getTangent( t, optionalTarget = new Vector3() ) {

		const tangent = optionalTarget;

		const u = this.knots[ 0 ] + t * ( this.knots[ this.knots.length - 1 ] - this.knots[ 0 ] );
		const ders = NURBSUtils.calcNURBSDerivatives( this.degree, this.knots, this.controlPoints, u, 1 );
		tangent.copy( ders[ 1 ] ).normalize();

		return tangent;

	}
Returns a unit vector tangent for the given interpolation factor.

@param {number} t - The interpolation factor.
@param {Vector3} [optionalTarget] - The optional target vector the result is written to.
@return {Vector3} The tangent vector.