How can I know at compile-time whether a type is an enum?

Issue

I want to write a function that validates that a value is part of an enum, and throw if not. From this answer, I wrote:

    private ensureValueInEnum(enumType: any, value: string): void {
        if (!Object.values(enumType).includes(value)) {
            throw new Exception(`${value} is invalid value. Valid values are: ${Object.values(enumType).join(' / ')}`);
        }
    }

But, I don't like that enumType is any. Is there something I can write that says "enumType is an enum"?

Solution

Not really, no.

Once compiled, there's nothing magic about enums. They are just objects with string keys and constant values. That type would be Record<string, unknown>.

So I think this type is as tight as your going to get:

function ensureValueInEnum(enumType: Record<string, unknown>, value: unknown): void {
  //
}

If you only want to support enums with string values, then you could do:

function ensureValueInEnum(enumType: Record<string, string>, value: string): void {
  //
}

But there no way to distinguish an enum from, say, this:

const testObj = {
    C: 'c',
    D: 'd',
}

ensureValueInEnum(testObj, 'c') // works
ensureValueInEnum(testObj, 'q') // error

If you want testObj to be a type error, and only enums declared with the enum keyword to be allowed, then I don't think that's possible.

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Answered By - Alex Wayne