setLength( length, headLength = length * 0.2, headWidth = headLength * 0.2 ) {

		this.line.scale.set( 1, Math.max( 0.0001, length - headLength ), 1 ); // see #17458
		this.line.updateMatrix();

		this.cone.scale.set( headWidth, headLength, headWidth );
		this.cone.position.y = length;
		this.cone.updateMatrix();

	}
Sets the length of the helper.

@param {number} length - Length of the arrow in world units.
@param {number} [headLength=length*0.2] - The length of the head of the arrow.
@param {number} [headWidth=headLength*0.2] - The width of the head of the arrow.