平时开发中，有些场景总有种蹩手蹩脚的感觉，看到 Utility Types 才知道自己 native 了，很多场景 ts 都帮我们想好了。
Partial
设置 Type 某个类型为可选，并返回部分 Type 类型。
源码：
type Partial<T> = { [P in keyof T]?: T[P]; };
例
interface Todo {
title: string;
description: string;
}
function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
return { ...todo, ...fieldsToUpdate };
}
const todo1 = {
title: "organize desk",
description: "clear clutter",
};
const todo2 = updateTodo(todo1, {
description: "throw out trash",
});
Required
Type 类型里面的某个类型为必要类型（必选项）。
源码
type Required<T> = {
[P in keyof T]-?: T[P];
};
-? 表示移除可选性?，这里移除指把可选项改为必选项，而不是移除可选项。
例
interface Props {
a?: number;
b?: string;
}
const obj: Props = { a: 5 };
// 报错，确少 b 类型
const obj2: Required<Props> = { a: 5 };
Readonly
只读属性，不可改写。
源码
type Readonly<T> = { readonly [P in keyof T]: T[P]; };
例
interface Todo {
title: string;
}
const todo: Readonly<Todo> = {
title: "Delete inactive users",
};
// 报错，不能被重新赋值
todo.title = "Hello";
Record<Keys,Type>
将 Kyes 的类型转化为 Type 的类型，类似于类型扩展。
源码
type Record<K extends keyof any, T> = {
[P in K]: T;
};
例
interface CatInfo {
age: number;
breed: string;
}
type CatName = "miffy" | "boris" | "mordred";
const cats: Record<CatName, CatInfo> = {
miffy: { age: 10, breed: "Persian" },
boris: { age: 5, breed: "Maine Coon" },
mordred: { age: 16, breed: "British Shorthair" },
};
// const cats: Record<CatName, CatInfo>
cats.boris;
Pick<Type, Keys>
将 Type 的部分类型 Keys 挑出来，返回这部分类型。（拣选属性）
源码
type Pick<T, K extends keyof T> = {
[P in K]: T[P];
};
例
interface Todo {
title: string;
description: string;
completed: boolean;
}
// type TodoPreview = { title: string, completed: boolean }
type TodoPreview = Pick<Todo, "title" | "completed">;
const todo: TodoPreview = {
title: "Clean room",
completed: false,
};
// const todo: TodoPreview
todo;
Omit<Type, Keys>
移除 Type 类型中的 Keys 类型，返回新的类型。和 Pick 含义相对。
源码
type Omit<T, K extends keyof any> = Pick<T, Exclude<keyof T, K>>;
例
interface Todo {
title: string;
description: string;
completed: boolean;
createdAt: number;
}
type TodoPreview = Omit<Todo, "description">;
const todo: TodoPreview = {
title: "Clean room",
completed: false,
createdAt: 1615544252770,
};
todo;
const todo: TodoPreview
type TodoInfo = Omit<Todo, "completed" | "createdAt">;
const todoInfo: TodoInfo = {
title: "Pick up kids",
description: "Kindergarten closes at 5pm",
};
// const todoInfo: TodoInfo
todoInfo;
Exclude<Type, ExcludedUnion>
从 Type 中排除可分配给 ExcludedUnion 的属性，剩余的属性构成新的类型
源码
type Exclude<T, U> = T extends U ? never : T;
例
// T0 = "b" | "c"
type T0 = Exclude<"a" | "b" | "c", "a">;
Extract<Type, Union>
从 Type 中抽出可分配给 Union 的属性构成新的类型(交集)。与Exclude相反。
源码
type Extract<T, U> = T extends U ? T : never;
例
// T0 = "a"
type T0 = Extract<"a" | "b" | "c", "a" | "f">;
NonNullable
Type 类型中移除 null 和 undefined
源码
type NonNullable<T> = T extends null | undefined ? never : T;
例
// T0 = string | number
type T0 = NonNullable<string | number | undefined>;
Parameters
Type 是函数类型，返回函数的参数。
源码
type Parameters<T extends (...args: any) => any> = T extends (...args: infer P) => any ? P : never;
例
declare function f1(arg: { a: number, b: string }): void
type T0 = Parameters<() => string>; // []
type T1 = Parameters<(s: string) => void>; // [string]
type T2 = Parameters<(<T>(arg: T) => T)>; // [unknown]
type T4 = Parameters<typeof f1>; // [{ a: number, b: string }]
type T5 = Parameters<any>; // unknown[]
type T6 = Parameters<never>; // never
ReturnType
Type 是函数类型，返回 Type 的返回值
源码
type ReturnType<T extends (...args: any) => any> = T extends (...args: any) => infer R ? R : any;
例
declare function f1(): { a: number; b: string };
// T0 = string
type T0 = ReturnType<() => string>;
// T1 = void
type T1 = ReturnType<(s: string) => void>;
// T2 = unknown
type T2 = ReturnType<<T>() => T>;
// T3 = number[]
type T3 = ReturnType<<T extends U, U extends number[]>() => T>;
// T4 = { a: number; b: string }
type T4 = ReturnType<typeof f1>;
InstanceType
返回构造函数类型 Type 的实例类型
源码
type InstanceType<T extends new (...args: any) => any> = T extends new (...args: any) => infer R ? R : any;
例
class C {
x = 0;
y = 0;
}
// type T0 = C
type T0 = InstanceType<typeof C>;
// type T1 = any
type T1 = InstanceType<any>;
// type T2 = never
type T2 = InstanceType<never>;
// Type 'string' does not satisfy the constraint 'abstract new (...args: any) => any'.
type T3 = InstanceType<string>;
ThisParameterType
返回函数的 this 参数类型，如果不是 this，返回 unknow 类型。
源码
type ThisParameterType<T> = T extends (this: infer U, ...args: any[]) => any ? U : unknown;
例
function toHex(this: Number) {
return this.toString(16);
}
// n:number
function numberToString(n: ThisParameterType<typeof toHex>) {
return toHex.apply(n);
}
function add(a: number, b: number) {
return a + b
}
// unknown
type addParamerType = ThisParameterType<typeof add>
OmitThisParameter
如果一个函数有指定的 this 类型，那么返回一个不带 this 类型的函数类型，否则还是返回原来的函数。
源码
type OmitThisParameter<T> = unknown extends ThisParameterType<T> ? T : T extends (...args: infer A) => infer R ? (...args: A) => R : T;
例
function toHex(this: Number) {
return this.toString(16);
}
// () => string
const fiveToHex: OmitThisParameter<typeof toHex> = toHex.bind(5);
ThisType
ThisType 不会返回一个转换之后的类型，提供基于上下文的 this 类型。注意，需要开启 --noImplicitThis 特性。
源码
interface ThisType<T> { }
例
type ObjectDescriptor<D, M> = {
data?: D;
methods?: M & ThisType<D & M>; // Type of 'this' in methods is D & M
}
function makeObject<D, M>(desc: ObjectDescriptor<D, M>): D & M {
let data: object = desc.data || {};
let methods: object = desc.methods || {};
return { ...data, ...methods } as D & M;
}
let obj = makeObject({
data: { x: 0, y: 0 },
methods: {
moveBy(dx: number, dy: number) {
this.x += dx; // Strongly typed this
this.y += dy; // Strongly typed this
}
}
});
obj.x = 10;
obj.y = 20;
obj.moveBy(5, 5);