in src/Transition.js [209:222]
updateStatus(mounting = false, nextStatus) {
if (nextStatus !== null) {
// nextStatus will always be ENTERING or EXITING.
this.cancelNextCallback();
if (nextStatus === ENTERING) {
this.performEnter(mounting);
} else {
this.performExit();
}
} else if (this.props.unmountOnExit && this.state.status === EXITED) {
this.setState({ status: UNMOUNTED });
}
}