Validate shape of cacheMap on initialization (#86)

* Validate shape of cacheMap on initialization

* Follow up review:

* Functions should avoid being defined within another function body if possible
* Naming consistency
* Flow typing
* Arrow functions with a single return statement should not use block format
* Check typeof 'function' instead of non-falsey
* filter(Boolean) to remove falsey values

* Update abuse-test.js

* map().filter() can be replaced by .filter()

* 80ch

* Flow

src/__tests__/abuse-test.js

@@ -150,4 +150,17 @@ describe('Provides descriptive error messages for API abuse', () => {
     );
   });
 
+  it('Cache should have get, set, delete, and clear methods', async () => {
+    class IncompleteMap {
+      get() {}
+    }
+
+    expect(() => {
+      var incompleteMap = new IncompleteMap();
+      var options = { cacheMap: incompleteMap };
+      new DataLoader(keys => keys, options); // eslint-disable-line no-new
+    }).to.throw(
+      'Custom cacheMap missing methods: set, delete, clear'
+    );
+  });
 });

src/index.js

@@ -54,8 +54,7 @@ export default class DataLoader<K, V> {
     }
     this._batchLoadFn = batchLoadFn;
     this._options = options;
-    this._promiseCache =
-      options && options.cacheMap || (new Map(): Map<K,Promise<V>>);
+    this._promiseCache = getValidCacheMap(options);
     this._queue = [];
   }
 
@@ -306,6 +305,24 @@ function failedDispatch<K, V>(
   });
 }
 
+function getValidCacheMap<K, V>(
+  options: ?Options<K, V>
+): CacheMap<K, Promise<V>> {
+  var cacheMap = options && options.cacheMap;
+  if (!cacheMap) {
+    return new Map();
+  }
+  var cacheFunctions = [ 'get', 'set', 'delete', 'clear' ];
+  var missingFunctions = cacheFunctions
+    .filter(fnName => cacheMap && typeof cacheMap[fnName] !== 'function');
+  if (missingFunctions.length !== 0) {
+    throw new TypeError(
+      'Custom cacheMap missing methods: ' + missingFunctions.join(', ')
+    );
+  }
+  return cacheMap;
+}
+
 // Private
 type LoaderQueue<K, V> = Array<{
   key: K;