The number type defines objects that represent numbers and include the IEEE 754 NaN values in the set if the implementation is fully ECMA-compliant.
The number range is huge, representing the double precision 64 bit IEEE 754 set of values.
The Number type includes the values positive and negative Infinity and, internally at least, positive and negative zero are represented as two distinct values.
ECMA 262 edition 2 - section - 4.3.19
ECMA 262 edition 2 - section - 8.5
ECMA 262 edition 3 - section - 4.3.20
ECMA 262 edition 3 - section - 8.5