Accept a fetch option for dependency injection

README.md

@@ -113,6 +113,14 @@ fetch('https://example.com', {
 })
 ```
 
+### options.fetch
+
+```js
+options.fetch = window.fetch
+```
+
+It is possible to pass an `options.fetch` function, which this library will use in place of the browser's `fetch` method. This may be useful for unit-testing.
+
 ## Environment
 
 Node and browser (es2015)

src/apigen.js

@@ -70,7 +70,12 @@ function fetchMethod (methodName, url, definition, config) {
     const fetchConfiguration = {body, method: 'POST'}
     Object.assign(fetchConfiguration, config.fetchConfiguration)
 
-    fetch(url, fetchConfiguration).then(response => {
+    // We have to use the naked `fetch` on Safari (or `this` will be wrong),
+    // unless the config has provided one to use instead:
+    const promise = config.fetch != null
+      ? config.fetch(url, fetchConfiguration)
+      : fetch(url, fetchConfiguration)
+    promise.then(response => {
       if (response.status >= 200 && response.status < 300) {
         return response.json()
       } else {