Slice three.js geometry with a plane.
var plane = new THREE.Plane(new THREE.Vector3(0, 0, 1), 0);
var geom = new THREE.BoxGeometry(1, 1, 1);
geom = sliceGeometry(geom, plane);
var material = new THREE.MeshBasicMaterial({ wireframe: true });
var mesh = new THREE.Mesh(geom, material);
scene.add(mesh);