feat(createEpicMiddleware): warn about reusing middleware

Warn about #389

Author: Max Kostow

src/createEpicMiddleware.js

@@ -29,6 +29,10 @@ export function createEpicMiddleware(rootEpic, options = defaultOptions) {
   let store;
 
   const epicMiddleware = _store => {
+    if (process.env.NODE_ENV !== 'production' && store) {
+      // https://github.com/redux-observable/redux-observable/issues/389
+      require('./utils/console').warn('this middleware is already associated with a store. createEpicMiddleware should be called for every store.\n\n See https://goo.gl/2GQ7Da');
+    }
     store = _store;
 
     return next => {

src/utils/console.js

@@ -8,3 +8,9 @@ export const deprecate = (typeof console === 'object' && typeof console.warn ===
     }
   }
   : () => {};
+
+export const warn = (typeof console === 'object' && typeof console.warn === 'function')
+  ? msg => {
+    console.warn(`redux-observable | WARNING: ${msg}`);
+  }
+  : () => {};

test/createEpicMiddleware-spec.js

@@ -29,6 +29,22 @@ describe('createEpicMiddleware', () => {
     store.dispatch({ type: 'FIRST_ACTION_TO_TRIGGER_MIDDLEWARE' });
   });
 
+  it('should warn about reusing the epicMiddleware', () => {
+    sinon.spy(console, 'warn');
+    const reducer = (state = [], action) => state.concat(action);
+    const epic = (action$, store) => action$
+      .ofType('PING')
+      ::map(() => store.dispatch({ type: 'PONG' }))
+      ::ignoreElements();
+
+    const middleware = createEpicMiddleware(epic);
+    createStore(reducer, applyMiddleware(middleware));
+    createStore(reducer, applyMiddleware(middleware));
+    expect(console.warn.callCount).to.equal(1);
+    expect(console.warn.getCall(0).args[0]).to.equal('redux-observable | WARNING: this middleware is already associated with a store. createEpicMiddleware should be called for every store.\n\n See https://goo.gl/2GQ7Da');
+    console.warn.restore();
+  });
+
   it('should warn about improper use of dispatch function', () => {
     sinon.spy(console, 'warn');
     const reducer = (state = [], action) => state.concat(action);