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                ~~~ /* * 在ES6中基于LET/CONST等方式創建變量或者函數,不存在變量提升機制 * * =>切斷了全局變量和WINDOW屬性的映射機制 * * =>在相同的作用域中,基于LET不能聲明相同名字的變量 * (不管用什么方式在當前作用域下聲明了變量,再次使用LET創建都會報錯) * 雖然沒有變量提升機制,但是在當前作用域代碼自上而下執行之前, * 瀏覽器會做一個重復性檢測(語法檢測): * 自上而下查找當前作用域下所有變量, * 一旦發現有重復的,直接拋出異常, * 代碼也不會在執行了(雖然沒有把變量提前聲明定義,但是瀏覽器已經記住了,當前作用域下有哪些變量) */ // console.log(a);//=>Uncaught ReferenceError: a is not defined // let a = 12; // console.log(window.a);//=>undefined // console.log(a);//=>12 /*let a = 12; console.log(a); let a = 13;//=>Uncaught SyntaxError: Identifier 'a' has already been declared console.log(a);*/ /*b = 12; console.log(b);//=>12 a = 12;//=>Uncaught ReferenceError: a is not defined console.log(a); let a = 13; console.log(a);*/ let a = 10, b = 10; let fn = function () { console.log(a, b);//=>Uncaught ReferenceError: a is not defined let a = b = 20; /* * let a=20; * b=20; //=>把全局中的 b=20 */ console.log(a, b); //20 20 }; fn(); console.log(a, b); //10 20 ~~~
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