simple middleware with sync && async functions support

middleware.js

const middy = (middList) => {
    return (event) => {
        let middStack = [...middList];
        let lastRetValue;
        const next = () => {
            const curr = middStack.shift();
            if (curr) {
               const retVal = curr(event, next);
               lastRetValue = retVal;
               if (retVal instanceof Promise && middStack.length){
                  return retVal.then(next);
               }
               return retVal;
            }
            return lastRetValue;
        }
        return next();
    };
};