Why does JavaScript variable declaration at console results in “undefined” being printed?

It prints the result of this expression – which is undefined. And yes, var a is a valid expression on its own.

Actually, you should rather be amused by why console prints undefined when you write var a = 3 or something like this. It also prints undefined if function anyFunctionName() {} statement is processed. In fact, all the var and function declaration (!) statements seem to be ignored if there’s another statement with some ‘real’ result:

>>> var a = 3;
undefined

>>> var a = 3; a = 4;
4

>>> var a = 3; a = 4; var a = 5; function f() {};
4 // !!!

Now, I suppose the real reason behind is behaviour of eval statement, as described here:

• Let result be the result of evaluating the program prog.
• If result.type is normal and its completion value is a value V, then return the value V.
• If result.type is normal and its completion value is empty, then return the value undefined.

So now the question is, what does var a = 4 statement return? Guess what: it’s not 4.

The production VariableStatement : var VariableDeclarationList; is
evaluated as follows:

• Evaluate VariableDeclarationList.
• Return (normal, empty, empty).

Now the most interesting part: what happened in the last example, why 4 is the result? That’s explained in this section:

The production Program : SourceElements is evaluated as follows:

• Let result be the result of evaluating SourceElements.

[…]

The production SourceElements : SourceElements *SourceElement* is evaluated as follows:

• Let headResult be the result of evaluating SourceElements.
• If headResult is an abrupt completion, return headResult.
• Let tailResult be result of evaluating SourceElement.
• If tailResult.value is empty, let V = headResult.value, otherwise let V = > tailResult.value.
• Return (tailResult.type, V, tailResult.target)

Both function f() {} and var a = 5 statements‘ return values were (normal, empty, empty). So the script ended up with giving out the result of the first statement (starting from the script’s end, so technically it’s the last one) that’s not (normal, empty, empty). That is the result of a = 4 assignment statement – which is 4.

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P.S. And now for some icing on the cake: consider the following:

>>> function f() {}
undefined

>>> (function f() {})
function f() {}

The difference is quite subtle: the first input is treated as a Function Declaration statement, which, according to this rule…

The production SourceElement : FunctionDeclaration is evaluated as
follows:

• Return (normal, empty, empty).

… will eventually produce undefined when eval-ed, as we already know.

The second input, however, is treated as a Function Expression, which is evaluated to the function itself. That means it’ll be passed through eval and eventually returned to the console (in its format).