Exploring the topic of Inferring Within Conditional Types, an illustrative example is provided:
type GetReturnType<Type> = Type extends (...args: never[]) => infer Return
? Return
: never;
type Num = GetReturnType<() => number>;
type Str = GetReturnType<(x: string) => string>;
type Bools = GetReturnType<(a: boolean, b: boolean) => boolean[]>;
But the choice of never[] in this context raises questions.
The purpose behind
type GetReturnType<Type> = Type extends (...args: never[]) => infer Return
? Return
: never;
is to extract the return type of any function without discriminating based on parameter types. Any function signature passed in as Type should be assignable to
(...args: never[]) => infer Return(...args: never[]) => infer ReturnConsidering that the parameters of Type are disregarded and no actual function call of type Type is made, there are various ways to define a function top type in TypeScript for this purpose. Here are different options listed in descending order of preference:
(...args: never) => infer Return(...args: any) => infer Return(...args: any[]) => infer Return(...args: never[]) => infer ReturnFunction types exhibit contravariance in their parameter types, meaning a function type should align with fewer parameter types to match a wider range of function types. Consequently, for a top type that encompasses all function types, a bottom type for the parameters (i.e., an intersection of all parameter types) is required.
The "never" type serves as TypeScript's bottom type, making
(...args: never) => infer ReturnAlternatively, you can utilize any type for this purpose, such as
(...args: any) => infer Returnany type makes it suitable for scenarios where type is inconsequential and errors need to be avoided.
Another option is to use
(...args: any[]) => infer Returnany[] can accommodate any array type.
Lastly, there's
(...args: never[]) => infer Returnnever[] signifies "an unindexable array," covering nearly all array types but not the precise intersection of all array types. Although it aligns with most functions, it should behave distinctively when called. While TypeScript treats (...args: never) => ? and (...args: never[]) => ? as identical, there are intricacies and bugs related to these types, as indicated in microsoft/TypeScript#48840.
However, given that the function is not invoked in this scenario, the distinction between never[] and never is mainly theoretical. For the purpose of GetReturnType, never[] is equally suitable as never.
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