How to loop over enum without values on TypeScript?

Question

I have the

enum EthereumNetwork {
  MAIN_NET,
  ROPSTEN,
  KOVAN
}

class EthereumHDWallet {
  constructor(network: EthereumNetwork) { }
}

The task is to write a test which goes over all possible networks and trying to create EthereumHDWallet. Something like that:

  for (const network in EthereumNetwork) {
      const wallet = new EthereumHDWallet(network);
      it('constructor', () => {
        expect(wallet).toBeDefined();
      });
  }

Unfortunately, the code above doesn't work because of error TS2345: Argument of type 'string' is not assignable to parameter of type 'EthereumNetwork'. Does anyone know way around?

It's important to point out that enums in TypeScript boil down to both the numeric index and the string representation. That's what you're seeing in the examples you've provided. — jhpratt
– jhpratt, Commented Jul 3, 2019 at 22:27
turned out, the example I've provided doesn't actually work either. — lebed2045
– lebed2045, Commented Jul 3, 2019 at 22:39

Sean Vieira · Accepted Answer · 2019-07-04 15:05:20Z

1

Iterating over all the keys will give you too much (it will also give you the keys "0", "1", "2"). You can filter it down and get the type-safety you want, but you have to tell TypeScript that this is safe (with the as K[] in enumKeys below):

function enumKeys<O extends object, K extends keyof O = keyof O>(obj: O): K[] {
  return Object.keys(obj).filter(k => Number.isNaN(+k)) as K[];
}

for (const networkName of enumKeys(EthereumNetwork)) {
  const network = EthereumNetwork[networkName];
  const wallet = new EthereumHDWallet(network);
}

answered Jul 4, 2019 at 15:05

Sean Vieira

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2 Comments

lebed2045 Over a year ago

this is excellent! Is there any reason why TypeScript shouldn't treat operators "for in" and "for of" in a similar way out of the box for enums?

Sean Vieira Over a year ago

for ... of they could (because the generated enum could implement the iterator protocol), but they can't change for ... in behavior with enums without more runtime emit (which is counter to the design goals of the language)

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