Why is this code valid?

Question

typescript

Hello
Code:

let err = new Error();
let data = {};

let iTakeSomethingAndPassItAnErr
    = (x: (err: Error, data: any) => void) => { /* do something */ };

iTakeSomethingAndPassItAnErr(() => null) // Okay
iTakeSomethingAndPassItAnErr((err) => null) // Okay
iTakeSomethingAndPassItAnErr((err, data) => null) // Okay

Why are we declaring that the input function must have two parameters, we have that the first two calls are valid, while there is an incorrect number of call arguments?