Evaluate multiple conditional statements with possible undefined variables

In Javascript, what would be the best way to evaluate several conditions when taking into consideration variables that could be undefined? For instance, I want to evaluate if x > y only if x is NOT undefined (has a value), and if it is undefined, continue to proceed to the next condition.

Something like:

if(x && x > y && a && a > b && c < d && e > f) ....

Here I want to see if x is defined, and then evaluate if x is greater than y, then go to evaluate if a is defined and evaluate if a is greater than b, then evaluate if c is less than d and etc. So here I want it so that if x or a is undefined CONTINUE to proceed to evaluate c < d and e > f

Here if the variables are are undefined then the whole statement is all false... what are some solutions for this that's clean?