TypeScript 函數-可選和默認參數

//可選參數
// function buildName(firstName:string,lastName:string) {
//  return firstName + " " + lastName;
// }
// let result1 = buildName('xiaochuan','xiaoming');//'xiaochuan xiaoming'
// let result2 = buildName('xiaochuan');//這里在編譯時直接就報錯了  Functions.ts(5,15): error TS2554: Expected 2 arguments, but got 1.
// let result3 = buildName('xiaochuan','xiaoming','xiaohong');//這里在編譯時直接就報錯了  Functions.ts(6,15): error TS2554: Expected 2 arguments, but got 3.

//上面的話傳遞參數的數量就必須得是 兩個

//下面是不確定幾個參數時的寫法
//在參數名稱的后面加一個 ? 號 使這個參數變為可選項
// function buildName(firstName:string,lastName?:string) {
//  //在這里做判斷返回想應的返回值
//  if(lastName){
//      return firstName + " " + lastName;
//  }else{
//      return firstName;
//  }
// }
// let result1 = buildName('xiaochuan','xiaoming');//'xiaochuan xiaoming'
// let result2 = buildName('xiaochuan');//這里編譯時就直接通過了
// let result3 = buildName('xiaochuan','xiaoming','xiaohong');//三個參數還是同樣的報錯，因為超出了參數的定義數量



//默認參數
function buildName(firstName:string,lastName="xiaochuan"){
    return firstName + " " + lastName;
}
let result1 = buildName('xiaochuan');//編譯通過  返回 'xiaochuan xiaochuan'
let result2 = buildName('xiaochuan','xiaoming');//編譯通過  返回 'xiaochuan xiaoming'
// let result3 = buildName('xiaochuan','xiaoming','xiaohong');//編譯失敗  Functions.ts(33,15): error TS2554: Expected 1-2 arguments, but got 3.
document.getElementById('pid').innerHTML = result1 + "<br/>" + result2;