fix: propagate batchFn sync throws to the loader instead of crashing

src/__tests__/abuse.test.js

@@ -97,6 +97,26 @@ describe('Provides descriptive error messages for API abuse', () => {
     );
   });
 
+  it('Batch function must return a Promise, not error synchronously', async () => {
+    // $FlowExpectError
+    const badLoader = new DataLoader<number, number>(() => {
+      throw new Error("Mock Synchronous Error")
+    });
+
+    let caughtError;
+    try {
+      await badLoader.load(1);
+    } catch (error) {
+      caughtError = error;
+    }
+    expect(caughtError).toBeInstanceOf(Error);
+    expect((caughtError: any).message).toBe(
+      'DataLoader must be constructed with a function which accepts ' +
+      'Array<key> and returns Promise<Array<value>>, but the function ' +
+      `errored synchronously: Error: Mock Synchronous Error.`
+    );
+  });
+
   it('Batch function must return a Promise, not a value', async () => {
     // Note: this is returning the keys directly, rather than a promise to keys.
     // $FlowExpectError

src/index.js

@@ -304,7 +304,16 @@ function dispatchBatch<K, V>(
 
   // Call the provided batchLoadFn for this loader with the batch's keys and
   // with the loader as the `this` context.
-  var batchPromise = loader._batchLoadFn(batch.keys);
+  var batchPromise;
+  try {
+    batchPromise = loader._batchLoadFn(batch.keys);
+  } catch (e) {
+    return failedDispatch(loader, batch, new TypeError(
+      'DataLoader must be constructed with a function which accepts ' +
+      'Array<key> and returns Promise<Array<value>>, but the function ' +
+      `errored synchronously: ${String(e)}.`
+    ));
+  }
 
   // Assert the expected response from batchLoadFn
   if (!batchPromise || typeof batchPromise.then !== 'function') {