标签:RT TypeScript any extends ReturnOf type infer
Res1: https://www.typescript-training.com/course/making-typescript-stick/08-type-challenges/#returnoff
Res2: https://learntypescript.dev/09/l2-conditional-infer
There is an infer
keyword that can be used within a condition in a conditional type to put the inferred type into a variable. That inferred variable can then be used within the conditional branches.
type ArrayElementType<T> = T extends (infer E)[] ? E : T;
// type of item1 is number
type item1 = ArrayElementType<number[]>
// type of item2 is `{name: string}`
type item2 = ArrayElementType<{name: string}>
For item1, T is number[], so (infer E)[] would match number[], therefore E is number type
For item2, T is {name: string}, so (infer E)[] doesn't match (not array of any type), therefore E would be T which is {name: string}
Infer function return type example:
type ReturnOf<F> = F extends (...args: any[]) => infer RT ? RT: F
// F extends (...args: any[]) => any: F should be a function type
// F extends (...args: any[]) => infer RT: the function return type is assigned to a variable RT
// infer RT ? RT: F: is RT a truth type? if yes return RT, otherwise return F
// the following test case can be pass
type cases = [
// simple 1
Expect<Equal<boolean, ReturnOf<() => boolean>>>,
// simple 2
Expect<Equal<123, ReturnOf<() => 123>>>,
Expect<
Equal<ComplexObject, ReturnOf<() => ComplexObject>>
>,
Expect<
Equal<
Promise<boolean>,
ReturnOf<() => Promise<boolean>>
>
>,
Expect<Equal<() => "foo", ReturnOf<() => () => "foo">>>,
Expect<
Equal<"heads" | "tails", ReturnOf<typeof flipCoin>>
>,
Expect<
Equal<
"rock" | "paper" | "scissors",
ReturnOf<typeof rockPaperScissors>
>
>
]
Enforce the `F` to be a function type:
type ReturnOf<F> = F extends (...args: any[]) => infer RT ? RT: F
const person = { name: "Fred" };
// PersonType is {name: string}
type PersonType = ReturnOf<typeof person>
It would be nice to enforce ReturnOf only accept function as type:
type ReturnOf<F extends (...args: any[]) => any> = F extends (...args: any[]) => infer RT ? RT: F
Another way to write ReturnOf<F> is:
type ReturnOf<F extends (...args: any[]) => any> = F extends {(...args: any[]): infer RT} ? RT: never
// F extends {(): any}: F should be any callable
// F extends {(...args: any[]): infer RT}: the return type of the callable should be RT
// F extends {(...args: any[]): infer RT} ? RT: never : if the return type is true, then return RT otherwise return never
标签:RT,TypeScript,any,extends,ReturnOf,type,infer 来源: https://www.cnblogs.com/Answer1215/p/16494555.html
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