var dir = new THREE.Vector3( 1, 0, 0 );
var origin = new THREE.Vector3( 0, 0, 0 );
var length = 1;
var hex = 0xffff00;
var arrowHelper = new THREE.ArrowHelper( dir, origin, length, hex );
scene.add( arrowHelper );
dir -- Vector3 -- direction from origin. Must be a unit vector.
origin -- Vector3
length -- scalar
hex -- hexadecimal value to define color ex:0xffff00
headLength -- The length of the head of the arrow
headWidth -- The length of the width of the arrow
This creates an arrow starting in origin in the direction dir for a certain length. It is also possible to change color.