Question

本书JavaScript for PHP Developers包含以下注释代码（to 我添加了alert（）来显示单变量表达式语句的值形式

variable;

我还添加了'use strict'指令，看看是不是这样造成这个问题。我无法使用Firefox重现code on JSFiddle。我已将自己的评论添加到大型大写的代码中：

'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

事实上，在following small program最后一次警报函数调用永远不会到达：

window.jane = "JJ";
delete window.jane;
alert(jane); // Program Crashes
alert('Got Here');

我已经在这里再次测试了所有案例，这说明了所有案例。

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，这种行为在所有浏览器中都是一致的或者是一个浏览器与另一个浏是代码来自预订错误，或者只是针对不同的浏览器运行我自己的（Firefox 33.0.1）？

如果有人可以解释各种情况，或许指向相关的 ECMA规范的各个部分，将不胜感激。

感谢。

好的，我看到的结果是这样的，因为我在JSFiddle中运行了代码正如所指出的那样，onload函数而不是全局范围。这是结果在我的本地计算机上提供的网页中运行综合测试。 JSFiddle with the script inside the body html element上的结果是相同的：

<!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。而我知道在严格模式下声明一个没有var的变量会导致引用错误我不确定我能否理解输出的其余部分，特别是为什么脚本执行在某些地方终止：

<!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>

如果有人可以帮我解释脚本执行在某些地方终止的原因我们非常感谢他们采用严格模式的方式。

感谢。

关于本书中的origninal代码，从脚本中正确运行时在文档的头部标记，我得到以下输出，我们可以看到一个由于访问变量jane而引发的ReferenceError实例被抛出。 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

Answer 1

查看jsfiddle界面的这一部分：

jsfiddle screenshot

“onLoad”选择器意味着您在界面的JavaScript象限中键入的代码将被包装在一个函数中，并且该函数将充当窗口的“加载”事件处理程序。因为你的代码在一个函数中，你在出现所声明的变量是全局级别的，实际上并不是全局的;它们是函数中的局部变量。

要使您的代码真正全局化，请将该选择更改为“无包装”设置之一。

＆＃39; PHP for PHP Developers - ＆gt;内置API - ＆gt;全局对象＆＃39;：无法在浏览器中重现代码

1 个答案: