The eval function should not be used if it can be avoided, the use of the eval function can slow things down, and can also open up some security concerns. I can not say that I use eval often, and even when I am in a situation in which I seems like I need to use it I do what I can to look for other options. Still this is a post on js eval, so then this will be a post on some of the ins and outs of the js eval function for what it is worth.
This is one weird thing about the use of eval that a developer should be ware of when using it. Also again about using eval, if you can every thing of any way to go about not using it do that instead, and not just for this reason.