11、类型别名和类型断言

使用type类型别名

function sum(x:number,y:number):number{
    return x+y
}
const sum1:(x:number,y:number)=>number = sum
type PlusType = (x:number,y:number)=>number
const sum2:PlusType = sum

类型断言

type NameResolver = ()=>string
type NameOrResolver = string | NameResolver

function getName(input:NameOrResolver):string{
    if(typeof input==='input'){
      return input
  }else{
      return input()
  }
}

使用as做类型转换

function getLength(input:string|number):number{
    let str = input as String
  if(str.length){
      return str
  }else{
      let num = input as Number
    return num.toString().length
  }
}

使用泛型来转换

function getLength(input:string|number):number{
    if((<string>input).length){
      return (<string>input).length
  }else{
         return input.toString().length
  }
}