Old name ""EJS"" Easy Java Simulations or new name since 2014 EJSS, or Easy JavaScript Simulations, is an open-source software tool, part of the Open Source Physics project, designed for the creation of discrete computer simulations.A discrete computer simulation, or simply a computer simulation, is a computer program that tries to reproduce, for pedagogical or scientific purposes, a natural phenomenon through the visualization of the different states that it can have.