null and undefined are nothingness, that is,
var a;
var b = null;
Here a
and b
do not have values. Whereas, 0, false and '' are all values. One thing common beween all these are that they are all falsy values, which means they all satisfy falsy conditions.
So, the 0, false and '' together form a sub-group. And on other hand, null & undefined form the second sub-group. Check the comparisons in the below image. null and undefined would equal. The other three would equal to each other. But, they all are treated as falsy conditions in JavaScript.
This is same as any object (like {}, arrays, etc.), non-empty string & Boolean true are all truthy conditions. But, they are all not equal.