Check type of payload creator

README.md

@@ -22,7 +22,9 @@ If you don’t use [npm](https://www.npmjs.com), you may grab the latest [UMD](h
 import { createAction } from 'redux-actions';
 ```
 
-Wraps an action creator so that its return value is the payload of a Flux Standard Action. If no payload creator is passed, or if it's not a function, the identity function is used.
+Wraps an action creator so that its return value is the payload of a Flux Standard Action. 
+
+`payloadCreator` must be a function or `undefined`. If `payloadCreator` is `undefined`, the identity function is used.
 
 Example:
 

src/__tests__/createAction-test.js

@@ -23,7 +23,21 @@ describe('createAction()', () => {
       });
     });
 
-    it('uses identity function if payloadCreator is not a function', () => {
+    it('should throw an error if payloadCreator is not a function or undefined', () => {
+      const wrongTypePayloadCreators = [1, false, null, 'string', {}, []];
+
+      wrongTypePayloadCreators.forEach(wrongTypePayloadCreator => {
+        expect(() => {
+          createAction(type, wrongTypePayloadCreator);
+        })
+        .to.throw(
+          Error,
+          'Expected payloadCreator to be a function or undefined'
+        );
+      });
+    });
+
+    it('uses identity function if payloadCreator is undefined', () => {
       const actionCreator = createAction(type);
       const foobar = { foo: 'bar' };
       const action = actionCreator(foobar);
@@ -35,7 +49,7 @@ describe('createAction()', () => {
     });
 
     it('accepts a second parameter for adding meta to object', () => {
-      const actionCreator = createAction(type, null, ({ cid }) => ({ cid }));
+      const actionCreator = createAction(type, undefined, ({ cid }) => ({ cid }));
       const foobar = { foo: 'bar', cid: 5 };
       const action = actionCreator(foobar);
       expect(action).to.deep.equal({
@@ -70,7 +84,7 @@ describe('createAction()', () => {
     });
 
     it('sets error to true if payload is an Error object and meta is provided', () => {
-      const actionCreator = createAction(type, null, (_, meta) => meta);
+      const actionCreator = createAction(type, undefined, (_, meta) => meta);
       const errObj = new TypeError('this is an error');
 
       const errAction = actionCreator(errObj, { foo: 'bar' });
@@ -94,7 +108,7 @@ describe('createAction()', () => {
       });
 
       const baz = '1';
-      const actionCreator = createAction(type, null, () => ({ bar: baz }));
+      const actionCreator = createAction(type, undefined, () => ({ bar: baz }));
       expect(actionCreator()).to.deep.equal({
         type,
         meta: {

src/createAction.js

@@ -1,10 +1,13 @@
 import identity from 'lodash/identity';
+import isFunction from 'lodash/isFunction';
 import isUndefined from 'lodash/isUndefined';
+import invariant from 'invariant';
 
-export default function createAction(type, payloadCreator, metaCreator) {
-  const finalPayloadCreator = typeof payloadCreator === 'function'
-    ? payloadCreator
-    : identity;
+export default function createAction(type, payloadCreator = identity, metaCreator) {
+  invariant(
+    isFunction(payloadCreator),
+    'Expected payloadCreator to be a function or undefined'
+  );
 
   const actionCreator = (...args) => {
     const hasError = args[0] instanceof Error;
@@ -13,7 +16,7 @@ export default function createAction(type, payloadCreator, metaCreator) {
       type
     };
 
-    const payload = hasError ? args[0] : finalPayloadCreator(...args);
+    const payload = hasError ? args[0] : payloadCreator(...args);
     if (!isUndefined(payload)) {
       action.payload = payload;
     }
@@ -23,7 +26,7 @@ export default function createAction(type, payloadCreator, metaCreator) {
       action.error = true;
     }
 
-    if (typeof metaCreator === 'function') {
+    if (isFunction(metaCreator)) {
       action.meta = metaCreator(...args);
     }