for(...in...) with mapped types cannot see index type (noImplicitAny)

[jameswilddev]

TypeScript Version: 2.2.1
Code

// noImplicitAny must be enabled.

type Options = "A" | "B" | "C"
const MappedOptions: {[option in Options]: number } = {
    A: 2,
    B: 3,
    C: 4
}

for (const key in MappedOptions) {
    alert(MappedOptions[key])
}

Expected behavior:
This compiles, as the index type of MappedOptions is Options.

Actual behavior:
A compile-time error occurs as MappedOptions has no index signature.

[ikatyang]

The type of key is considered a string, you have to convert it to Options manually, since there is no index signature, e.g. { [key: string]: number }.

solution:

for (const key in MappedOptions) {
  alert(MappedOptions[<Options>key]);
}

or

Object.keys(MappedOptions).forEach((key: Options) => {
  alert(MappedOptions[key]);
});

I recommend the second one since it is clearer.

[jameswilddev]

Thanks. I'm already doing that (MappedOptions[key as Options]).

[mhegazy]

Duplicate of #20503 or at least the same rational why it works this way. Please see #12253 (comment) for more context.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.