I had the same question here: Thicker lines on Cube and/or applying TubeFilter to CubeSource?
It seems like this feature does not exist in vtk.js at this moment, unfortunately 
I was somewhat able to work around it for my use case by defining a fill color and setting the opacity low so instead of a wireframe of a cube, it’s a lightly colored, semi-transparent cube.
cubeActor.getProperty().setColor(0, 1, 0);
cubeActor.getProperty().setOpacity(0.15);