The type null has exactly one value, called null.
You can use this value when testing for the undefined state of variables and objects in browsers that do not support an explicit undefined value, to test for in logical expressions.
ECMA 262 edition 2 - section - 4.3.12
ECMA 262 edition 2 - section - 8.2
ECMA 262 edition 3 - section - 4.3.12
ECMA 262 edition 3 - section - 8.2