Typescript Generic Syntax < P extends U | undefined = undefined >

Question

I bumped to this definition when I was browsing Next-auth.js type definition.

export declare function signIn<P extends RedirectableProviderType | undefined = undefined>(provider?: LiteralUnion<BuiltInProviderType>, options?: SignInOptions, authorizationParams?: SignInAuthorizationParams): Promise<P extends RedirectableProviderType ? SignInResponse | undefined : undefined>;

What does undefined = undefined part of the definition mean?

you should read it like `P extends RedirectableProviderType | undefined` , the default value is `undefined` — coglialoro, Apr 21 '22 at 09:53

score 2 · Accepted Answer · answered Apr 21 '22 at 10:01

P extends RedirectableProviderType | undefined = undefined means that RedirectableProviderType or undefined types are allowed to be asserted when invoking signIn (e.g. signIn<RedirectableProviderType>), however when no assertion type is provided - P will be defaulted to undefined.

This can alternatively be viewed as P (extends RedirectableProviderType | undefined) = undefined

Typescript Generic Syntax < P extends U | undefined = undefined >

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