functions are accepted as first argument instead of promises

Author: lsimone

README.md

@@ -68,12 +68,12 @@ If you are using [Create-React-App](https://github.com/facebookincubator/create-
 * `fetch.once(bodyOrFunction, init): fetch` - Alias for mockResponseOnce
 * `fetch.mockResponses(...responses): fetch` - Mock multiple fetch calls independently
   * Each argument is an array taking `[bodyOrFunction, init]`
-* `fetch.mockReject(error): fetch` - Mock all fetch calls, letting them fail directly
-* `fetch.mockRejectOnce(error): fetch` - Let the next fetch call fail directly
+* `fetch.mockReject(errorOrFunction): fetch` - Mock all fetch calls, letting them fail directly
+* `fetch.mockRejectOnce(errorOrFunction): fetch` - Let the next fetch call fail directly
 
-### Promises
+### Functions
 
-Instead of passing body, it is possible to pass a function that returns a promise too.
+Instead of passing body, it is also possible to pass a function that returns a promise.
 The promise should resolve with an object containing body and init props
 
 i.e:

src/index.js

@@ -28,34 +28,34 @@ function ResponseWrapper (body, init) {
   return new ActualResponse(body, init)
 }
 
-const isAPromise = obj => obj && obj.then && (typeof obj.then === 'function')
+const isFn = unknown => typeof unknown === 'function'
 
-const resolve = (bodyOrFunction, init) => () => isAPromise(bodyOrFunction) ?
-      bodyOrFunction.then((res) => new ResponseWrapper(res.body, res.init))
-      : Promise.resolve(new ResponseWrapper(bodyOrFunction, init))
+const normalizeResponse = (bodyOrFunction, init) => () => isFn(bodyOrFunction) ?
+    bodyOrFunction().then(({body, init}) => new ResponseWrapper(body, init)) :
+    Promise.resolve(new ResponseWrapper(bodyOrFunction, init))
+
+const normalizeError = errorOrFunction => () => isFn(errorOrFunction) ?
+    errorOrFunction :
+    () => Promise.reject(errorOrFunction)
 
 const fetch = jest.fn()
 fetch.Headers = Headers
 fetch.Response = ResponseWrapper
 fetch.Request = Request
-fetch.mockResponse = (bodyOrFunction, init) => fetch.mockImplementation(resolve(bodyOrFunction, init))
+fetch.mockResponse = (bodyOrFunction, init) => fetch.mockImplementation(normalizeResponse(bodyOrFunction, init))
 
-fetch.mockReject = error => {
-  return fetch.mockImplementation(() => Promise.reject(error))
-}
+fetch.mockReject = errorOrFunction => fetch.mockImplementation(normalizeError(errorOrFunction))
 
-const mockResponseOnce = (bodyOrFunction, init) => fetch.mockImplementationOnce(resolve(bodyOrFunction, init))
+const mockResponseOnce = (bodyOrFunction, init) => fetch.mockImplementationOnce(normalizeResponse(bodyOrFunction, init))
 
 fetch.mockResponseOnce = mockResponseOnce
 
 fetch.once = mockResponseOnce
 
-fetch.mockRejectOnce = errorOrPromise => {
-  return fetch.mockImplementationOnce(() => isAPromise(errorOrPromise) ? errorOrPromise : Promise.reject(error))
-}
+fetch.mockRejectOnce = errorOrFunction => fetch.mockImplementationOnce(normalizeError(errorOrFunction))
 
 fetch.mockResponses = (...responses) => {
-  responses.forEach(([bodyOrFunction, init]) => fetch.mockImplementationOnce(resolve(bodyOrFunction, init)))
+  responses.forEach(([bodyOrFunction, init]) => fetch.mockImplementationOnce(normalizeResponse(bodyOrFunction, init)))
   return fetch
 }