three.js/src/extras/curves/LineCurve.js

/**************************************************************
 *	Line
 **************************************************************/

THREE.LineCurve = function ( v1, v2 ) {

	this.v1 = v1;
	this.v2 = v2;

};

THREE.LineCurve.prototype = Object.create( THREE.Curve.prototype );
THREE.LineCurve.prototype.constructor = THREE.LineCurve;

THREE.LineCurve.prototype.getPoint = function ( t ) {

	var point = this.v2.clone().sub(this.v1);
	point.multiplyScalar( t ).add( this.v1 );

	return point;

};

// Line curve is linear, so we can overwrite default getPointAt

THREE.LineCurve.prototype.getPointAt = function ( u ) {

	return this.getPoint( u );

};

THREE.LineCurve.prototype.getTangent = function( t ) {

	var tangent = this.v2.clone().sub(this.v1);

	return tangent.normalize();

};