Why the type returned by function ref is not equal to Ref

[nieyuyao]

As shown in the doc, the type of ref and Ref are as follows,

function ref<T>(value: T): Ref<UnwrapRef<T>>

interface Ref<T> {
  value: T
}

Ref<UnwrapRef<T>> is not equal to Ref<T>. Some issues also seem to be related to this, such as #10476 and #10862 .
On the other hand, if we don't pass a parameter to ref, the returned type is Ref<T | undefined>.

Why the returned type of ref can't be written as Ref<T> but needs to be wrapped by UnwrapRef.

[4refael]

I'd assume due to the value passed to ref might be a Ref itself.
So a simple example

const msg1 = ref('Hello World!')
const msg2 = ref(msg1)

[LinusBorg]

nested refs will be unwrapped, so the return type is different than the type you pass in. One example:

const initial = {
  x: ref('x'),
}
const myref = ref(initial)

// reading x from initial:
initial.x.value // x is a ref, so we need to use x.value to get the value of that ref

//reading x from the ref:
myref.value.x // no x.value needed, because the ref is unwrapped by Vue's reactivity

// so this is the shape of initial:
Interface Initial = {
  x: Ref<string>
}

// and this is the shape of myref's value:
interface MyRefsValue {
  x: string
}

// To get from one to the other, we use a type that does this nested unwrapping on the type level.

[nieyuyao]

thank you for your reply. but I have another question. why the return type is not written as Ref<UnWrapRef<T> | undefined> when not passing params.

function ref<T = any>(): Ref<UnWrapRef<T> | undefined>