Creating an experimental stimuli in JsPsych

Question

I am a beginner in jsPsych and am trying to build an experimental task in which there will 3 boxes within which there will be a bar with varying values. The boxes should be occluded and when the participant hovers their mouse over the boxes with their mouse, the boxes should get un-occluded and the participant should be able to see the contents of the box (i.e., the bar) - I can see that the serial-reaction-time-mouse allows us to make a grid of boxes i.e., squares and we can use the mouse to click on each of the boxes but, I don't know how to get the boxes to do the occluding and un-occluding and how to make the bars visible upon un-occlusion.

Thank you for the help in advance!