函數及預編譯

1.聲明方式

函數聲明存在提升，函數表達式不存在提升

函數聲明

alert(sum(10,10));//20
function sum(num1, num2){
    return num1 + num2;
};

函數表達式

alert(sum(10,10)); //sum is not a function
var sum = function(num1, num2){
    return num1 + num2;
};

2. 函數的參數

function sum(a,b,c){//a,b,c爲形參
    if(sum.length > arguments.length){//arguments爲實參列表，類型是僞數組
        console.log("形參多")
    }else{
        console.log("實參多")
    }
}
sum(1,2);//1,2爲實參


function sum(a,b,c){
    arguments[1] = 333;
    c = 999;
    console.log(arguments[1]);//333
    console.log(b);//333
    console.log(arguments[2]);//undefined
}
sum(1,2);

3.函數預編譯

1. 創建AO對象 Activation Object（執行期上下文，也就是作用域）
2. 找形參和變量聲明，將變量和形參名作爲AO屬性名，值爲undefined
3. 將實參值和形參統一
4. 在函數體裏面找到函數聲明，值賦予函數體

function fn(a){
    console.log(a);//ƒ a(){}
    var a = 123;
    console.log(a);//123
    function a(){}
    console.log(a);//123
    var b = function(){}
    console.log(b);//ƒ (){}
}

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(a);//3
    console.log(b);//2
}
test(1);

function test(a,b){
    console.log(a);//ƒ a(){}
    console.log(b);//undefined
    var b = 234;
    console.log(b);//234
    a = 123;
    console.log(a);//123
    function a(){}
    var a;
    b = 234;
    var b = function(){}
    console.log(a);//123
    console.log(b);//ƒ (){}
}
test(1);

4.全局預編譯

1. 創建GO對象 Global Object（執行期上下文，也就是作用域）
2. 找全局的變量聲明，將變量名作爲GO屬性名，值爲undefined
3. 在全局 裏面找到函數聲明，值賦予函數體

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

fn();
var global;