react-ssr-thunk.js

function dispatch(action) {
    // Promise tracker middleware
    const nextAction = reduxThunkMiddleware(action)

    // Because react-thunk returns the result of the
    // thunk, our async function returns a promise
    // Which means this statement will be true
    if (Promise.resolve(nextAction) === nextAction) {
        // Tracking a promise is a side effect of our middleware
        track(nextAction)
    }

    return nextAction
}

function reduxThunkMiddleware(action) {
    // If the action is a function, double dispatch
    // and do not pass action to next middleware
    if (typeof action === 'function') {
        // Return the result of the thunk
        return action(dispatch, getState)
    }

    return next(action)
}