slerp( qb, t ) {

		if ( t === 0 ) return this;
		if ( t === 1 ) return this.copy( qb );

		const x = this._x, y = this._y, z = this._z, w = this._w;

		// http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/slerp/

		let cosHalfTheta = w * qb._w + x * qb._x + y * qb._y + z * qb._z;

		if ( cosHalfTheta < 0 ) {

			this._w = - qb._w;
			this._x = - qb._x;
			this._y = - qb._y;
			this._z = - qb._z;

			cosHalfTheta = - cosHalfTheta;

		} else {

			this.copy( qb );

		}

		if ( cosHalfTheta >= 1.0 ) {

			this._w = w;
			this._x = x;
			this._y = y;
			this._z = z;

			return this;

		}

		const sqrSinHalfTheta = 1.0 - cosHalfTheta * cosHalfTheta;

		if ( sqrSinHalfTheta <= Number.EPSILON ) {

			const s = 1 - t;
			this._w = s * w + t * this._w;
			this._x = s * x + t * this._x;
			this._y = s * y + t * this._y;
			this._z = s * z + t * this._z;

			this.normalize(); // normalize calls _onChangeCallback()

			return this;

		}

		const sinHalfTheta = Math.sqrt( sqrSinHalfTheta );
		const halfTheta = Math.atan2( sinHalfTheta, cosHalfTheta );
		const ratioA = Math.sin( ( 1 - t ) * halfTheta ) / sinHalfTheta,
			ratioB = Math.sin( t * halfTheta ) / sinHalfTheta;

		this._w = ( w * ratioA + this._w * ratioB );
		this._x = ( x * ratioA + this._x * ratioB );
		this._y = ( y * ratioA + this._y * ratioB );
		this._z = ( z * ratioA + this._z * ratioB );

		this._onChangeCallback();

		return this;

	}
Performs a spherical linear interpolation between quaternions.

@param {Quaternion} qb - The target quaternion.
@param {number} t - The interpolation factor in the closed interval `[0, 1]`.
@return {Quaternion} A reference to this quaternion.