Switch.js

class Switch extends React.Component {
    render() {
        return (
            <RouterContext.Consumer>
                {context => {
                    invariant(
                        context,
                        'You should not use <Switch> outside a <Router>'
                    );

                    const location = this.props.location || context.location;

                    let element, match;

                    // We use React.Children.forEach instead of React.Children.toArray().find()
                    // here because toArray adds keys to all child elements and we do not want
                    // to trigger an unmount/remount for two <Route>s that render the same
                    // component at different URLs.
                    React.Children.forEach(this.props.children, child => {
                        if (match == null && React.isValidElement(child)) {
                            element = child;

                            const path = child.props.path || child.props.from;

                            match = path
                                ? matchPath(location.pathname, {
                                      ...child.props,
                                      path
                                  })
                                : context.match;
                        }
                    });

                    return match
                        ? React.cloneElement(element, {
                              location,
                              computedMatch: match
                          })
                        : null;
                }}
            </RouterContext.Consumer>
        );
    }
}