Typescript 从入门到放弃系列(十二)-条件类型

一.条件类型基本使用

可以使用extends关键字和三元表达式，实现条件判断

interface Fish {
  name1: string;
}
interface Water {
  name2: string;
}
interface Bird {
  name3: string;
}
interface Sky {
  name4: string;
}
type Condition<T> = T extends Fish ? Water : Sky;
let con1: Condition<Fish> = { name2: "水" };

二.条件类型分发

let con2: Condition<Fish | Bird> = { name2: "水" };

这里会用每一项依次进行分发,最终采用联合类型作为结果,等价于:

type c1 = Condition<Fish>;
type c2 = Condition<Bird>;
type c = c1 | c2;

三.内置条件类型

• 1.Exclude排除类型

type Exclude<T, U> = T extends U ? never : T;
type MyExclude = Exclude<"1" | "2" | "3", "1" | "2">;
// type MyExclude = "3"

• 2.Extract抽取类型

type Extract<T, U> = T extends U ? T : never;
type MyExtract = Extract<"1" | "2" | "3", "1" | "2">;
// type MyExtract = "1" | "2"

• 3.NoNullable 非空检测

type NonNullable<T> = T extends null | undefined ? never : T;
type MyNone = NonNullable<"a" | null | undefined>;
// type MyNone = "a"

四.infer 类型推断

• 1.ReturnType获取函数的返回值类型

export {};
function getUser(a: number, b: number) {
  return { name: "james", age: 18 };
}
type ReturnType<T> = T extends (...args: any) => infer R ? R : never;
type MyReturn = ReturnType<typeof getUser>;
// type MyReturn = {
//   name: string;
//   age: number;
// }

• 2.Parameters 获取函数的参数类型

export {};
function getUser(a: number, b: number) {
  return { name: "james", age: 18 };
}
type Parameters<T> = T extends (...args: infer R) => any ? R : any;
type MyParams = Parameters<typeof getUser>;
// type MyParams = [a: number, b: number]

• 3.ConstructorParameters构造函数参数类型

export {};
class Person {
  constructor(name: string, age: number) {}
}
type ConstructorParameters<T> = T extends { new (...args: infer R): any }
  ? R
  : never;
type MyConstructor = ConstructorParameters<typeof Person>;
// type MyConstructor = [name: string, age: number]

• 4.InstanceType 实例类型

export {};
class Person {
  constructor(name: string, age: number) {}
}

type InstanceType<T> = T extends { new (...args: any): infer R } ? R : any;
type MyInstance = InstanceType<typeof Person>;
// type MyInstance = Person

五.infer 实践

将数组类型转化为联合类型

export {};
type ElementOf<T> = T extends Array<infer E> ? E : never;
type TupleToUnion = ElementOf<[string, number, boolean]>;
// type TupleToUnion = string | number | boolean

将两个函数的参数转化为交叉类型

type T1 = { name: string };
type T2 = { age: number };
type ToIntersection<T> = T extends [(x: infer U) => any, (x: infer U) => any]
  ? U
  : never;
type T3 = ToIntersection<[(x: T1) => any, (x: T2) => any]>;
// type T3 = T1 & T2

表示要把T1、T2赋予给 x，那么 x 的值就是T1、T2的交集。（参数是逆变的可以传父类）

TS 的类型：TS 主要是为了代码的安全性来考虑。所以所有的兼容性问题都要从安全性来考虑!