assignabiltity between interfaces and types

[wclr]

TypeScript Version: 2.2.1 / nightly (2.3.0)

I believe my issue can be narrowed down:

export interface Params {
  [name: string]: string
}

function doSmth(params: Params) { }

interface MyParams {
  prop: string
}

let myParams: MyParams = { prop: 'X' }

let params: Params = myParams // NOT OK

Actual behavior:

Type 'MyParams' is not assignable to type 'Params'.
  Index signature is missing in type 'MyParams'.

if to replace interface with type there is no error:

type MyParams = {
  prop: string
}

[ahejlsberg]

This is working as intended. See #7029 for the rationale.

[wclr]

Anders Hejlsberg (@ahejlsberg)
Can you please explain, what is wrong with this particular case of usage? How is it supposed to be fixed correctly? Is type should be used for this case instead of interface? Why?

[kitsonk]

MyParams interface needs an index signature that is assignable to Params.

I believe the justification, reading #7029, is that a type is more like a type derived from an object literal, and therefore is assignable without an explicit index. This is to keep things more "usable" in TypeScript, but when using an interface, you are being more explicit about the typing and so the assignability does not work and index signature assignability is enforced.

[wclr]

MyParams interface needs an index signature that is assignable to Params.

I see this. I've modified the code to narrow the issue down. I need to understand more about differences between interface and type in terms of the issue and in general.

Is there a strong recommendations where it is more appropriate to use interface or type for defining object shape?

[kitsonk]

Back in my time we didn't have type, though the internals of the compiler obviously had access to the concept. I don't think there are any formal guidelines, so I will give my personal opinion.

type is a way to name a type that you are going to re-use multiple places and almost always contains some sort of type operator (e.g. a union or intersection operator). This then creates a shorthand (or reusable) type name that can be used in a type location.

interface is the way to specify an abstract shape of an Object, Function or Class which will be the formal contract that people will consume your code with. Classes (or constructor Functions) have two interfaces, the constructor and the instance interfaces.

[wclr]

Kitson Kelly (@kitsonk) Ok thanks for the explanation.

[ahejlsberg]

Generally, given a type T with an index signature, a type S is assignable to T only if S has a compatible index signature. This rule ensures in a transitive manner that S is as least as constrained as T. Consider:

interface Params {
    [x: string]: string;
}

interface MyParams {
    log: string;
}

const logWithNumber = {
    log: "foo",
    count: 42
};

let myParams: MyParams = logWithNumber;  // Ok, source permitted to have additional properties
let params: Params = myParams;  // Error

Without the rule that requires a matching index signatures, the last line in the example above wouldn't be an error and we would have violated the index signature constraint of the target. Effectively, because of subtype substitutability, an instance of an interface type without an index signature may have additional properties that we know nothing about.

However, when it comes to object literals, we do know that they don't have additional properties. Therefore, when the source is an object literal is it safe to permit assignment as long as all of the properties meet the constraint of the target. Now, the way we track that something is an object literal is by checking that its type originated in an object literal expression or an object literal type. The latter technically doesn't guarantee anything about additional properties (as you've discovered) but is included such that anything that applies to an inferred object literal type also applies to an explicitly specified object literal type. So it's a compromise we have to make for object literal types, but not for interface types.

[wclr]

So I probably may close this one. Thanks for explanation.