What does `1..something` mean in JavaScript?

<script>
1..z
<开发者_Go百科/script>

This gives no syntax or runtime error. Looks like number and variable name can be any other (123..qwerty). I'm wondering what does this statement mean?

Is not a range, the 1..z expression will simply return undefined.

Why?

The first dot ends a representation of a Numeric Literal, giving you a Number primitive:

var n = 1.;

The grammar of a Numeric Literal is expressed like this:

DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt 

As you can see the DecimalDigits part after the dot is optional (opt suffix).

The second dot is the property accessor, it will try only to get the z property, which is undefined because it doesn't exist on the Number.prototype object:

1..z; // undefined
1..toString(); // "1"

Is equivalent to access a property with the bracket notation property accessor:

1['z']; // or
1['toString'](); 

Combine these:

alert(1.foo); // --> parse error
alert(1.4.foo); // --> undefined - number 1.4 doesn't have the property foo
alert(1.); // --> 1 (?)

To the conclusion:

alert(1..foo); // --> undefined