//調用函數(shù)時傳入的實際參數(shù)
  function add(num1, num2) {
    return num1 + num2;
  }
  //一切正常的調用方式
  console.info(add(1, 2)); //3
  //不檢查參數(shù)的類型，字串和數(shù)字可以混用
  console.info(add("1", 2)); //12
  //多余參數(shù)被忽略
  console.info(add(1, 2, 3)); //3
  //少的參數(shù)被視為undefined
  //1+undefined=NaN
  console.info(add(1)); //NaN

  //檢測參數(shù)是否缺失
  function sayHello(name, message) {
    if (typeof message === 'undefined') {
      message = '你好!';
    }
    console.info(name + "，" + message);
  }
  sayHello("賈君鵬", "你媽喊你吃飯"); 
  sayHello("賈君鵬");    //賈君鵬,你好!

  //保存參數(shù)的arguments對象
  function sumNumbers() {
    var result = 0;
    for (var i = 0; i < arguments.length; i++) {
      result += arguments[i];
    }
    return result;
  }
 console.info(sumNumbers(1, 2)); //3
 console.info(sumNumbers(1, 2, 3)); //6

 //函數(shù)對象的length屬性
 function sayName(name){
   console.info(name);
 }
 function sum(num1, num2){
   return num1 + num2;
 }
 function sayHi(){
   console.info("hi");
 }
 console.info(sayName.length); //1
 console.info(sum.length); //2
 console.info(sayHi.length); //0 
 

 //作為參數(shù)的函數(shù)對象
 function callSomeFunction(func, argu) {
   return func(argu);
 }
 function getGreeting(name) {
   return "Hello, " + name;
 }
 var result = callSomeFunction(getGreeting, "jxl");
 console.info(result); //Hello,jxl

  var callTwice = function (otherFunc) {
    otherFunc();
    otherFunc();
  };
  callTwice(function () {
    console.info("this is a function");
  });

<script>
  var button = document.getElementById('btnClick');
  var result = document.getElementById('result');
  var clickCount = 0;
  button.addEventListener('click', function () {
    clickCount++;
    result.setAttribute('value', clickCount+': Hello,world!');
  })
</script>