Ah, I have a memory that thisN doesn’t work certain versions of PsychoJS, but it should in more recent versions, see the crib sheet: https://docs.google.com/document/d/13jp0QAqQeFlYSjeZS0fDInvgaDzBXjGQNe4VNKbbNHQ/edit
What I’d do in your case first is add some debugging script so you can see what happens. I added some console.log statements to your original script. These produce messages in the browser console (see the Crib Sheet again).
if ((loop_pract.thisN === 0)) {
console.log('generate jitters'); // Print that we're here now
jitters = ([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0] * 2);
shuffle(jitters);
console.log(jitters); // Print output of shuffle
}
// Print we're popping and print jitters again
console.log('pop jitters');
console.log(jitters);
current_jitter = jitters.pop();
thisExp.addData("jitter", current_jitter);