Flow: Add support for ES6 symbol types

Created on 17 Sep 2015 · 5Comments · Source: facebook/flow

Code along this lines causes an error:

Class List {
  $$typeof: symbol;
  constructor() {
     this.$$typeof = Symbol.for('mylib/symbol')
  }
}

ES2015+ incompleteness

Source

Gozala

👍49

Most helpful comment

Has Symbol.iterator support been implemented?

eranimo on 20 Mar 2017

👍14

All 5 comments

I did not quite figured out how to add a primitive type support and at this point I don't have enough time to digg into to do it. So I have created this libdef for my use:

// See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol
declare class Symbol {
  static (value?:any): symbol;
  static for(key:string): symbol;
  static keyFor(symbol:symbol): string;
  toString(): string;
  valueOf(): symbol;
  // Well-known symbols
  static iterator: symbol;
  static match: symbol;
  static replace: symbol;
  static search: symbol;
  static split: symbol;
  static hasInstance: symbol;
  static isConcatSpreadable: symbol;
  static unscopables: symbol;
  static species: symbol;
  static toPrimitive: symbol;
  static toStringTag: symbol;
}

type symbol = Symbol;

I'm happy to send a pull to include it into core.js until proper support is built.

Gozala on 17 Sep 2015

Has Symbol.iterator support been implemented?

eranimo on 20 Mar 2017

👍14

This would be really good to have because it is a common use-case with React Redux to use symbols for action namespacing (and slightly better performance, too).

Our current codebase massively fouls with flow and we've got a lot of $FlowFixMes caused by lack of Symbol support and support for symbols as map keys. ie

export const SOME_FOO = Symbol('SOME_FOO');

// ...
import { SOME_FOO } from './some/action/type';

export const {
    SOME_FOO: doSomeFoo,
    // ^ flow error, symbol not supported / can't use as map key
    ...
}