How to find if element exists in jquery?

this part of the code is not working.

$partofcode ='
userid=8;
if ($(".btn-follow[userid='+userid+']").length) {
alert("ok found");
}';

this part of the code is working

$partofcode ='
if ($(".btn-follow[userid='8']").length) {
alert("ok found");
}';

what is wrong here?