Typescript: is short hand for union of undefined and string is optional property type?

Question

I find it is weird, so can someone confirm either these two are the same.

interface TypeA {
 time: string | undefined;
}

interfaceTypeB {
 time?: string;
}

Answer 1

They are similar, but not identical. Try the following for example:

interface TypeA {
 time: string | undefined;
}

interface TypeB {
 time?: string;
}

const exampleA: TypeA = {}

const exampleB: TypeB = {}

exampleA will give an error, because it needs the undefined time to be explicit. Merely omitting the property does not suffice. exampleB gives no error.

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