x = nameof y
> const y = 1;
> ```
>
> At line 1 adjacent to ```nameof``` how does the user even know that there is a variable that will be declared named ```y```?
>
> What is the output of ```x``` where there is no variable named ```y``` later declared?
>
> ```
> const x = nameof y
> const z = 1;
> ```
>
> On Sun, Jun 16, 2019 at 12:03 AM Ron Buckton <Ron.Buckton at microsoft.com> wrote:
>>
>> > What should occur where the code is
>>
>> It would be "y" in all 3 places.
>>
>> > ... is a proposal for _more_ than only getting the _name_ of an _declared_ and _initialized_ variable?
>>
>> It is a proposal for getting the name of a _declared_ variable. Whether it is _initialized_ does not matter.
>>
>> > Should a ```RefefenceError``` _not_ be thrown simple because ```nameof``` is used?
>>
>> No, an error is not thrown. ECMAScript is much more nuanced. Block scoped variables from 'let' or 'const' exist and can be *referenced* (via closure or export, currently) anywhere within the same block scope, even before they are initialized. Until they have been *initialized* (the line of code contain the declaration has been reached and evaluated), they exist in a "Temporal Dead Zone" (TDZ). Attempting to *dereference* them (i.e. access or store a value) while in this TDZ is what results in the ReferenceError.
>>
>> At no point does the `nameof` operator *dereference* the variable, so no error need be thrown.
>>
>> From: guest271314
>> Sent: Saturday, June 15, 4:29 PM
>> Subject: Re: Re: What do you think about a C# 6 like nameof() expression for
>> To: Ron Buckton
>> Cc: es-discuss at mozilla.org
>>
>>
>>
>>
>> What should occur where the code is
>>
>> ```
>> const x = nameof y
>> await new Promise(resolve => setTimeout(resolve, 100000)); // should x be "y" here?
>> await new Promise(resolve => setTimeout(resolve, 200000)); // should x be "y" here?
>> await Promise.all([new Promise(resolve => setTimeout(resolve, 300000)), ...doStuff()]); // should x be "y" here?
>> const y = 1;
>> ```
>>
>> ?
>>
>> The immediately invoked arrow function example (where a ```RefeferenceError``` is thrown) appears to demonstrate that to output the expected result of ```nameof``` within the context of the code example
>>
>> ```
>> const x = nameof y
>> const y = 1;
>> ```
>>
>> is a proposal for _more_ than only getting the _name_ of an _declared_ and _initialized_ variable?
>>
>> Should a ```RefefenceError``` _not_ be thrown simple because ```nameof``` is used?
>>
>> On Sat, Jun 15, 2019 at 11:16 PM Ron Buckton <Ron.Buckton at microsoft.com> wrote:
>>
>> ```
>> const x = nameof y
>> const y = 1;
>> ```
>>
>> `x` would have the value “y”. It would not matter if `y` were initialized or had yet been reached during execution. It does not deviate from the purpose of `let` or `const`, because you are not accessing the value of the identifier.
>>
>> Also consider that this is legal ECMAScript in a module:
>>
>> ```
>> export { y }
>> const y = 1;
>> ```
>>
>> The binding for `y` exists within the same block scope, it just has not yet been initialized. Exporting it via `export { y }`, closing over it via `() => y`, or accessing it via `nameof y` would all be the same. In all three cases you are accessing the *binding* of `y`, not the *value* of `y`. Even in the `() => y` case, you don’t access the *value* of `y` until you execute the function.
>>
>> From: guest271314 <guest271314 at gmail.com>
>> Sent: Saturday, June 15, 2019 3:57 PM
>> To: Ron Buckton <Ron.Buckton at microsoft.com>
>> Cc: es-discuss at mozilla.org
>> Subject: Re: Re: What do you think about a C# 6 like nameof() expression for
>>
>> > Sorry, I meant to say “not entirely correct”.
>>
>> You have not yet confirmed if in fact the expected output is referencing a variable declared using ```const``` on the current line _before_ initialization _on the next line_.
>>
>> That example appears to deviate from the purpose and usage of ```const```, beyond the scope of ```nameof```, and if were implemented, a ```ReferenceError``` should _not_ be thrown when a ```const``` variable that has yet to be initialized _on the next line_ is referred to _on the current line_?
>>
>> Aside from that example, the code which essentially already implements ```nameof``` should be able to be found in the code which implements ```ReferenceError``` relevant to ```const```.
>>
>> On Sat, Jun 15, 2019 at 10:47 PM Ron Buckton <Ron.Buckton at microsoft.com> wrote:
>>
>> Sorry, I meant to say “not entirely correct”.
>>
>> From: Ron Buckton
>> Sent: Saturday, June 15, 2019 3:03 PM
>> To: guest271314 <guest271314 at gmail.com>
>> Cc: es-discuss at mozilla.org
>> Subject: RE: Re: What do you think about a C# 6 like nameof() expression for
>>
>> > At that point in the example code the identifer ```y``` does not exist.
>>
>> That is not entirely incorrect. The identifier `y` exists, but its binding has not been initialized, otherwise you couldn’t refer to y in this case:
>>
>>
>>
>>
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