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  • JavaScript基础-var

    使用var 的变量会在js开头先申明,故此var变量申明顺序不影响,

    1 alert("aa" in window); // false
    2 alert("bb" in window); // true
    3 aa=5;
    4 var bb=2;

    变量使用var(显式申明)为局部变量,变量不使用var(隐式申明)为全局变量,

    1 function test(){
    2     a = "12";
    3     var b = "eaa";
    4 }
    5 test();
    6 alert(a); // 12
    7 alert(b); // b is not defined

    隐式申明 创建了一个全局对象的属性,可以删除,

    1 var a = 5;
    2 b=4;
    3 delete a;
    4 delete b;
    5 alert(a); // 5
    6 alert(b); // b is not defined

    var a=1; 申明的是本地变量a并赋值1;

    b=2; 先寻找是否有变量b申明,有则使用该变量,没有则申明一个全局变量

     1 var num1 = 1;
     2 num2 = 2;
     3 console.log("1--"+num1+","+num2); // 1,2
     4 +function(){
     5     num1 = 11;
     6     var num2 = 22;
     7     console.log("2--"+num1+","+num2); // 11,22
     8     (function(){
     9         console.log("3--"+num1+","+num2); // undefined,22 
    10       var num1 = 111;
    11       num2 = 222;
    12       num3 = 333;
    13       console.log("4--"+num1+","+num2+","+num3); // 111,222,333
    14     }())
    15     console.log("5--"+num1+","+num2+","+num3); // 11,222,333
    16 }();
    17 console.log("6--"+num1+","+num2+","+num3); // 11,2,333
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  • 原文地址:https://www.cnblogs.com/kkcodin/p/5413521.html
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