How can I use a property key (object.key) in a TypeScript Enum?

Question

I'd like to convert some JS arrays into TypeScript enums.

I know I can do:

export enum FRUIT {
  PEACH,
  BANANA
}

But in this case 'FRUITS' is exported from a separate file, and has the format:

const FRUIT = {
  PEACH: "delicious peach",
  BANANA: "wow a banana that's awesome",
  PEAR: "gross pear do not allow pear using the Enum"
};

(I want to ensure only a subset of fruits, eg "delicious peach" can be set for ACCEPTABLE_FRUIT )

I need to do:

export enum ACCEPTABLE_FRUIT {
  FRUITS.PEACH,
  FRUITS.BANANA
}

However this is a syntax error:

Only numeric enums can have computed members, but this expression has type 'string'. If you do not need exhaustiveness checks, consider using an object literal instead.

How can I use a property key (object.key) in a TypeScript Enum? Alternatively, if I can't, how can I get around the syntax error?

What _is_ `FRUITS`? You seem to be trying to use the _values_ from it as _names_, which doesn't make much sense - do you want e.g. `export enum FRUIT_TYPES { PEACH = FRUIT.PEACH, ... }`? Or maybe something where you can only use e.g. `keyof typeof FRUITS` (like `"PEACH"`) as a value? How would you use this once you've created it? — jonrsharpe, May 07 '21 at 12:45
And is that set of strings `keyof typeof FRUIT`? Please put this in the question, along with an example showing how you intend to _use_ it. — jonrsharpe, May 07 '21 at 12:52
It sounds like you want a `valueof` equivalent, see e.g. https://stackoverflow.com/a/49286056/3001761, an enum isn't the right direction. — jonrsharpe, May 07 '21 at 12:53
Try it in the playground - `type FRUIT_NAMES = keyof typeof FRUIT` would be `"PEACH" | "BANANA" | "PEAR"`. — jonrsharpe, May 07 '21 at 12:54
[Try it in the playground](https://www.typescriptlang.org/play?#code/MYewdgzgLgBAYgJQKoEkAqMC8MDeAoGGABQFEBBAYQAkAuGAIgBMBTAGwEth2QBXCGAA7MAhsAAW9ADR4AvgG48eKAE8h8ZOgD6AOTIBZEgGUsMANbNlIAGYwVQ6+tRoFQA) — jonrsharpe, May 07 '21 at 12:57
[Edit] the question. Please provide the actual context, this seems to be an [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem) - for example, if you're asking about a subset, having an example with only one member isn't helpful. — jonrsharpe, May 07 '21 at 12:58
Then yes, you probably want a "valueof" with either `Pick` ("only peach or banana") or `Omit` ("not pear") - see https://www.typescriptlang.org/docs/handbook/utility-types.html. That's not going to get you to an enum, though. — jonrsharpe, May 07 '21 at 13:07

score 2 · Accepted Answer · answered May 07 '21 at 13:16

This doesn't get you an enum, but if your goal is just to be able to able to express "either FRUIT.PEACH or FRUIT.BANANA" you don't need one.

First, you need the values of FRUIT to be typed more specifically than string, which you can do with as const (see literal inference):

const FRUIT = {
  PEACH: "delicious peach",
  BANANA: "wow a banana that's awesome",
  PEAR: "gross pear do not allow pear using the Enum"
} as const;

Second, you want a "valueof" type as shown in jcalz's answer to Is there a `valueof` similar to `keyof` in TypeScript?:
```
type ValueOf<T> = T[keyof T];
```

Finally you can use the Pick or Omit utility types to select the props from typeof FRUIT you want to include:

type ACCEPTABLE_FRUIT = ValueOf<Pick<typeof FRUIT, "PEACH" | "BANANA">>;
// type ACCEPTABLE_FRUIT = "delicious peach" | "wow a banana that's awesome";

Playground

Just wanted to acknowledge you were right earlier when you said this was an y/x problem. I thought enums were the only way to restrict values in this situation but obviously not. — mikemaccana, May 08 '21 at 14:55

How can I use a property key (object.key) in a TypeScript Enum?

1 Answers1