Get access to current trial number

Online experiments
1

There are plenty of questions and hints online about this kind of requirement, probably most relevantly here, but I still can’t get it to work, suggesting that I’m missing something simple.

In the experiment builder, I’,m editing the .js in the Begin Routine section. I would like to set some parameters each routine, conditioned on the trial number. I therefore need the trial number, but I just can’t get hold of it.

Here is the function declaration and the first line

function Stim_PresentationRoutineBegin(snapshot) {
  return async function () {
    TrialHandler.fromSnapshot(snapshot);

Here is the list of things I have tried so far

trials.thisN;
trials.thisTrialN;
TrialHandler.thisN;
TrialHandler.thisTrialN;
TrialHandler.getSnapshot().thisTrialN;
snapshot.thisN;

and then reporting the output with console.log()
Trying getSnapshot() declares that the function does not exist. For the other options I always get undefined, except for the final option where ‘snapshot’ is NaN.

I’m not clear what my mistake is then. Do I need to do something fancy to get the current trial number?

2

What version of PsychoPy are you using? trials.thisN works in the newest release but did not work in earlier releases. So it might be that you need an update.

Becca

3

Hmm, okay

This is running using PsychoJS 2021.2.0 . I’ll try and find the newer version and test with that.

4

yes that will be the problem then. the release downloadable here will have trials.thisN

Becca

5

Hi Becca,

Thanks for this - this is certaintly a more elegant method that what I was using in the Python script, but it still doesn’t work for the onliny experiments in PsychoJS 2021.2.0. I have looked for the newer JS release but it won’t let me select one on Pavlovia.

Is there another way of doing this in Javascript?

Thanks,

Peter

6

Big oops from me, using trials.thisN IS actually working. I looked back at a different version of my code from yesterday when it wasn’t working and it seems I was accidentally overwriting it earlier in the script when I forgot to remove that previous attempt to access the trial number.

Sorry!