TypeScript 泛型

泛型基础

泛型函数

function identity<T>(arg: T): T {
    return arg;
}

// 使用
let output = identity<string>("myString");
let output2 = identity("myString"); // 类型推断

泛型接口

interface GenericIdentityFn<T> {
    (arg: T): T;
}

let myIdentity: GenericIdentityFn<number> = identity;

泛型类

class GenericNumber<T> {
    zeroValue: T;
    add: (x: T, y: T) => T;
}

let myGenericNumber = new GenericNumber<number>();
myGenericNumber.zeroValue = 0;
myGenericNumber.add = function(x, y) { return x + y; };

泛型约束

extends关键字

interface Lengthwise {
    length: number;
}

function loggingIdentity<T extends Lengthwise>(arg: T): T {
    console.log(arg.length);  // 现在我们知道它有length属性了
    return arg;
}

在泛型约束中使用类型参数

function getProperty<T, K extends keyof T>(obj: T, key: K) {
    return obj[key];
}

let x = { a: 1, b: 2, c: 3, d: 4 };
getProperty(x, "a"); // okay
getProperty(x, "m"); // error: Argument of type 'm' isn't assignable to 'a' | 'b' | 'c' | 'd'.

实用泛型工具类型

Partial<T>

interface Todo {
    title: string;
    description: string;
}

function updateTodo(todo: Todo, fieldsToUpdate: Partial<Todo>) {
    return { ...todo, ...fieldsToUpdate };
}

Pick<T,K>

interface Todo {
    title: string;
    description: string;
    completed: boolean;
}

type TodoPreview = Pick<Todo, "title" | "completed">;

Omit<T,K>

interface Todo {
    title: string;
    description: string;
    completed: boolean;
}

type TodoPreview = Omit<Todo, "description">;