TypeScript 函数-可选和默认参数

//可选参数
// function buildName(firstName:string,lastName:string) {
//  return firstName + " " + lastName;
// }
// let result1 = buildName('xiaochuan','xiaoming');//'xiaochuan xiaoming'
// let result2 = buildName('xiaochuan');//这里在编译时直接就报错了  Functions.ts(5,15): error TS2554: Expected 2 arguments, but got 1.
// let result3 = buildName('xiaochuan','xiaoming','xiaohong');//这里在编译时直接就报错了  Functions.ts(6,15): error TS2554: Expected 2 arguments, but got 3.

//上面的话传递参数的数量就必须得是 两个

//下面是不确定几个参数时的写法
//在参数名称的后面加一个 ? 号 使这个参数变为可选项
// function buildName(firstName:string,lastName?:string) {
//  //在这里做判断返回想应的返回值
//  if(lastName){
//      return firstName + " " + lastName;
//  }else{
//      return firstName;
//  }
// }
// let result1 = buildName('xiaochuan','xiaoming');//'xiaochuan xiaoming'
// let result2 = buildName('xiaochuan');//这里编译时就直接通过了
// let result3 = buildName('xiaochuan','xiaoming','xiaohong');//三个参数还是同样的报错，因为超出了参数的定义数量



//默认参数
function buildName(firstName:string,lastName="xiaochuan"){
    return firstName + " " + lastName;
}
let result1 = buildName('xiaochuan');//编译通过  返回 'xiaochuan xiaochuan'
let result2 = buildName('xiaochuan','xiaoming');//编译通过  返回 'xiaochuan xiaoming'
// let result3 = buildName('xiaochuan','xiaoming','xiaohong');//编译失败  Functions.ts(33,15): error TS2554: Expected 1-2 arguments, but got 3.
document.getElementById('pid').innerHTML = result1 + "<br/>" + result2;