While I like the first answer and I hate eval, I'd like to add that there's another way (similar to eval) so if you can go around it and not use it, you better do. But in some cases you may want to call some javascript code before or after some ajax call and if you have this code in a custom attribute instead of ajax you could use this:
var executeBefore = $(el).attr("data-execute-before-ajax");
if (executeBefore != "") {
var fn = new Function(executeBefore);
fn();
}
Or eventually store this in a function cache if you may need to call it multiple times.
Again - don't use eval or this method if you have another way to do that.