渡一 9-1预编译&作用域&闭包

预编译 步骤

1.创建A0
2.找形参和var写入属性，值undefined
3.给形参赋值
4.找function定义，赋值的不算，找到用名字创建属性并赋值

function fn(a){
    console.log(a); 

    var a = 123;

    console.log(a);

    function a(){};

    console.log(a);

    var b = function(){var a=1} //这个是赋值不是定义

    console.log(b);

    function b(){}
}
/*
    1.AO{}
    2.找形参和var
    A0{
        a:undefined,
        b:undefined
    }
    3.给形参传值
    A0{
        a:1,
        b:undefined
    }
    4.找定义的function
    A0{
        a:function a(){},
        b:function b(){}
    }
*/
fn(1)
/*
    执行
    输出：function a(){}        123        123        function(){var a=1}
*/

a=100
function demo(e){
    function e(){}
    arguments[0] = 2;
    console.log(e);//2
    if(a){
        var b=123;
        function c(){}
    }
    var c;
    a=10;
    var a;
    console.log(b);//undefined
    f=123;
    console.log(c);//fn
    console.log(a);//10
}
var a;
demo(1);

GO{
    a:100,
    demo:function(){...}
    f:123
}
A0{
    e:2,
    b:undefined,
    c:fn,
    a:10
}
console.log(a);//100
console.log(f);//123

作用域 步骤

function a(){
    function b(){
        var b=234;
    }
    var a = 123;
    b();
}

var glob =100;
a();
/*
    产生一个[[scope]]集合
    [[scope]]里包括A0 GO
    同时如果A0里还包括FN，那会包含FN b的AO,a的AO,GO
    这里b里包含的a的A0是一个地址引用，和上一层的是同一个变量
    
*/

function a(){
    function b(){
        function c(){

        }
        c();
    }
    b();
}
a();

/*

a defined a.[[scope]] --> 0:GO
a doing   a.[[scope]] --> 0:aAO
                          1:GO

b defined b.[[scope]] --> 0:aAO
                          1:G0

b doing   b.[[scope]] --> 0:bAO
                          1:aAO
                          2:GO

c defined c[[scope]]  --> 0:bAO
                          1:aAO
                          2:GO

c doing   c[[scope]] -->  0:cAO
                          1:bAO
                          2:aAO
                          3:GO

*/

闭包——里面的fn被return到外面

闭包作用

1.实现公有变量，eg函数累加器
2.可以做缓存（存储结构）
3.实现封装，属性私有化
4.模块化开发，防止污染全局变量

function a(){
    function b(){
        var bbb = 234;
        console.log(aaa)
    }

    var aaa = 123;
    return b;
}

var glob = 100;
var demo = a();
demo();

示例

var a(){
    var num = 100;
    function b(){
        num++;
        console.log(num)
    }
    return b;
}
var demo = a();
demo();//101
demo();//102

function test(){
    var num = 100;
    function a(){
        num++;
        console.log(num);
    }
    function b(){
        num--;
        console.log(num);
    }
    return [a,b]
}
var myArr = test();
myArr[0]();//101
myArr[1]();//100

function eater(){
    var food = "";
    var obj ={
        eat:function(){
            console.log("i am eating"+food);
        },
        push:function(myFood){
            food=myFood;
        }
    }
    return obj;
}

var eater1 = eater();
eater1.push('banana')
eater1.eat();