Does the three.js library come out of the box with method to detect non watertight mesh and method to close holes?
¹ The closest I can think of is
mergeVertices from the Three.js addon
BufferGeometryUtils. It can be used to cover-up small gaps. Also, there is another add-on
ConvexGeometry, which will transform a geometry into a convex polyhedron, which is guaranteed to be watertight … but you won’t want it.