条件类型

在最有用的程序的核心，我们必须根据输入做出决定。JavaScript 程序也不例外，但考虑到值很容易内省，这些决定也基于输入的类型。条件类型有助于描述输入和输出类型之间的关系。

interface Animal {
  live(): void;
}
interface Dog extends Animal {
  woof(): void;
}

type Example1 = Dog extends Animal ? number : string;

type Example2 = RegExp extends Animal ? number : string;

条件类型的形式看起来有点像 JavaScript 中的条件表达式 (condition ? trueExpression : falseExpression)：

type SomeType = any;
type OtherType = any;
type TrueType = any;
type FalseType = any;
type Stuff =
  SomeType extends OtherType ? TrueType : FalseType;

当 extends 左侧的类型可以分配给右侧的类型时，您将获得第一个分支（"true" 分支）中的类型；否则，您将在后一个分支（"false" 分支）中获得类型。

从上面的例子中，条件类型可能不会立即有用——我们可以告诉自己是 Dog extends Animal 还是选择 number 或 string！但是条件类型的强大之处在于将它们与泛型一起使用。

例如，让我们以下面的 createLabel 函数为例：

interface IdLabel {
  id: number /* some fields */;
}
interface NameLabel {
  name: string /* other fields */;
}

function createLabel(id: number): IdLabel;
function createLabel(name: string): NameLabel;
function createLabel(nameOrId: string | number): IdLabel | NameLabel;
function createLabel(nameOrId: string | number): IdLabel | NameLabel {
  throw "unimplemented";
}

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  1. 如果一个库必须在其 API 中一遍又一遍地做出相同的选择，这将变得很麻烦。
  2. 我们必须创建三个重载：一个用于确定类型的每种情况（一个用于 string，一个用于 number），一个用于最一般的情况（采用 string | number）。对于 createLabel 可以处理的每个新类型，重载的数量呈指数增长。

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interface IdLabel {
  id: number /* some fields */;
}
interface NameLabel {
  name: string /* other fields */;
}
type NameOrId<T extends number | string> = T extends number
  ? IdLabel
  : NameLabel;

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interface IdLabel {
  id: number /* some fields */;
}
interface NameLabel {
  name: string /* other fields */;
}
type NameOrId<T extends number | string> = T extends number
  ? IdLabel
  : NameLabel;
function createLabel<T extends number | string>(idOrName: T): NameOrId<T> {
  throw "unimplemented";
}

let a = createLabel("typescript");

let b = createLabel(2.8);

let c = createLabel(Math.random() ? "hello" : 42);

条件类型约束

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type MessageOf<T> = T["message"];

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type MessageOf<T extends { message: unknown }> = T["message"];

interface Email {
  message: string;
}

type EmailMessageContents = MessageOf<Email>;

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type MessageOf<T> = T extends { message: unknown } ? T["message"] : never;

interface Email {
  message: string;
}

interface Dog {
  bark(): void;
}

type EmailMessageContents = MessageOf<Email>;

type DogMessageContents = MessageOf<Dog>;

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

// Extracts out the element type.
type Str = Flatten<string[]>;

// Leaves the type alone.
type Num = Flatten<number>;

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在条件类型中推断

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type Flatten<Type> = Type extends Array<infer Item> ? Item : Type;

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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[]>;

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declare function stringOrNum(x: string): number;
declare function stringOrNum(x: number): string;
declare function stringOrNum(x: string | number): string | number;

type T1 = ReturnType<typeof stringOrNum>;

分布式条件类型

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type ToArray<Type> = Type extends any ? Type[] : never;

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type ToArray<Type> = Type extends any ? Type[] : never;

type StrArrOrNumArr = ToArray<string | number>;

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type StrArrOrNumArr =
  string | number;

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type ToArray<Type> = Type extends any ? Type[] : never;
type StrArrOrNumArr =
  ToArray<string> | ToArray<number>;

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type StrArrOrNumArr =
  string[] | number[];

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type ToArrayNonDist<Type> = [Type] extends [any] ? Type[] : never;

// 'StrArrOrNumArr' is no longer a union.
type StrArrOrNumArr = ToArrayNonDist<string | number>;