Why is it possible to override a function that requires a different signature as input?

Question

typescript

The code:

let foo: ({ x: number }) => void

foo = () => null
foo = function () {}

In theory, the function signature is not respected (argument "x" is not required in the assigned function), but there is no error.
Why is this code valid?

Answer 1

firstly, your code is wrong, it should be visible because type checking is not just comparing function signatures - it is checking that one type can be cast to another. a function that does not have a parameter can be called in a place where a parameter is passed to it, it will simply ignore it. That is, the type of a function without parameters is converted to a type with parameters. In fact, if you call it, it will work fine, which the type system takes into account. try to do the opposite and immediately get an error: The same if the parameters do not match the types let foo: ( x: number ) => void