Js预编译

a = 123
console.log(a); // 123

var a = b = 123
console.log(a, b);    // 123 123

function test() {
    var a = b = 123
}
test()
console.log(b)  // 123

<script>
// a = 123
// console.log(a); // 123
// var a = b = 123
// console.log(a, b);  // 123 123

function test() {
    var a = b = 123
}
test()
console.log(b)  // 123
// console.log(a);  //报错Uncaught ReferenceError: a is not defined
console.log(window.a);  // undefined

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

    var a = 123;

    console.log(a); //123

    function a() { }

    console.log(a); //123

    var b = function () { }

    console.log(b)  //function () { }

    function d() { };
}

/*  ***预编译阶段***
    1，创建AO对象
        AO{}
    2，找形参和变量声明，将变量和形参名作为AO属性名，值为undefined
        AO{
            a:undefined;
            b:undefined;
        }
    3，将实参值和形参统一
        AO{
            a:1;
            b:undefined;
        }
    4，在函数体里面找函数声明，值赋予函数体
        AO{
            a:function a() { };
            b:undefined;
            d:function d() { };
        }
*/
fn(1)
/*  ***执行阶段***
    AO{
        a:function a() { };     --->123
        b:undefined;            --->function () { }
        d:function d() { };
    }
*/

function test(a, b) {
    console.log(a); //1
    c = 0;
    var c;
    a = 3;
    b = 2;
    console.log(b); //2
    function b() { }
    function d() { }
    console.log(b); //2
}

/*  ***预编译阶段***
    AO{
        c:undefined;
        a:undefined;    --->1
        b:undefined;    --->function b() { }
        d:function d() { }

    }
*/

test(1)
/*  ***执行阶段***
    AO{
        c:undefined;                            ===》0
        a:undefined;    --->1                   ===》3
        b:undefined;    --->function b() { }    ===》2
        d:function d() { }
    }

*/

/* 
    GO{
        say:undefined--->function say(){}
    }
*/

function say(say) {
    console.log(say);   //function say() { var num = 10 }
    var say = 234
    console.log(say);   //234
    function say() { var num = 10 }
}

/* 
    AO{
        say:undefined;--->1--->function say() { }--->234

    }        
*/

say(1)
console.log(say);   //function say(say){...}
var say = 123
console.log(say);   //123

show = 100
function bye() {
    console.log(show);  //undefined
    show = 200
    console.log(show);  //200
    var show = 300
    console.log(show);  //300
}

bye()
var show

/* 
    GO{
        aa:undefined,--->10
        car:function(){}
        cc:234
    }
*/

function car() {
    console.log(bb);    //undefined
    if (aa) {
        var bb = 100
    }
    console.log(bb);    //undefined
    cc = 234;
    console.log(cc);    //234
}
var aa
/* 预编译不看条件语句，直接看里面的语句
    AO{
        bb:undefined,
    }
*/
car()
aa = 10;
console.log(cc);    //234

console.log(show);  //function show(){}
var show = 10
console.log(show);  //10
function show() {
    console.log(show);
    console.log('show');
}
show()  //Uncaught TypeError: show is not a function
console.log(show);  //10

console.log(show);  //function show(){}
var show = 10
var hidden = 100
console.log(show);  //10
function show() {
    console.log(show);
    hidden = 200
    console.log('show');
}
// show()  //Uncaught TypeError: show is not a function
console.log(show);  //10
console.log(hidden);  //100

console.log(hidden);    //undefined
console.log(show);    //undefined
var hidden = 100;
var show = 200;
/* 自执行函数不会提升 */
(function hidden() {
    // console.log(display);    //Uncaught ReferenceError: display is not defined
    console.log(show);    //200
    console.log(hidden);    //function hidden(){}
    hidden = 200    //这里的hidden是IIFE非匿名函数，不能给它重新赋值！严格模式下会报错
    show = 300
    display = 999
    console.log(hidden);    //function hidden(){}
    console.log(show);    //300
    console.log(display);    //999
})()
console.log(hidden);    //100
console.log(display);   //999

console.log(hidden);    //undefined
console.log(show);    //undefined
var hidden = 100;
var show = 200;
/* 自执行函数不会提升 */
(function hidden() {
    console.log(hidden);    //undefined
    console.log(show);    //200
    hidden = 200
    show = 300
    var hidden
    console.log(hidden);    //200
    console.log(show);    //300
})()
console.log(hidden);    //100