What's going on with this rather peculiar JavaScript syntax

Question

The following JavaScript code:

alert(2 .x);

Alerts 'undefined' (See it here: http://jsfiddle.net/Rp4wk/)

(Note: The space between the '2' and '.x' is intended)

Simple question: Why? Especially when the following yield syntax errors:

alert(2.x);
alert(2. x);

Anyone?

Answer 1

The . is an operator. The 2 is a number. The x is (treated as) a property name.

A floating-point numeric constant must not have embedded spaces. Thus, 2 .x is an expression calling for the constant 2 to be promoted to a Number object, and then the property called "x" is examined. There isn't one, of course, so the value is undefined.

You can get the same effect a little more explicitly with

alert((2).x);

Note that

alert("Hello".x);

is somewhat similar: in that case, it's not a numeric constant, it's a string constant. It's less peculiar because there's no syntactic funny business involved, but otherwise the interpreter does similar things when evaluating. The string constant is first converted to a String object, and then the "x" property is fetched.

edit — to clarify a little, 2.x is an error because it's parsed as a numeric constant ("2.") followed by the identifier "x", and that's a syntax error; two values placed next to each other like that with no intervening operator do not form any sort of construct in the language.