Support typeof x === "symbol"

[mroch]

not sure what flow's support for Symbol in general is, but typeof x === "symbol" is not currently supported so we should fix that and whatever else is necessary for that to work properly.

[samwgoldman]

Support for well-known symbols would be really handy. For example, if we understood Symbol.iterator (and plugged that into dynamic object keys), it would be possible to define custom iterators for object types (Generators let us define iterables today, but in a more limited way.)

[ghost]

It would be a welcome improvement for Flow to properly support typeof x === "symbol" and related stuff for ES2015 symbols in general. TypeScript seems to be well-ahead in this regard.

[chyzwar]

I think there are two things,

One is proper support for the primitive data type of symbol.

  6: export type EventName = string | symbol;
                                      ^^^^^^ symbol. Could not resolve name

Second is support for common symbols:
@@iterator

[bradennapier]

Any update here?

[devsnek]

so when will this actually happen?

[bradennapier]

:(

[kevinSuttle]

https://github.com/facebook/flow/blob/master/lib/core.js#L84 ?

[devsnek]

@kevinSuttle the Symbol primitive if supported, but specifically using typeof with "symbol" is not.

[villesau]

Just a side note: Importing pull request does not mean that it will be merged. AFAIK FB has their internal review tools that they use instead of github. If importing means something, it can be interpreted in a way that they are considering the proposed change. If you are more interested in that particular issue ( #7271 ), there is pretty interesting, even philosophical conversation about the topic: #7128 If you think that my opinion is the correct one, you can show support by thumbing it up :)

If you are willing to contribute but don't now how to climb the OCaml mountain, this might help you forward: #4181 (comment)
They have some (pretty sparse though) documentation about flow internals in their wiki as well: https://github.com/facebook/flow/wiki/The-Flow-Codebase:-Introduction

[TrySound]

@villesau Well, yes. But at least being imported it will be considered.

[goodmind]

#7405

[TrySound]

Merged. Will be in upcoming release.

[a7madgamal]

https://flow.org/en/docs/types/primitives/
I suppose someone needs to remove the mentioned ticket

[bugb]

agree with @a7madgamal, the doc seem has not been updated for a while!

[kevinbarabash]

I noticed that while:

function testSymbol(x: mixed) {
  if (typeof x === "symbol") { // ok
    (x: string); // error
  }
}

errors correctly, the following does not:

let x: mixed;
if (typeof x === "symbol") {
  (x: string);  // no error, should there be?
}

Is this difference in behavior intentional?