JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdn.rawgit.com/mrdoob/three.js/master/build/three.min.js"></script>

CSS

body {
	  margin: 0;
}

JavaScript

const A = new THREE.Vector3( 1, 0, 0 );
const B = new THREE.Vector3( 0, 1, 0 );
const C = new THREE.Vector3( - 1, 0, 0 );

const B_ON_AC = new THREE.Vector3();
const direction = new THREE.Vector3();

const lineSegment = new THREE.Line3( A, C );
lineSegment.closestPointToPoint( B, true, B_ON_AC );

direction.subVectors( B, B_ON_AC ).normalize();
B.add( direction.multiplyScalar( 10 ) );

console.log( B );