为什么控制台中的变量声明总是返回“未定义”？
我使用的是最新的 Firefox (4.0.1) 和 Firebug (1.7.2)。
每当我在控制台中输入变量声明时，都会返回斜体“未定义”警告。
例如，如果我输入“var x = 5;”那么响应是“未定义”，而不是“5”。
之后，如果我在控制台中输入“x”，则会返回正确的值 5。然而，错误/警告有点麻烦，我真的很想知道原因和解决方案，以及我是否是唯一经历过这种情况的人。
有趣的是，如果我不使用“var”，而只是使用“x=5”声明值，则会显示正确的行为，并在控制台中返回“5”。
I am using the latest Firefox (4.0.1) and Firebug (1.7.2).
Any time I enter a variable declaration into the console, an italicized "undefined" warning is returned.
So for example if I enter "var x = 5;" then the response is "undefined", rather than "5".
Afterward if I enter "x" into the console, the proper value of 5 is returned. However the error/warning is a bit of a nuisance, would really like to know the cause and resolution, and if I'm the only one experiencing this.
Interestingly if I don't use "var" but just declare the value using "x=5" then the correct behavior exhibits and "5" is returned in the console.
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（这只是一个猜测，我不是 Javascript 语言规则细节或 Firebug 的专家。）
控制台给您的反馈是您输入的行的评估结果。我假设声明
var x = ...是一个没有值的语句，而简单的赋值 (x = ...>) 符合 C 的传统和函数式语言的“一切都是表达式”态度，是一个计算指定值的表达式。(This is just a guess, I'm not an expert on the details of Javascript's language rules or on Firebug.)
The feedback the console gives you is the result of the evaluation of the line you entered. I assume the declaration
var x = ...is a statement that doesn't have a value, while simple assignment (x = ...) is, in line with the C heritage and the "everything is an expression" attitude of functional languages, an expression that evaluates to the assigned value.与其他一些语言不同，JavaScript 中的每段代码要么是表达式，要么是语句。
表达式总是返回一个值。语句始终返回未定义。什么是语句、什么是表达式在 1997 年的原始 JavaScript 规范。
例如，假设这是我们的程序：
您会注意到，如果您在控制台中逐行输入此内容，第一行返回未定义，而第二行返回“红色”。
这是因为，正如您可能已经猜到的那样，变量声明 (
var Something = Something) 是一个语句，而变量赋值 (something = Something) 是一个表达式。如果您好奇，请尝试阅读 11.13.1（第 50 页）中 JavaScript 如何评估分配，位于我上面链接的规范中的“简单分配”部分。
Unlike some other languages, in JavaScript every piece of code is either an expression or a statement.
Expressions always return a value. Statements always return undefined. What is a statement and what is an expression is defined in the original JavaScript specification from 1997.
For example, say this is our program:
You will notice that if you enter this line-by-line into your console, the 1st line returns undefined, while the 2nd line returns "red".
This is because, as you may have guessed, a variable declaration (
var something = something) is a statement, whereas a variable assignment (something = something) is an expression.If you're curious, try reading through how JavaScript evaluates an assignment in 11.13.1 (page 50), under the "Simple Assignment" section in the spec I linked above.
Firebug 报告计算表达式的结果，相当于：
typeof eval("var x = 5;");
“未定义”
typeof eval("x = 5;");
“数字”
Firebug is reporting the result of evaluating the expression, equivalent to:
typeof eval("var x = 5;");
"undefined"
typeof eval("x = 5;");
"number"