Reusable components that are shared between isotopes-and-atomic-mass, build-a-nucleus and build-an-atom.
By PhET Interactive Simulations https://phet.colorado.edu/
The PhET Development Overview is the most complete guide to PhET Simulation Development. This guide includes how to obtain simulation code and its dependencies, notes about architecture & design, how to test and build the sims, as well as other important information.
See the license.