关于TypeScript中的infer的用法理解

提到TypeScript中的infer关键字，我们需要联想到【条件类型】(Conditional Types)

具体可以查看：https://www.typescriptlang.org/docs/handbook/2/conditional-types.html

SomeType extends OtherType ? TrueType : FalseType;

相关的条件类型示例

1.

interface Animal {
  live(): void;
}

interface Cat extends Animal {
  sleep(): void;
}

type CondType = Cat extends Animal ? number : string;
// CondType => number

2.

interface IdLabel {
  id: number;
}

interface NameLabel {
  name: string;
}

type NameOrId = T extends number ? IdLabel : NameLabel;

function createLabel (idOrName: T): NameOrId {
  throw "unimplemented";
}

3.

type MessageOf = T['message'];

interface Email {
  message: string;
}

type EmailMessageContents = MessageOf;
// EmailMessageContents => string

4.

type Flatten = T extends any[] ? T[number] : T;

type Str = Flatten;
// Str => string

type Num = Flatten;
// Num => number

使用类型约束来条件性地选择、提取目标类型是相对常规且比较容易的。

infer 类型推导

条件类型还提供了一种使用infer关键字在类型比较的true分支中进行目标类型推断的方法。

(可以将infer当作一种标记符，后续可以得到其标记的类型)

示例

type Flatten = Type extends Array ? Item : Type;

type Str = Flatten;
// Str => string

type Num = Flatten;
// Num => number

type GetReturnType = T extends (...args: never[]) => infer Return ? Return : never;

type Num = GetReturnType<() => number>;
// Num => number

type Bools = GetReturnType<(a: boolean, b: boolean) => boolean[]>;
// Bools => boolean[]

type ToArray = T extends any ? T[] : never;

type StrArrOrNumArr = ToArray;
// StrArrOrNumArr => string[] | number[];


type ToArrayNonDist = [T] extends [any] ? T[] : never;

type StrArrOrNumArr = ToArrayNonDist;
// StrArrOrNumArr => (string | number)[]

end.