The WASM/Typescript/JavaScript version of Rapier is fully cross-platform deterministic. This means that running the same simulation (with the same initial conditions) using the same version of Rapier, on two different machines (even with different browsers, operating systems, and processors, will give the exact same results. In particular, creating a snapshot of the World with world.createSnapshot() and taking a MD5 hash of the resulting byte array will return the exact same hash on different machines (assuming the snapshot itself is taken after the same number of timesteps).


Two simulations run with the same initial conditions if all the simulation parameters are initialized with the same values, rigid-bodies/colliders/joints are constructed the same way, and they are added/removed in the exact same order.

Keep in mind that all the values used to initialize the physics simulation must result from cross-platform deterministic operations to in order to preserve the determinism of the physics engine itself. Otherwise, you won't get the exact same initial conditions on all the platfroms. In particular, transcendental functions like Math.sin, Math.cos are not cross-platform determinism and may give different results on different platforms.