Allowing you to check membership of an enum

[aboyton]

Suggestion

I'd like to write a function to if a value is a member of an enum.

Use Cases

This is useful anytime you have external data you want to validate is a member of an enum, such as loading data from a network or a config file.

Examples

export enum Environment {
  DEV = 'dev',
  STAGING = 'staging',
  SANDBOX = 'sandbox',
  PRODUCTION = 'production',
}

function isValidEnvironment(env: string): env is Environment {
  // Argument of type 'string' is not assignable to parameter of type 'Environment'.ts(2345)
  return Object.values(Environment).includes(env);
}

This code used to work in older versions of TypeScript, but with the stricter definitions for Object.values the compiler now (correctly) says that the return value of Object.values is Environment[], and so I can't check if an enum includes a string without casting in some way:

return Object.values<string>(Environment).includes(env);
// or 
return Object.values(Environment).includes(env as any);

Alternatively I tried making a generic enum membership function so I can limit my casting to a single place, but that didn't work very well. My TypeScript foo appears to be lacking.

function inEnum<E = Object>(value: any, e: E): value is E {
  return Object.values(e).includes(value as any);
}

inEnum('foo', Environment) // `value is typeof Environment` which is wrong
inEnum<Environment>('foo', Environment) // Argument of type 'typeof Environment' is not assignable to parameter of type 'Environment'

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. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[RyanCavanaugh]

Duplicate #10717 / #1394 - absent a more concrete suggestion, I'm assuming you want Array#includes or similar functions to allow supertypes of the element type (which is entirely reasonable).

[aboyton]

Ah yes, I wasn't thinking about it from that perspective, making Array#includes to be able to take in more values. I was instead thinking of it from the point of view of being able to say that a enum was of a particular type in a generic. Happy to either leave this open or be closed as a duplicate of those tickets, whichever you think is more helpful.

[RyanCavanaugh]

Yep, any alternate to those would be out of scope anyway. Thanks!