'type' aliases of string can not be used as object index signature.

[jlennox]

type Guid = string;
var foo: { [deviceId: Guid]: DateTime } = {};

Generates the error:

An index signature parameter type must be 'string' or 'number'.

It would be nice to be able to type indexes using string-aliases.

[aelr]

+1

[MicahZoltu]

This includes the alias String.

class Map<T> {
    [key: String]: T
}

[MicahZoltu]

It would seem you also can't pass aliased types to an indexer either:

class Map<T> {
    [key: string]: T
}

let map: Map<number> = ...
let key: String = ...
let value = map[key]; // error

An index expression argument must be of type 'string', 'number', 'symbol, or 'any'

And yes, there is a missing ' in the error message.

[RyanCavanaugh]

String is not an alias for string. It refers to the non-primitive type String in JavaScript.

[MicahZoltu]

Ah I didn't know that, thanks! I suppose this means I should be using string, number and boolean (lowercase) everywhere in my code rather than String, Number and Boolean. Most unfortunate.

[zpdDG4gta8XKpMCd]

what is the point in aliasing string?

[RyanCavanaugh]

aleksey's question is probably the most relevant.

This falls under the category of not appearing to do something you don't. Specifically, all type aliases of a type are equivalent, so you could still index foo by a CustomerName.

There's already a place to write the intended index semantics (deviceId should probably be deviceGuid ?). Adding a second place to write that same information doesn't help because it makes it look like the compiler will enforce the specific indexing type, when in reality it won't.

One nascent proposal at the moment is "Fresh types" which would allow an enforcement of the indexing type. If that ends up happening, we'd definitely revisit this.