Interceptor middleware with Promises support

interceptor-with-promises.js

const createInterceptorMiddleware = (interceptors) => (store) => (next) => (action) => {
  Promise.all(
    interceptors
      .filter(interceptor => interceptor.type === action.type)
      .map(interceptor => {
        const result = interceptor.handler(action, store.dispatch, store.getState);
        return result instanceof Promise ? result : Promise.resolve(result);
      })
  )
    .then(() => next(action))
    .catch(e => console.error(e));
}

const interceptors = [
  {type: 'INCREMENT', handler: () => doSomeApiStuff()},
  {type: 'DECREMENT', handler: () => doOtherApiStuff()}
];

const middleware = createInterceptorMiddleware(interceptors);

In this snippet the line:

return result instanceof Promise ? result : Promise.resolve(result);

in context of Promise.all is equivalent to:

return result;

Try this:

const getDelayedPromisedValue = value => new Promise((resolve, reject) => {
  setTimeout(() => resolve(value), 1000);
});

const getImmediatePromisedValue = value => Promise.resolve(value);

const getValue = value => value;

Promise.all([getDelayedPromisedValue(10), getImmediatePromisedValue(5), getValue(1)]).then(values => console.log(values));