Querying Enums containing string initializers with a variable throws TS7015

[CSchulz]

TypeScript Version: 2.4.2

Code

enum Colors {
  Red = 'RED',
  Green = 'GREEN',
  Blue = 'BLUE',
}
const test: string = 'Red';
Colors[test];

Workaround

enum Colors {
  Red = 'RED',
  Green = 'GREEN',
  Blue = 'BLUE',
}
const test: any = 'Red';
Colors[test];

Expected behavior:
Should compile

Actual behavior:

error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.

There is an issue #11773 demonstrating it with a "normal" enum.

[ajafff]

just remove the type annotation:

const test = 'Red';
// alternative
const test: keyof Colors = 'Red';

[RyanCavanaugh]

Seems we never did anything different with the indexing rules for string enums, which is bad. Colors[32] is extremely suspect.

[fmpierce]

I found the fix!

const test: keyof typeof Colors = 'Red';

I had the same problem and that fixed it.

[duluca]

I've a different repro case.

Playground link: https://www.typescriptlang.org/play/index.html#src=enum%20ContentType%20%7B%0D%0A%20%20Article%20%3D%20'a'%2C%0D%0A%20%20Video%20%3D%20'v'%0D%0A%7D%0D%0A%0D%0Aconst%20args%20%3D%20%7B%20type%3A%20%5B%20'Article'%2C%20'Video'%20%5D%20%7D%0D%0A%0D%0Awindow.alert(args.type.map(t%20%3D%3E%20ContentType%5Bt%5D))

Source:

enum ContentType {
  Article = 'a',
  Video = 'v'
}

const args = { type: [ 'Article', 'Video' ] }

window.alert(args.type.map(t => ContentType[t]))

Workaround:
As @fmpierce suggests, rewriting t as (t: keyof typeof ContentType) does fix the compilation issue. Obviously, the amount of code that needs to be written for the workaround is unacceptable.

Expected behavior:
The compiler should be able to figure out that the intent is keyof typeof ContentType.

Actual behavior:

error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.

[hotdang-ca]

Thanks, @fmpierce! Used your solution in the following:

Object.keys(Colors).map((color: keyof typeof Color): JSX.Element => {
    return (
        <div
            key={color}
            onClick={() => this._methodThatTakesAnEnum(Color[color])}
        >
            { Util.functionThatTakesAString(color) }
        </div>
    );
});

The anon function signature color: string causes the same error when trying to access Color[color].

[RyanCavanaugh]

I don't really see this as a bug at this point - enums don't have index signatures, it'd be a huge break to add one now, and it doesn't seem to merit a special case in the indexing rules.