The suggestion is to pre-set a variable to a null if that same variables is intended to be used as an object later on in the code.
Essentially null is related to undefined (null == undefined) but is not the same. As explained above, the type null is an empty object pointer.
This example shows the relationship between these two data types:
alert(null == undefined); // result is TRUE
alert(null === undefined); // result is FALSE
To understand better the comparisons shown above, visit the chapter "Equality Operators".