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

Ron Buckton Ron.Buckton at microsoft.com
Sun Jun 16 00:03:37 UTC 2019


> 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<mailto: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<mailto:guest271314 at gmail.com>>
Sent: Saturday, June 15, 2019 3:57 PM
To: Ron Buckton <Ron.Buckton at microsoft.com<mailto:Ron.Buckton at microsoft.com>>
Cc: es-discuss at mozilla.org<mailto: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<mailto: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<mailto:guest271314 at gmail.com>>
Cc: es-discuss at mozilla.org<mailto: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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