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

Question

<script>
1..z
</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?

Answer 1

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']();