Strange behavior of mapped types when using with string

[lorefnon]

TypeScript Version: 3.7.0-dev.20190831

Search Terms: mapped types

Code

type M<T> = {
  [K in keyof T]: never;
}

type MStr = M<string>;

Expected behavior:

MStr is an object type where the corresponding value type for all properties of string type ("toString" | "charAt" | "charCodeAt" | "concat" | "indexOf" ...) will be never

Actual behavior:

type of MStr is string

Playground Link: Link

Related Issues:

• When creating a mapped type of an indexed type it extends too liberal #32484

[jack-williams]

This is working as intended (though I'm not sure where this behaviour is explicitly documented...other than the source code [1]).

Your alias M defines what is known as a homomorphic mapped type, indicated by the key constraint being keyof T for some T. When a homomorphic mapped type is instantiated with a primitive type it passes through unmodified.

To get your desired behaviour you can write: type MStr = Record<keyof string, never>. The type alias Record defines a normal mapped type, not a homomorphic one.

[1]

// For a homomorphic mapped type { [P in keyof T]: X }, where T is some type variable, the mapping
// operation depends on T as follows:
// * If T is a primitive type no mapping is performed and the result is simply T.

[fatcerberus]

From a practical perspective, I would probably find it a bit odd if the mapped type silently promoted string to an object type, since string is not assignable to object (but is assignable to {} so... hm...)

[jack-williams]

string is assignable to Object, however.

[AnyhowStep]

I'm on mobile and can't test but what about, { [k in Extract<keyof T, string|number|symbol>] : never }

I think that should work. If you only care about string keys, change it to extracting string.

I think that should make it no longer resolve to string

[lorefnon]

Your alias M defines what is known as a homomorphic mapped type, indicated by the key constraint being keyof T for some T. When a homomorphic mapped type is instantiated with a primitive type it passes through unmodified.

@jack-williams Thanks for clarifying on this.

I didn't know about the above, and assumed that because mapped types use object notation { [...] : ... } they can be expected to always be object types.

To get your desired behaviour you can write: type MStr = Record<keyof string, never>. The type alias Record defines a normal mapped type, not a homomorphic one.

I had simplified my usage to come up with a reproducer, so the above is not exactly applicable.

However the solution by @AnyhowStep worked quite well and solves my use case.

[fatcerberus]

Yeah, mapped types are essentially a form of type operator that just happens to use object notation. It’s kind of confusing, I would agree. TS gets a lot of mileage out of a small amount of syntax. 😄

[lorefnon]

I'll close this as the team believes that this behavior is intended. Thanks a lot for the help.