javascript - 为什么这个js代码有效？

Question

if (!wysiwyg_toolbarButtons) {
    var wysiwyg_toolbarButtons = new Array(
        //command, display name, value, title, prompt/function, default text
        ["bold", "Strong", WYSIWYG_VALUE_NONE, "Give text strength"],
        ["italic", "Emphasis", WYSIWYG_VALUE_NONE, "Give text emphasis"],
        ["createlink", "Link", WYSIWYG_VALUE_PROMPT, "Create a hyperlink", "Enter the URL:", "http://"],
        ["unlink", "Unlink", WYSIWYG_VALUE_NONE, "Remove hyperlink"],
        ["insertimage", "Image", WYSIWYG_VALUE_PROMPT, "Insert an image", "Enter the URL of the image:", "http://"],
        ["inserthorizontalrule", "Rule", WYSIWYG_VALUE_NONE, "Insert horizontal rule"],
        ["div"], // place a toolbar divider
        ["formatblock", "Headling 1", "<H1>", "Make top level heading"],
        ["formatblock", "Headling 2", "<H2>", "Make 2nd level heading"],
        ["formatblock", "Headling 3", "<H3>", "Make 3rd level heading"],
        ["formatblock", "Paragraph", "<P>", "Make a paragraph"],
        ["formatblock", "Monospace", "<PRE>", "Make paragraph monospaced text"],
        ["insertunorderedlist", "List", null, "Make an unordered list"],
        ["insertorderedlist", "Ordered List", null, "Make an ordered list"],
        ["div"], // place a toolbar divider
        ["toggleview", "Source", "Compose", "Switch views"]
    );
}

来自this file ，带演示here

我的问题是:为什么不报“ReferenceError: wysiwyg_toolbarButtons is not defined”？

Answer 1

Web 浏览器中的 JavaScript 搜索窗口对象 中的属性。访问未知属性不会抛出错误，因此实际上它的计算方式如下:

if( !window.wysiwyg_toolbarButtons ) { }

尝试 if( !wtf ) { alert('error'); } 和 if( !window.wtf ) { alert('no error'); 在 Firebug 控制台中。

编辑

目前 firebug 中的控制台使用 with( window ) { ..console..code.. } 评估代码。但是“with”语句很棘手，例如:

>>> alert(location);
= eval( "with( window ) { alert(location); }" );
OK, "location" attribute found in window

>>> alert(wtf);
= eval( "with( window ) { alert(wtf); }" );
ERROR, "wtf" not found in window, and not in global scope, throws ReferenceError

浏览器中的隐式窗口对象的行为不同于它与“with”语句一起使用时的行为。