Is there a good way to accomplish this Javascript?

Basically I want something where the values of the fields are concatenate into a single field with comma between the values. Figure something simple like this

var a  

var b  

var c

allfieldvalue = a + ","b + ","c

My issue is I need to build condition in order for them to be put into allfieldvalue. So if var a = true, var b = false, and var c = true, then allfieldvalue = a +","c. Keep in mind, there could 20 different var. Hopefully that makes sense.