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soft_body_attachments

A particle can also be attached directly to a rigid-body (SoftBody.attachParticle), which is the simplest way of combining the two kinds of bodies: a rope tied to a swinging ball, a flag attached to its pole, etc. Unlike pinning, an attachment is a two-way point-to-point constraint: the rigid-body holds the particle, and the particle pulls the rigid-body back. The anchor is the position the particle has when the attachment is created, expressed in the local frame of the rigid-body. Note that a particle attached twice keeps both of its attachments, and that an attachment is undone with SoftBody.detachParticle:

// Attach the last particle of a rope to a rigid box, at the particle's position.
let ropeDesc = RAPIER.SoftBodyDesc.rope({ x: 8.0, y: 9.0 }, { x: 12.0, y: 9.0 }, 25)
    .setPinnedParticles([0])
    .setSoftness(40.0, 1.0);
let rope = world.createSoftBody(ropeDesc);
let last = rope.particlePosition(24);
let weight = world.createRigidBody(RAPIER.RigidBodyDesc.dynamic().setTranslation(last.x, last.y - 0.4));
world.createCollider(RAPIER.ColliderDesc.cuboid(0.3, 0.3).setDensity(2.0), weight);
rope.attachParticle(24, weight);