function functionName(arg){
   //函数体
}

helloworld(); //在代码执行之前会先读取函数声明
function helloworld(){
  console.log("hello world");
}

var functionName=function(arg){
   //函数体
}

helloworld(); //错误，还未赋值，函数不存在

var helloworld=function(){
  console.log("hello world");
}

var helloworld; //声明
if(condition){ //条件
  helloworld=function(){ //赋值
    console.log("hello world"); 
  }
}
else{
  helloworld=function(){ //赋值
    console.log("你好，世界");
  }
}

   function foo(n) {
      if (n <= 0)
        return 0;
      else if (n > 0 && n <= 2)
        return 1;
      else
        return foo(n - 1) + foo(n - 2);
    }

   var foo1 = foo;
    foo = null;
    console.log(foo1(34));

 function foo(n) {
      if (n <= 0)
        return 0;
      else if (n > 0 && n <= 2)
        return 1;
      else
        return arguments.callee(n - 1) + arguments.callee(n - 2);
    }

 var foo = (function f(n) {
      if (n <= 0)
        return 0;
      else if (n > 0 && n <= 2)
        return 1;
      else
        return f(n - 1) + f(n - 2);
    });

 var n = 100; //定义一个全局变量
    function fn() {
      console.log(n); //函数内部直接读取全局变量
    }

    fn();

    function fn() {
      var n = 100;
    }

    console.log(n); //n is not defined

 function fn() {
       n = 100;
    }
    fn();
    console.log(n); //100

   function fn() {
      n = 100;

      function fn1() {
        console.log(n);
      }

      fn1();
    }
    fn(); //100

function fn() {
      n = 100;

      function fn1() {
        console.log(n);
      }

      return fn1;
    }
    
    var result=fn();
    result(); //100

function fn() {
      n = 100;

      nadd = function () {
        n += 1;
      }

      function fn1() {
        console.log(n);
      }

      return fn1;
    }

    var result = fn();
    result(); //100
    nadd();
    result(); //101

(function(){
   //块级作用域
})();

(function () {
      var now = new Date();
      if (now.getMonth() == 0 && now.getDate() == 1) {
        alert("新年快乐");
      }
    })();