SOURCE

const p1 = new THREE.Vector3(0,0,0);
const p2 = new THREE.Vector3(5,0,0);
const p3 = new THREE.Vector3(-1,0,1);

const line1 = new THREE.Line3(p1, p2);
const res = line1.closestPointToPointParameter(p3, false);

console.log(res);