You mean a falsy value. null value is null value, it's not "equal to undefined, false, 0", this only adds to the confusion.
EDIT: the default value in JS is undefined.
Again, a falsy value, a true value it's not.
The discussion here is about three things: undeclared, undefined or null.
Using x without declaring it first (without using an explicit var statement) denotes a variable which declaration will be hoisted by the interpreter. This also usually leads to unexpected behavior, where shadowing a variable leads to a completely different result. The recommendation is to always declare your variables at the top of your scope.
Declaring x outside any function denotes a global variable (usually not a pattern), where, if no value is assigned, a undefined value is the default value. So using var x = null overrides the default undefined value.
Explicitly using var x = null in comparison with undefined, ensures that, when using "===" instead of "==", you'll get the right false value, false.
>console.log(null == undefined);
> console.log(null === undefined);