typescript之泛型

泛型是什么，为什么要使用泛型
使用泛型定义数组中的元素类型，promise返回的数据类型等

const names:Array<string>  =  ['1']

const promise: Promise<string> = new Promise((resolve, reject) => {
  setTimeout(()=>{
    resolve('it is ok')
  },1000)
})

promise.then(data=>{
  const list = data.split('o')
  console.log(list)
},()=>{})

创建泛型函数

// function merge<T, U>(objA: T, objB: U): T & U
function merge<T, U>(objA:T, objB: U){
  return Object.assign(objA, objB)
}

const data = merge({name:'max'}, {age: 30})
console.log(data.name)
// you can this
const data2 = merge<{name: string}, {age: number}>({name:'max'}, {age: 30})

extends 限制传的参数必须时某一类型的任意结构

function merge<T extends object, U extends object>(objA:T, objB: U){
  return Object.assign(objA, objB)
}

keyof 限制类型为一个对象的属性

function extractAndCovert<T extends object, U extends keyof T> (obj: T, key: U) {
  return obj[key]
}

console.log(extractAndCovert({name: 'max'}, 'name'))

创建class泛型

class DataStorage<T>{
  private data:T[] = [];

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

  getItem() {
    return [...this.data];
  }
}

const textStorage = new DataStorage<string>();

textStorage.addItem("max");
console.log(textStorage.getItem())

const numberStorage = new DataStorage<number>();

numberStorage.addItem(2);
console.log(numberStorage.getItem())`

Readonly

const names: Readonly<string[]> = ['max', 'anna']