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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
                # [Block scoping](https://babeljs.cn/docs/plugins/transform-es2015-block-scoping) 就是塊級作用域,在 ES6 中可以使用 let 創建塊級作用域,可以使用 `babel-plugin-transform-es2015-block-scoping` 進行語法轉換。 ``` npm install --save-dev babel-plugin-transform-es2015-block-scoping ``` ## .babelrc 配置 ```json { "plugins": ["transform-es2015-block-scoping"] } ``` ## 使用 ### 循環輸出 我們最熟悉的 var 并不存在塊級作用域,比如: ```js for (var i = 0; i < 5; i++) { setTimeout(() => { console.log(i) }, 1000); } ``` 現象是間隔一秒后,連續輸出5次5 而 let 的出現正好解決了這一問題: in ```js for (let i = 0; i < 5; i++) { setTimeout(() => { console.log(i) }, 1000); } ``` out ```js "use strict"; var _loop = function _loop(i) { setTimeout(function () { console.log(i); }, 1000); }; for (var i = 0; i < 5; i++) { _loop(i); } ``` 實際上,編譯出來的代碼實際上是一個立即執行函數,可以簡寫為: ```js "use strict"; for (var i = 0; i < 5; i++) { (function (i) { setTimeout(function () { console.log(i); }, 1000); })(i) } ``` 現象是間隔一秒后,連續輸出 0, 1, 2, 3, 4 ### 控制流程 var 并不存在塊級作用域: ```js if (true) { var a = 0 } console.log(a) // 0 ``` let 卻存在塊級作用域: in ```js if (true) { let b = 1 } console.log(b) // ERROR: b is not defined ``` out ```js "use strict"; if (true) { var _b = 1; } console.log(b); // b is not defined ``` 可以看到,編譯出的代碼實際上是使用 `_` 替換了原變量,因此在外部訪問不到對應變量,以達到塊級作用域的效果。
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