use batchLoadFn.apply instead of directly invoking batchLoadFn (#182)

* use batchLoadFn.apply instead of directly invoking batchLoadFn

use batchLoadFn.apply(loader,[keys]) instead of batchLoadFn(keys) to allow acces to the actual dataloader. Needed for example to use dataloader.prime() in the batch loader function.

* Add tests

README.md

@@ -90,7 +90,19 @@ minimal outgoing data requests.
 #### Batch Function
 
 A batch loading function accepts an Array of keys, and returns a Promise which
-resolves to an Array of values. There are a few constraints that must be upheld:
+resolves to an Array of values or Error instances. The loader itself is provided
+as the `this` context.
+
+```js
+async function batchFunction(keys) {
+  const results = await db.fetchAllKeys(keys)
+  return keys.map(key => results[key] || new Error(`No result for ${key}`))
+}
+
+const loader = new DataLoader(batchFunction)
+```
+
+There are a few constraints this function must uphold:
 
  * The Array of values must be the same length as the Array of keys.
  * Each index in the Array of values must correspond to the same index in the Array of keys.
@@ -116,7 +128,7 @@ with the original keys `[ 2, 9, 6, 1 ]`:
 [
   { id: 2, name: 'San Francisco' },
   { id: 9, name: 'Chicago' },
-  null,
+  null, // or perhaps `new Error()`
   { id: 1, name: 'New York' }
 ]
 ```

src/__tests__/dataloader.test.js

@@ -30,6 +30,19 @@ describe('Primary API', () => {
     expect(value1).toBe(1);
   });
 
+  it('references the loader as "this" in the batch function', async () => {
+    let that;
+    const loader = new DataLoader(async function (keys) {
+      that = this;
+      return keys;
+    });
+
+    // Trigger the batch function
+    await loader.load(1);
+
+    expect(that).toBe(loader);
+  });
+
   it('supports loading multiple keys in one call', async () => {
     const identityLoader = new DataLoader(keys => Promise.resolve(keys));
 

src/index.js

@@ -253,7 +253,8 @@ function dispatchQueueBatch<K, V>(
 
   // Call the provided batchLoadFn for this loader with the loader queue's keys.
   var batchLoadFn = loader._batchLoadFn;
-  var batchPromise = batchLoadFn(keys);
+  // Call with the loader as the `this` context.
+  var batchPromise = batchLoadFn.call(loader, keys);
 
   // Assert the expected response from batchLoadFn
   if (!batchPromise || typeof batchPromise.then !== 'function') {