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Translating PsychoPy code to Javascript & general documentation


Hi there,

I am completely new to Pavlovia and just tried running an experiment I programmed in Builder 3.1.0 online, but it fails and creates a ReferenceError. I read in a related post that this is because I need to translate code components into JavaScript.

Where can I find documentation on:

  • where I need to replace the code (in Builder or on Pavlovia / Github, for example
  • what bits need to be replaced
  • how to translate them

Many thanks for indulging my ignorance!

1 Like

Hi @Marc_Buehner, the documentation is currently being updated. The reference error is probably because you have added some Python code in a code component, but have not translated this into JavaScript. This means that any variables created in the code component will not be carried through to the JS, and so you would get the equivalent of a Python NameError - a variable is being used that has not been defined. If you have a code component, open up the dialog and select the code type as “Both”, so you can see Python and JavaScript panels on the left and right, respectively. JS is not so different from Python, so translating the code will not be too challenging if you have already written some Python code.

If there are no code components in your task, please paste the full error so we can see what is happening, and also provide the URL so we can help debug.


HI @dvbridges, thanks that makes sense. Yes, there are code components and I have started translating them.
One thing I am stuck in is that some of the code reads in csv files using pandas pd.read_csv, and then later on uses values.tolist() to read the content of a column into a list.

Any pointers in how this can be done in javascript — the reading of csv file and then some pandas equivalent to basically convert columns in the csv file into lists?

Many thanks



I think the easiest method would be to use trialhandler code, then you can refer to your columns as in a list of dicts/JS objects. E.g.,

myList = new TrialHandler({
    psychoJS: psychoJS,
    nReps: 1, method: TrialHandler.Method.SEQUENTIAL,
    extraInfo: expInfo, originPath: undefined,
    trialList: "myConditions.xlsx",
    seed: undefined, name: 'myList'});

aValue = myList.trialList[0]['variableName']  // Get value from row 0

You just need to make sure you conditions Excel file is in your resources folder. Just coding it like this will mean that PsychoPy will not recognise it as a necessary resource, and so will not move it to the resources folder for you.


HI David - Thanks for this. I have tried as best as I can but now the experiment just won’t launch at all on Pavlovia. Still fine on Builder, but Pavlovia gets stuck on ‘loading the experiment’ page. Because I don’t get an error message I have no pointer as to where I am going wrong. Any ideas what I can do?
Thank you!


Sure, would you mind sharing the URL so I can take a look?


You’re a star!
The URL is and I have set the permissions to Internal



Thanks. The first issue that arises how you defined nTrials as type int. You do not need to define your variables like this, or at all using var. This is because PsychoPy will do this for you, to make your variables global scope. If you do actually want your variables to be defined with a local scope e.g., to a particular function, then use var.

Your next issue will be with the use of the shuffle function. Shuffle is not part of the imported methods in JavaScript, as it is in Python. Instead, you will have to make your own. E,g., try this to get a global scope function called shuffle:

shuffle = function (a) {
    var j, x, i;
    for (i = a.length - 1; i > 0; i--) {
        j = Math.floor(Math.random() * (i + 1));
        x = a[i];
        a[i] = a[j];
        a[j] = x;
    return a;

HI David,

Thanks so much for this.

I have eliminated all the variable declarations and added the shuffle function to the “Begin Experiment” tab of the relevant routine.

But it still won’t start.

I played around with the originPath parameter — the .xlsx files that I need to read in reside in the subfolder auxFiles.

I have tried both originPath: “auxFiles” and originPath: “/auxFiles” as well as originPath “\auxFiles” — which is the correct one?

Anyway, it still does not work.

Would you have any other pointers as to what might need fixing?




Looks like a couple of syntax errors. E.g., for loops are different in JS.

for (let thisRater of catchList) {
  // code

Also, boolean types are lower case, e.g., False becomes false.

If you press F12, you can see the developers tools in a browser. These are useful for debugging and finding out what went wrong with your task.


Ah OK — Fixed these now but still no luck.

Did you mean press F12 in PsychoPy or in the browser window? Either way, nothing happens when I press F12.

I also tried from within the gitHub page.

Which is the correct syntax for the originPath for the .xlsx files? I assume I denote subdirectories with / but do I need to just provide the directory or also the filename?

So, if the file is ’names.xlsx’ in subdirectory. ‘auxFiles’, do I need to set originPath: “/auxFiles” or “/auxFiles/names.xlsx”?

Many thanks!