The function (a: number) => {} is incompatible with the type (a?: number) => void and cannot be assigned

type Func = (param?: number) => void

const func: Func = (param: number) => {}

I encountered the error:
Type '(param: number) => void' is not assignable to type 'Func'.
Types of parameters 'param' and 'param' are incompatible.
Type 'number | undefined' is not assignable to type 'number'.
Type 'undefined' is not assignable to type 'number'.(2322)

However, my understanding is that I'm assigning a variable to a Type, so I'm assigning 'number' to 'number | undefined', not the other way around.

Interestingly, the reverse assignment does not show any error:

type Func = (param: number) => void

const func: Func = (param?: number) => {}

But it appears that it should generate an error because it allows a function without any parameter to be assigned to a type that requires 'param' to be a number.

What am I missing? I would appreciate your help in clarifying this for me.

typescript

Answer №1

When defining types, it is important to understand that they can only limit the possible call signatures for a function. For example, if you have a function f and you assign it the type F, the valid call signatures of F must be a subset of the valid call signatures of f.

To illustrate this concept further: If your definition F allows for an optional number parameter, then any function foo classified as type 

F</code must allow for <code>foo()
as a valid call.

In contrast, if your function f requires a number to be passed in, then f() is not a valid call and cannot be considered of type F.

Looking at it from the opposite perspective: If F mandates that a number must be included, then f can be of type 

F</code, as it restricts the type of <code>f
: normally, both f() and f(n) would be valid calls, but by specifying f as type F</code, only <code>f(n) would be allowed.

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