Assign a known symbol to a constant should retain type information

[KSXGitHub]

Search Terms

Suggestion

Assigning a known symbol constant to another constant should retain type information.

const foo = Symbol() // known symbol
const bar = foo // expecting 'typeof foo' but received 'symbol'

interface UseFoo {
  [foo]: 'value'
}

// This should work
const useBar: UseFoo = {
  [bar]: 'value'
}

Use Cases

Creating aliases of symbol constants without losing type information.

Examples

Symbol.iterator alias

const { iterator } = Symbol

const iterable: Iterable<number> = {
  * [iterator] () {
    yield * [0, 1, 2, 3]
  }
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

[DanielRosenwasser]

@rbuckton any thoughts here?

[rbuckton]

The first example (const x = Symbol(); const y = x;) we initially disallowed because we were not able to write the declaration to an output declaration file (const x: unique symbol; const y: typeof x;). I will have to investigate to see if that has changed.

The second example (const { iterator } = Symbol;) won't be feasible until we are able to redeclare iterator (and the other symbols) as unique symbol.

[KSXGitHub]

The second example (const { iterator } = Symbol;) won't be feasible until we are able to redeclare iterator (and the other symbols) as unique symbol.

I have realized this, so I made #27525