Prevent browser from making the same async calls as the server

I am following this tutorial: https://crypt.codemancers.com/posts/2017-06-03-reactjs-server-side-rendering-with-router-v4-and-redux/ which i think is the 'standard' way of doing server side rendering in react (?).

Basically what happens is i use react router (v4) to make a tree of all the components that are about to get rendered:

const promises = branch.map(({ route }) => {
    return route.component.fetchInitialData
        ? route.component.fetchInitialData(store.dispatch)
        : Promise.resolve();
});

Wait for all those promises to resolve and then call renderToString.

In my components i have a static function called fetchInitialData which looks like this:

class Users extends React.Component {
    static fetchInitialData(dispatch) {
        return dispatch(getUsers());
    }
    componentDidMount() {
        this.props.getUsers();
    }
    render() {
        ...
    }
}

export default connect((state) => {
    return { users: state.users };
}, (dispatch) => {
    return bindActionCreators({ getUsers }, dispatch);
})(Users);

And all this works great except that getUsers is called both on the server and the client.

I could of course check if any users are loaded and not call getUsers in componentDidMount but there must be a better, explicit way to not make the async call twice.

After getting more and more familiar with react i feel fairly confident i have a solution.

I pass a browserContext object along all rendered routes, much like staticContext on the server. In the browserContext i set two values; isFirstRender and usingDevServer. isFirstRender is only true while the app is rendered for the first time and usingDevServer is only true when using the webpack-dev-server.

const store = createStore(reducers, initialReduxState, middleware);

The entry file for the browser side:

const browserContext = {
    isFirstRender: true,
    usingDevServer: !!process.env.USING_DEV_SERVER
};

const BrowserApp = () => {
    return (
        <Provider store={store}>
            <BrowserRouter>
                {renderRoutes(routes, { store, browserContext })}
            </BrowserRouter>
        </Provider>
    );
};

hydrate(
    <BrowserApp />,
    document.getElementById('root')
);

browserContext.isFirstRender = false;

USING_DEV_SERVER is defined in the webpack config file using webpack.DefinePlugin

Then i wrote a HOC component that uses this information to fetch initial data only in situations where it is needed:

function wrapInitialDataComponent(Component) {
    class InitialDatacomponent extends React.Component {
        componentDidMount() {
            const { store, browserContext, match } = this.props;

            const fetchRequired = browserContext.usingDevServer || !browserContext.isFirstRender;

            if (fetchRequired && Component.fetchInitialData) {
                Component.fetchInitialData(store.dispatch, match);
            }
        }
        render() {
            return <Component {...this.props} />;
        }
    }

    // Copy any static methods.
    hoistNonReactStatics(InitialDatacomponent, Component);

    // Set display name for debugging.
    InitialDatacomponent.displayName = `InitialDatacomponent(${getDisplayName(Component)})`;

    return InitialDatacomponent;
}

And then the last thing to do is wrap any components rendered with react router with this HOC component. I did this by simply iterating over the routes recursively:

function wrapRoutes(routes) {
    routes.forEach((route) => {
        route.component = wrapInitialDataComponent(route.component);

        if (route.routes) {
            wrapRoutes(route.routes);
        }
    });
}

const routes = [ ... ];

wrapRoutes(routes);

And that seems to do the trick :)