existingAction( clip, optionalRoot ) {

		const root = optionalRoot || this._root,
			rootUuid = root.uuid,

			clipObject = typeof clip === 'string' ?
				AnimationClip.findByName( root, clip ) : clip,

			clipUuid = clipObject ? clipObject.uuid : clip,

			actionsForClip = this._actionsByClip[ clipUuid ];

		if ( actionsForClip !== undefined ) {

			return actionsForClip.actionByRoot[ rootUuid ] || null;

		}

		return null;

	}
Returns an existing animation action for the passed clip.

@param {AnimationClip|string} clip - An animation clip or alternatively the name of the animation clip.
@param {Object3D} [optionalRoot] - An alternative root object.
@return {?AnimationAction} The animation action. Returns `null` if no action was found.