blocks|key|1440232|text|{|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1440233|在语句的开头表示一个“语句块”(参见ECMA-262-3的12.1节)，其中包含一个语句列表。|unstyled|offset|length|1440234|}|1440235|立即结束不包含任何语句的语句块。这很好。但现在解析器正在寻找下一条语句：|1440236|==false|1440237|哈?这不是一个语句；语法错误。|1440238|语句块是用来做什么的？好吧，每次你说：|1440239|if+(something)+{
++++...
}|1440240|JavaScript将这些流控制语句定义为：|1440241|if+"("+<expression>+")"+<statement>+[else+<statement>]|1440242|即。在单语句形式中，不带大括号。然后，它允许您在任何可以使用单个语句的地方使用语句块，这意味着您可以使用if括号多个语句。但这也意味着您可以拥有一个单独的语句块，而无需关联的流控制语句。|1440243|这绝对没有实际意义！您可能会认为它为您提供了信息隐藏，但事实并非如此：|1440244|var+a=+1;
{
++++var+a=+2;
}
alert(a);|1440245|2中的...results，因为语句块本身不会创建新的作用域。|style|CODE|1440246|JavaScript以这种方式定义流控制和语句块，因为C(以及从它派生的其他语言)是这样做的。不过，这些语言并没有让{}作为对象文字表达式提供双重功能，因此它们不存在使这成为另一个JS错误特性的歧义。|1440247|即使是这个wannabe-字面：|1440248|{
+++a:+1
}|1440249|是一个有效的语句块，因为‘：’用于表示语句中的标签。(+1是一个无用的表达式语句，省略了分号。)标签是从C继承的另一个特性，在JavaScript中很少使用。它们并不像积木一样完全没有意义，但它们很少被需要，而且经常被认为品味不佳。|1440250|(具有两个属性的文本将导致语法错误，因为对象文本使用逗号分隔符，但带标签的语句必须用分号分隔。)|1440251|这就是not+the+only+place，它松散的语法可能会让你陷入困境，因为它会对你认为是表达式的东西做一些其他的陈述。|1440252|entityMap|0|LINK|mutability|MUTABLE|url|http://www.ecma-international.org/publications/standards/Ecma-262.htm|1|https://stackoverflow.com/questions/440739/what-do-parentheses-surrounding-a-javascript-objectfunctionclass-declaration/442408#442408^0|0|I|A|0|0|0|0|0|0|0|0|0|0|0|0|0|0|1|0|1M|2|0|0|0|S|1|0|0|3|I|1|0^^$0|@$1|2|3|4|5|6|7|1U|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|1V|8|@]|9|@$G|1W|H|1X|1|1Y]]|A|$]]|$1|I|3|J|5|6|7|1Z|8|@]|9|@]|A|$B|C]]|$1|K|3|L|5|F|7|20|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|21|8|@]|9|@]|A|$B|C]]|$1|O|3|P|5|F|7|22|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|23|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|24|8|@]|9|@]|A|$B|C]]|$1|U|3|V|5|F|7|25|8|@]|9|@]|A|$]]|$1|W|3|X|5|6|7|26|8|@]|9|@]|A|$B|C]]|$1|Y|3|Z|5|F|7|27|8|@]|9|@]|A|$]]|$1|10|3|11|5|F|7|28|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|29|8|@]|9|@]|A|$B|C]]|$1|14|3|15|5|F|7|2A|8|@$G|2B|H|2C|16|17]]|9|@]|A|$]]|$1|18|3|19|5|F|7|2D|8|@$G|2E|H|2F|16|17]]|9|@]|A|$]]|$1|1A|3|1B|5|F|7|2G|8|@]|9|@]|A|$]]|$1|1C|3|1D|5|6|7|2H|8|@]|9|@]|A|$B|C]]|$1|1E|3|1F|5|F|7|2I|8|@$G|2J|H|2K|16|17]]|9|@]|A|$]]|$1|1G|3|1H|5|F|7|2L|8|@]|9|@]|A|$]]|$1|1I|3|1J|5|F|7|2M|8|@]|9|@$G|2N|H|2O|1|2P]]|A|$]]|$1|1K|3|-4|5|F|7|2Q|8|@]|9|@]|A|$]]]|1L|$1M|$5|1N|1O|1P|A|$1Q|1R]]|1S|$5|1N|1O|1P|A|$1Q|1T]]]]

<pre><code>{
</code></pre>

at the start of a statement signals a ‘statement block’ (see <a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm" rel="nofollow noreferrer">ECMA-262-3</a> section 12.1), which contains a list of statements.

<pre><code>}
</code></pre>

immediately ends the statement block with no statements in it. That's fine. But now the parser is looking for the next statement:

<pre><code>==false
</code></pre>

Huh? That's not a statement; syntax error.

What are statement blocks for? Well, you are writing a statement block every time you say:

<pre><code>if (something) {
 ...
}
</code></pre>

JavaScript defines these flow-control statements as:

<pre><code>if "(" &lt;expression&gt; ")" &lt;statement&gt; [else &lt;statement&gt;]
</code></pre>

ie. in the single statement form with no braces. It then allows you to use a statement-block anywhere you can use a single statement, which means you can have if-braces-many-statements. But it also means you can have a statement-block on its own with no associated flow-control statement.

This serves absolutely no practical purpose! You might be tempted to think it gave you information-hiding, but no:

<pre><code>var a= 1;
{
 var a= 2;
}
alert(a);
</code></pre>

...results in <code>2</code>, because statement blocks don't in themselves create a new scope.

JavaScript defines flow control and statement blocks in this manner because C (and other languages derived from it) did. Those languages didn't make <code>{}</code> serve double-duty as an Object literal expression though, so they didn't have the ambiguity that makes this another JS misfeature.

Even this wannabe-literal:

<pre><code>{
 a: 1
}
</code></pre>

is a valid statement block, because ‘:’ is used to denote a label in a statement. (and <code>1</code> is a useless expression-statement, with the semicolon omitted.) Labels are another feature inherited from C that are rarely used in JavaScript. They're not totally pointless like the blocks, but they're seldom needed and often considered in poor taste.

(A literal with two properties will cause a syntax error, as object literals use comma separators, but labelled statements must be separated by semicolons.)

This is <a href="https://stackoverflow.com/questions/440739/what-do-parentheses-surrounding-a-javascript-objectfunctionclass-declaration/442408#442408">not the only place</a> where JavaScript's loose syntax can trip you up by making some other statement of something you think is an expression.

blocks|key|1225524|text|简单地说，{}==false是由Js编译器编译成{};==false的，所以这是语法错误。你应该写({})==false，它会返回false。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1225525|entityMap^0|5|9|O|A|1D|B|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

Simply to say, <code>{}==false</code> are compiled by Js compiler to <code>{};==false</code>, so it's syntax error. you should write <code>({})==false</code> and it'll return false.

In Firefox 3.5, I type this in the Firebug console :

<pre><code>false=={} // =&gt; evals to false
{}==false // syntax error
</code></pre>

What is the explanation for this ?

JavaScript: {}==false is a SyntaxError?

编译器

在Firefox3.5中，我在Firebug控制台中键入以下内容：false=={} // => evals to false{}==false // syntax error对此有何解释？

问JavaScript：{}==false是SyntaxError吗？
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