函数和变量声明

    console.log(a);

    var a = 1;
    var getNum = function(){
        a = 2;
    }
    function getNum(){
        a = 3;
    }
    console.log(a);
    getNum();
    console.log(a);

　　输出结果是 underfined, 1, 2

因为实际代码里面 a被提升在顶部声明了，而且函数声明先于变量申明;所以实际运行里面代码是这样的

     var a;  
    console.log(a);   //underfined

    var a = 1;

    function getNum(){
        a = 3;
    }
    var getNum = function(){
        a = 2;
    }
   
    console.log(a); //1
    getNum();
    console.log(a); //2