Explicit string indexing access of map type

[mikecann]

TypeScript Version: 3.1.0-dev.201xxxxx

Search Terms:

string index, explicit index

Code

type EventDefinitions<TEventPayload extends object> = {
    [eventName: string]: TEventPayload;
  };

type X = keyof EventDefinitions<object>;

Expected behavior:

The type of X to be string as I have explicitly said it will be.

Actual behavior:

The type of X is string | number.

Playground Link:

http://www.typescriptlang.org/play/#src=type%20EventDefinitions%3CTEventPayload%20extends%20object%3E%20%3D%20%7B%0D%0A%20%20%20%20%5BeventName%3A%20string%5D%3A%20TEventPayload%3B%0D%0A%20%20%7D%3B%0D%0A%0D%0Atype%20X%20%3D%20keyof%20EventDefinitions%3Cobject%3E%3B

Related Issues:

Maybe this: #26470 ?

[RyanCavanaugh]

In TypeScript, when a type can be indexed by string, it can also be indexed by number, so keyof is consistent by providing the legal key types for the provided operand.

[mikecann]

@RyanCavanaugh oh okay :S Thanks for that, it was certainly unexpected behaviour for me. Is there a way to explicitly say its going to be a string?

[unional]

On the other hand:

type R = Record<string, number>

type Y = keyof R // string

It caught me by surprise too. 🌷

[mikecann]

@unional yep Record is the way to do it I think 👍

[naddeoa]

Worth pointing out that the example in this issue is almost the exact example used in the docs here

https://www.typescriptlang.org/docs/handbook/advanced-types.html#index-types-and-string-index-signatures

That example claims that the keys will be of type string but they're actually of type string | number it seems.

https://www.typescriptlang.org/play/#src=interface%20Foo%3CT%3E%20%7B%0D%0A%20%20%20%20%5Bkey%3A%20string%5D%3A%20T%3B%0D%0A%7D%0D%0Alet%20keys%3A%20keyof%20Foo%3Cnumber%3E%3B%20%2F%2F%20string

Assuming I didn't miss something, I think those docs could use an update.

[mikecann]

@naddeoa you are correct the docs are incorrect.. They say this:

interface Map<T> {
    [key: string]: T;
}
let keys: keyof Map<number>; // string
let value: Map<number>['foo']; // number

But its not true:

I would submit a PR for the docs but I dont think I understand the reason why this doesnt work enough to comment on it. IMO its counter-intuitive and we shouldnt need a Record to do this which is why I opened this issue in the first place but shrug