week17Hw OK #26
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week17Hw OK #26
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aszx87410
reviewed
Sep 20, 2019
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| ``` 執行過程 | ||
| 1.`console.log(1)` 進入 Call Stack 直接執行，印出 1 後從 Call Stack pop | ||
| 2.`setTimeout(() => { console.log(2) }, 0)` 進入 Call Stack 看到是 setTimeout 把 setTimeout 放到 WebApi 並設置計時器給它，經過 0 秒後會把 `console.log(2)` 放到 callback queue。 |
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其實是把 () => { console.log(2) } 放到 callback queue
aszx87410
reviewed
Sep 20, 2019
| ``` | ||
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| `obj.inner.hello()` | ||
| 上面這個是語法糖的形式，實際上要使用 function 應該是 function call 這個形式，所以我們可以把這段程式碼轉成 `obj.inner.hello.call(obj.inner)`，call 裡面代的值就是 hello 這個 function 前的值，同時這個值就是 this。 |
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跟語法糖其實沒關係，我只是說 this 可以用這種形式來「代換」，但不等同於這是語法糖
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我是看this 的值到底是什么？一次说清楚 裡面有一段提到:
从看到这篇文章起，你一定要记住，第三种调用形式，才是正常调用形式：
func.call(context, p1, p2)其他两种都是语法糖，可以等价地变为 call 形式：
func(p1, p2) 等价于
func.call(undefined, p1, p2)
obj.child.method(p1, p2) 等价于
obj.child.method.call(obj.child, p1, p2)请记下来。（我们称此代码为「转换代码」，方便下文引用）
至此我们的函数调用只有一种形式：
func.call(context, p1, p2)裡面提到這其他兩種都是語法糖才這樣寫的
看到老師回答後，我去找了一下維基對語法糖的定義
語法糖，指電腦語言中添加的某種語法，這種語法對語言的功能沒有影響，但是更方便程式設計師使用。
以這段定義來說，純 function 沒辦法改變 this 的值，所以這樣不算語法糖只是可以代換，不知道這樣對不對?
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原來是文章裡面提到的，我自己是對這說法保持懷疑啦
因為要證明 JS 底層真的是轉成這種形式才能說是語法糖
aszx87410
reviewed
Sep 20, 2019
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| 另一個難題應該是 this 了，一開始看完 What's THIS in JavaScript? 覺得 this 也沒什麼，信心滿滿的接續看了老師的文章後馬上傻眼XD，範例只答的出一個馬上被打臉，後來也是一篇文章重複看了好幾遍，然後某天就突然通了， call 的方法懂了後真的簡單好用。 | ||
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| 第一次好好的理解程式語言的底層到底在幹嘛，看了老師的文章真的有了解滿多不同的東西，那個大安 star 的 code 實做出來我有嚇到XD，繼續邁進下一週。 |
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那個大安 star 的 code 實做出來我有嚇到
這是什麼？
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所有的函式都是閉包：談 JS 中的作用域與 Closure全域變數就是國際巨星這段，直接把說法轉成程式碼覺得很厲害XD
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麻煩老師了 感謝