Type narrowing on Function.length for either callable type

[mjbvz]

TypeScript Version: 2.5.2

Code

interface IFoo {
  a: ((x: number) => string) | ((x: number, y: boolean) => string)
}

const z: IFoo = { 
  a(x: number) { return '' }
}

if (z.a.length === 1) {
  z.a(1)
}

Expected behavior:
Type of z.a is narrowed to (x: number) => string) from the if statement

Actual behavior:
Error:

'Cannot invoke an expression whose type lacks a call signature. Type '((x: number) => string) | ((x: number, y: boolean) => string)' has no compatible call signatures.'

I'm trying to express an interface that can implement a method in one of two ways. Let me know if there is a better way to express this

[DanielRosenwasser]

I don't really know whether narrowing by length is the right thing since the underlying implementation might have nothing to do with how the signatures are described to TypeScript. In other words, you might have a parameter length of 0 if the function uses arguments under the cover.

I think it's worth considering whether it's the common enough case though, or whether it's fair game for single-signature types.

[mjbvz]

Ok, that makes sense. Is there a better way to express the idea of an interface that implement a method one of two ways. I also tried:

interface X { x: number }

interface Y { y: number }

interface IFoo {
  a(x: X): string
  a(x: Y, y: boolean): string
}

const z: IFoo = { 
  a(x: X) { return '' }
} 

if (z.a.length <= 1) {
  z.a({ x: 1 })
}

But this fails to compile since types X and Y are not compatible

[mhegazy]

The type system really does not have a good way to model function.length, in almost all its uses. The only way that works today is to have the signature part of type that that has a discriminant property, e.g.:

type IFoo = {
    kind: "x",
    a(x: X): string
} | {
    kind: "y"
    a(x: Y, y: boolean): string
};

const z: IFoo = {
    kind: "x",
    a(x: X) { return '' }
};

if (z.kind === "x") {
  z.a({ x: 1 })
}

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.