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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                ~~~ //=>在全局作用域下聲明一個變量,也相當于給WINDOW全局對象設置了一個屬性, 變量的值就是屬性值(私有作用域中聲明的私有變量和WINDOW沒啥關系) /* console.log(a);//=>undefined console.log(window.a);//=>undefined console.log('a' in window); //=>TRUE 在變量提升階段,在全局作用域中聲明了一個變量A, 此時就已經把A當做屬性賦值給WINDOW了,只不過此時還沒有給A賦值, 默認值UNDEFINED in:檢測某個屬性是否隸屬于這個對象 var a = 12;//=>全局變量值修改,WIN的屬性值也跟著修改 console.log(a);//=>全局變量A 12 console.log(window.a);//=>WINDOW的一個屬性名A 12 a = 13; console.log(window.a);//=>13 window.a = 14; console.log(a);//=>14 //=>全局變量和WIN中的屬性存在 “映射機制”(一個改,另一個也跟著改) */ //=>不加VAR的本質是WIN的屬性,是屬性 *** /* // console.log(a);//=>Uncaught ReferenceError: a is not defined console.log(window.a);//=>undefined // console.log('a' in window);//=>false a = 12;//=>window.a=12,給window設置屬性為a console.log(a);//=>12 不是變量 簡寫 console.log(window.a);//=>12 console.log('a' in window);//=>true */ 不加var不存在變量提升 *** /*var a = 12, b = 13;//=>這樣寫B是帶VAR的*/ /*var a = b = 12;//=>這樣寫B是不帶VAR的*/ 相當于: var a = 12; b = 12 console.log(a, b);//=>undefined undefined var a = 12, b = 12; function fn() { console.log(a, b);//=>undefined 12,a是私有變量,b不是私有變量,向上級找 var a = b = 13; /*var a=13; b=13;*/ console.log(a, b);//=>13 13 在webstorm中,a的私有變量和全局的顏色不同 } fn(); console.log(a, b);//=>12 13 ~~~ ![](https://img.kancloud.cn/4c/23/4c237793bd9ffecabf87a390ae64a6d9_1488x563.png) ![](https://img.kancloud.cn/f9/13/f9138655ec33d0b9cdaac220b7b35f81_493x365.png)
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                              哎呀哎呀视频在线观看