泛型

TIP

定义的时候，不关心具体的类型

在使用的时候，TypeScript的type system能够更加有效的检查，帮助开发者知道类型

const names: Array<string> = [];

const promise: Promise<string> = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("success:)");
  }, 3000);
});

promise.then((data) => {
  data.split("");
});

Function泛型

TIP

tsc会自动推断，可以让我们不用写多余的代码，如方法的返回值，调用方法时传入具体的类型

function merge<T, U>(objA: T, objB: U) {
  // return 的类型会推断为 T & U
  return Object.assign(objA, objB);
}
// merge<{name: string;hobbies: string[];}, {score: number;}>(参数。..)
const mergedObj = merge(
  { name: "Aeroxian", hobbies: ["Codeing", "Reading"] },
  { score: 100 }
);

console.log(mergedObj);

Constraints约束

TIP

improve function

function merge<T, U extends object>(objA: T, objB: U) {
  return Object.assign(objA, objB);
}
// error will not work
//const mergedObj = merge({ name: "Aeroxian", hobbies: ["Codeing", "Reading"] }, "");

extends interface⭐

TIP

约束接口，方法的内容操作聚焦接口规范

interface Lengthy {
  length: number;
}

function countAndDescribe<T extends Lengthy>(element: T) {
  let descriptionText = "Got no value.";
  if (element.length === 1) {
    descriptionText = "Got 1 element.";
  } else if (element.length > 1) {
    descriptionText = `Got ${element.length} elements.`;
  }
  return [element, descriptionText];
}

console.log(countAndDescribe(["Coding","Reading"]));

keyof⭐

TIP

在泛型中加强泛型之间的约束

function extractAndConvert<T extends object, U extends keyof T>(
  obj: T,
  key: U
) {
  return "Value: " + obj[key];
}
// error
// extractAndConvert({ name: "Aeroxian" }, "age");
console.log(extractAndConvert({ name: "Aeroxian" }, "name"));

Generic class

class DataStorage<T extends string | number | boolean> {
  private data:T[] = [];

  addItem(item: T) {
    this.data.push(item);
  }

  removeItem(item:T){
    const index = this.data.indexOf(item);
    if(index === -1){
        return ;
    }
    this.data.splice(index,1);
  }

  getItems(){
      return [...this.data]
  }
}

const textStorage = new DataStorage<string>();

Generic utils type

TIP

TypeScript中集成了许多内置的泛型类用于开发使用

ReadOnly

const cities: Readonly<string[]> = ["BeiJing", "GuiLin"];
// error
//cities.pop();

partial

function createCourseGoal(
  name: string,
  description: string,
  completeUtil: Date
) {
  // Partial的作用Make all properties in T optional
  //let courseGoal: CourseGoal = {};
  let courseGoal: Partial<CourseGoal> = {};
  courseGoal.name = name;
  courseGoal.description = description;
  courseGoal.completeUtil = completeUtil;
  return courseGoal as CourseGoal;
}