Typing (string[] | string[][]) not recognized properly

[rogierschouten]

Hi all,

I have two small examples that I expected to be equivalent but tsc accepts only one:

Working example:

let myArray: (string | string[])[];
myArray.forEach((element: string | string[]): void => {
}); // no compile error

Not-working example:

let myArray: string[] | string[][];
myArray.forEach((element: string | string[]): void => {
}); // compile error: Cannot invoke an expression whose type lacks a call signature

[ahejlsberg]

Methods are available on a union type only when each of the underlying methods have identical parameter lists (which isn't the case here). This is by design. From a typing perspective it technically isn't correct to do what you expected in all cases (i.e. union together the parameter types of each of the underlying methods). For example, say you had a method with two parameters of the array element type. The underlying signatures would be foo(x: string, y: string) and foo(x: string[], y: string[]), and we'd produce a combined signature foo(x: string | string[], y: string | string[]). That would permit you to call with x and y arguments of different types, which is incorrect.

[BairDev]

@ahejlsberg Maybe I'm at a loss, but why is foo(x: string, y: string)

a method with two parameters of the array element type

Of course, here you can't do x.forEach() in the function body.

I mean (and maybe OP, either), that here a is an array anyway.

let a: number[] | string[];
// later
a.forEach(); // compile error: Cannot invoke an expression whose type lacks a call signature

What is the technical difference between string[] | number[] and (string | number)[]?

[sledorze]

@BairDev

string[] | number[] means either an array of string OR an array of number.

(string | number)[] means an array of string OR number, in a non exclusive way.
so that: ["a", 5] is such an array that cannot be bound to string[] nor to number[].

[sledorze]

@ahejlsberg is there any notion of (co/contra)variance in typescript? I think something can come out from properly considering that aspect.

In your example, we may also consider the signature as a whole:
(x : string, y: string) => R | (x : string[], y : string[]) => R

[sledorze]

@mhegazy I think it could be reopened

[kitsonk]

Contra/covariance is covered in #1394 as well as somewhat related to variadic kinds proposed in #5453.