Why do variable declarations in the console keep returning "undefined"?

I am using the latest Firefox (4.0.1) and Firebug (1.7.2开发者_C百科).

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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(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.

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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:

    var color = "blue";
    color = "red"; 

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.

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Firebug is reporting the result of evaluating the expression, equivalent to:

typeof eval("var x = 5;");
"undefined"

typeof eval("x = 5;");
"number"