[types] body.json() return type

Currently body.json() returns any type. It’s aligned to JSON.parse I guess. However from types perspective it’s not very good.

For example without additional linters and configs we could do the following:

const foo: any = 10; // We can assign anything to any
const bar: boolean = foo; // Any is assignable to anything 
foo.exec(); // Ok; anything goes with any

But if we have unknown type there it will be much safer and force end-users manually cast it to type that they expect.

const foo: unknown = 10; // We can assign anything to unknown, but
const bar: boolean = foo; // Error, we can't assign unknown to any other type without an explicit assertion
foo.exec(); // Error, we can't access a property and call it on unknown

On the other hand it will be a breaking change from typescript point of view and maybe someone have other point of view regarding type safety in this case.

[SkeLLLa]

Yes, changing the type will cause issues in existing projects, so if it’s considered to be changed it should go in semver-major. But in general those issues will allow users to improve their projects.

For example, sometimes we use a customized ts compiler that treats all any as unknown to see weak points in code where any is used without casting to some type.

Here’s also related issue in TS: https://github.com/microsoft/TypeScript/issues/26188

[meyfa]

In my opinion it makes sense to mirror types from lib.dom.d.ts so that code written for both the browser and for Node doesn’t suffer from mismatched types. lib.dom.d.ts defines body.json() to be returning Promise<any>, as well: https://github.com/microsoft/TypeScript/blob/298b3a432ca099330108ed7bb18293ae79693672/lib/lib.dom.d.ts#L2448