I would also recommend using Builder to create your online studies. PsychoJS tries to mirror the PsychoPy API as much as possible, so code you write in Python does not need to change too much to work in PsychoJS. The crib sheet in the link above helps with the translation process if you are having issues translating code components from Builder.
However, if you want a working local copy of the PsychoJS libs because you want to see the code, or maybe even change it to see how things work, you can use the local debugging mode from the Runner (globe with bug icon). After you have created your Pavlovia project, clicking the local debug mode will download the PsychoJS libs into your projects HTML folder, so you can run your online task from a locally served browser.