applyMiddleware.js

// Apply a series of reducers, continuation passing style, to the state object,
// and return the transformed state
export function applyMiddleware(
    keysDown:Object,
    actions:Object,
    gameData:Object,
    oldState:Object,
    initialState:Object,
    ...reducers
):Object {

    // Keep track of what reducer we're on, because when a reducer calls next()
    // it needs to call the next reducer in the array
    let index = -1;

    const next = newState => {

        index = index + 1;

        if( !reducers[ index ] && index < reducers.length ) {
            console.error( 'One of your reducers is undefined: ', reducers );
            throw new Error( 'One of your reducers is undefined.' );
        }

        return reducers[ index ] ?
            // Either call the next reducer...
            reducers[ index ]( keysDown, actions, gameData, oldState, newState, next ) :
            // Or we're at the end
            newState;

    };

    return {
        ...oldState,
        ...next( initialState ),
    };

}