泛型

引入

不使用泛型

function createArray(value: any, count: number): any[] {
  const arr: any[] = []
  for (let index = 0; index < count; index++) {
    arr.push(value)
  }
  return arr
}

const arr1 = createArray(11, 3)
const arr2 = createArray('aa', 3)
console.log(arr1[0].toFixed(), arr2[0].split(''))

使用函数泛型

function createArray2 <T> (value: T, count: number) {
  const arr: Array<T> = []
  for (let index = 0; index < count; index++) {
    arr.push(value)
  }
  return arr
}
const arr3 = createArray2<number>(11, 3)
console.log(arr3[0].toFixed())
// console.log(arr3[0].split('')) // error
const arr4 = createArray2<string>('aa', 3)
console.log(arr4[0].split(''))
// console.log(arr4[0].toFixed()) // error

多个泛型参数的函数

function swap <K, V> (a: K, b: V): [K, V] {
  return [a, b]
}
const result = swap<string, number>('abc', 123)
console.log(result[0].length, result[1].toFixed())

泛型接口

在定义接口时, 为接口中的属性或方法定义泛型类型
在使用接口时, 再指定具体的泛型类型

interface IbaseCRUD <T> {
  data: T[]
  add: (t: T) => void
  getById: (id: number) => T
}

class User {
  id?: number; //id主键自增
  name: string; //姓名
  age: number; //年龄

  constructor (name, age) {
    this.name = name
    this.age = age
  }
}

class UserCRUD implements IbaseCRUD <User> {
  data: User[] = []
  
  add(user: User): void {
    user = {...user, id: Date.now()}
    this.data.push(user)
    console.log('保存user', user.id)
  }

  getById(id: number): User {
    return this.data.find(item => item.id===id)
  }
}


const userCRUD = new UserCRUD()
userCRUD.add(new User('tom', 12))
userCRUD.add(new User('tom2', 13))
console.log(userCRUD.data)

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 
}

let myGenericString = new GenericNumber<string>()
myGenericString.zeroValue = 'abc'
myGenericString.add = function(x, y) { 
  return x + y
}

console.log(myGenericString.add(myGenericString.zeroValue, 'test'))
console.log(myGenericNumber.add(myGenericNumber.zeroValue, 12))

泛型约束

// 没有泛型约束
function fn <T>(x: T): void {
  // console.log(x.length)  // error
}
我们可以使用泛型约束来实现

interface Lengthwise {
  length: number;
}

// 指定泛型约束
function fn2 <T extends Lengthwise>(x: T): void {
  console.log(x.length)
}
我们需要传入符合约束类型的值，必须包含必须 length 属性：

fn2('abc')
// fn2(123) // error  number没有length属性