变量提升,函数提升

var v = "hello world";
(function(){
    alert(v);
})    

弹出undefined

var v = "hello word";
(function(){
    alert(v);
    var v = 'I love you';
})()

函数提升,只有函数声明方式才能被提升

function myTest(){
    foo();
    function foo(){
           alert('foo')
    }
}

myTest();

//报错
function myTest(){
    foo();
    var foo = function(){
        alert('aaa')
    }
}
myTest();