Typescript ? vs undefined

Question

In TypeScript, is there any difference between the following?

private dog: TDog | undefined;

vs

private dog?: TDog;

Also, what is the ? called?

Does this answer your question? Typescript: What's the difference between an optional field and a union with undefined? — DBS
– DBS, Commented Nov 4, 2022 at 10:31

Skyone · Accepted Answer · 2022-11-04 10:40:30Z

1

? operator means that parameter is optional, it’s not necessary to pass it in component/function.

undefined means that parameter must be passed in but its value may be undefined.

A simple example:

type A = {
    a?: string
}
type B = {
    a: string | undefined
}

const a: A = { }
const b: B = { }

Paste it into VSCode and you can see:

I once read an article about this. Hope it helps you.

answered Nov 4, 2022 at 10:40

Skyone

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