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本書JavaScript for PHP Developers包含我添加警報（）■顯示下面的註釋代碼（以 單變量表達式語句形式「JavaScript的PHP開發 - >內置的API - >全局對象」：在瀏覽器中無法重現的代碼
variable;
和 的價值觀，我還增加了「使用嚴格的」指令來看看這是什麼 導致的問題。我無法重現code on JSFiddle與Firefox。 我已將自己的評論添加到大寫字母代碼中：
'use strict';
// Create a global variable
var john = "Jo";
alert(john); // "Jo"
alert(window.john); // "Jo", works as a property too
/* BUT I GET UNDEFINED HERE */
// Create a property of the global object
window.jane = "JJ";
alert(jane); // "JJ", works as a variable too
alert(window.jane); // "JJ"
// Delete them
alert(delete window.john); // false
/* BUT I GET true HERE */
alert(delete window.jane); // true
alert(john); // "Jo"
alert(jane); // undefined
/* BUT PROGRAM CRASHES HERE */
alert(this === window); // true
實際上在following small program最後警報函數調用從未達到：
window.jane = "JJ";
delete window.jane;
alert(jane); // Program Crashes
alert('Got Here');
我測試過的所有案件在這裏再一次這說明所有情況。
var a = 'John';
window.b = 'Jane';
c = 'Jack';
alert(a); // John
alert(b); // Jane
alert(c); // Jack
alert(window.a); // undefined
alert(window.b); // Jane
alert(window.c); // Jack
alert(delete a); // false
alert(delete b); // true
alert(delete c); // true
alert(a); // John
//alert(b); // would crash
//alert(c); // would crash
window.b = 'Jane';
c = 'Jack';
alert(delete window.a); // true
alert(delete window.b); // true
alert(delete window.c); // true
alert(window.a); // undefined
alert(window.b); // undefined
alert(window.c); // undefined
alert(a); // John
//alert(b); // would crash
//alert(c); // would crash
我想知道的是，這種行爲在所有瀏覽器 或者是一個瀏覽器和其他之間的差異是一致的。 本書的代碼是否有錯，或者是否僅僅是針對不同於我自己的（Firefox 33.0.1）的 運行的瀏覽器？
如果有人能夠解釋各種情況，可能指向ECMA規範的相關 部分，將不勝感激。
謝謝。
OK，我看到的結果是這樣的，因爲我已經運行的jsfiddle裏面的代碼 在onload功能，而不是在全球範圍內，如指出。在我的本地機器上運行的網頁內部運行綜合測試的結果爲 。 上JSFiddle with the script inside the body html element的結果是一樣的：
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<script type="text/javascript">
var a = 'John';
window.b = 'Jane';
c = 'Jack';
alert(a); // John
alert(b); // Jane
alert(c); // Jack
alert(window.a); // John
alert(window.b); // Jane
alert(window.c); // Jack
alert(delete a); // false
alert(delete b); // true
alert(delete c); // true
alert(a); // John
try { alert(b); } catch (e) { alert(e); } // throws ReferenceError: b is not defined
try { alert(c); } catch (e) { alert(e); } // throws ReferenceError: c is not defined
window.b = 'Jane';
c = 'Jack';
alert(delete window.a); // false
alert(delete window.b); // true
alert(delete window.c); // true
alert(window.a); // John
alert(window.b); // undefined
alert(window.c); // undefined
alert(a); // John
try { alert(b); } catch (e) { alert(e); } // throws ReferenceError: b is not defined
try { alert(c); } catch (e) { alert(e); } // throws ReferenceError: c is not defined
</script>
</body>
</html>
和here is what happens when the ECMAScript5 'use strict' directive is also used。雖然 我知道在嚴格模式下聲明一個沒有var的變量會導致引用錯誤 我不確定我能否理解其餘的輸出，特別是爲什麼腳本 在某些地方終止執行：
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<script type="text/javascript">
'use strict';
var a = 'John';
window.b = 'Jane';
try { c = 'Jack'; } catch (e) { alert(e); } // throws ReferenceError: assignment to undeclared variable c
alert(a); // John
alert(b); // Jane
try { alert(c); } catch (e) { alert(e); } // throws ReferenceError: c is not defined
alert(window.a); // John
alert(window.b); // Jane
alert(window.c); // undefined
try {
// Uncommenting any of these three following statements will cause the script to be exited
// during the parsing time; no statement from this script will be executed.
//alert(delete a); causes script to end during parsing at runtime even though try catch block present
//alert(delete b); causes script to end during parsing at runtime even though try catch block present
//alert(delete c); causes script to end during parsing at runtime even though try catch block present
} catch (e) { alert(e); }
alert(a); // John
try { alert(b); } catch (e) { alert(e); } // Jane
try { alert(c); } catch (e) { alert(e); } // throws ReferenceError: c is not defined
window.b = 'Jane';
c = 'Jack';
try {
//alert(delete window.a); // causes script to end during execution at runtime even though try catch block present
//alert(delete window.b); // causes script to end during execution at runtime even though try catch block present
//alert(delete window.c); // causes script to end during execution at runtime even though try catch block present
} catch (e) { alert(e); }
/* Script stops execution at this point. Why?????
alert(window.a); //
alert(window.b); //
alert(window.c); //
alert(a); //
try { alert(b); } catch (e) { alert(e); } //
try { alert(c); } catch (e) { alert(e); } //
</script>
</body>
</html>
如果任何人都可以幫我解釋爲什麼腳本執行終止在某些地方 他們做嚴格模式的方式將不勝感激。
謝謝。
至於從書，origninal代碼時從文件的頭部的腳本 標籤內正常運行，我得到下面的輸出，是我們可以看到， 的ReferenceError例如由於訪問變量jane被拋出。 Here is the JSFiddle for the code：
//'use strict'
// Create a global variable
var john = "Jo";
alert(john); // "Jo"
alert(window.john); // "Jo", works as a property too
// Create a property of the global object
window.jane = "JJ";
alert(jane); // "JJ", works as a variable too
alert(window.jane); // "JJ"
// Delete them
try { alert(delete window.john); } catch (e) { alert(e); }
// false
/* if strict mode were enforced would actually cause
the following fatal error:
TypeError:
property "john" is non-configurable and can't be deleted
*/
try { alert(delete window.jane); } catch (e) { alert(e); }
// true
alert(john); // "Jo"
try { alert(jane); } catch (e) { alert(e); } // undefined in book
/* but actually gives a:
ReferenceError: jane is not defined
which is a fatal error causing the script to exit
if not caught*/
alert(this === window); // true
我懷疑這隻發生在JSFiddle上，對吧？如果是這樣，那是因爲JSFiddle將你的JS代碼封裝在'onload'函數中。 – 2014-10-31 12:46:55
請仔細閱讀這些文檔：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var和here：https：//developer.mozilla.org/en-US/ docs/Web/JavaScript/Reference/Strict_mode – istos 2014-10-31 12:56:38
謝謝@istos的參考。有一件事我沒有得到。關於第一個鏈接，可配置屬性和不可配置屬性之間有什麼區別。謝謝。 – 2014-10-31 14:49:51