From 483f743683cdb793b33690e7b494a966a4759606 Mon Sep 17 00:00:00 2001
From: Peter Sieg <chasingmaxwell@gmail.com>
Date: Mon, 3 Aug 2020 17:09:45 -0400
Subject: [PATCH] [BREAKING] Update types to allow array of promises in
 batchLoadFn return

---
 README.md                        |  3 ++-
 src/__tests__/dataloader.test.js | 17 +++++++++++++++++
 src/index.js                     |  2 +-
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index e192a18..c2f5b28 100644
--- a/README.md
+++ b/README.md
@@ -389,7 +389,8 @@ access permissions and consider creating a new instance per web request.
 Create a new `DataLoader` given a batch loading function and options.
 
 - *batchLoadFn*: A function which accepts an Array of keys, and returns a
-  Promise which resolves to an Array of values.
+  Promise which resolves to an Array of values or promises which resolve to
+  values.
 
 - *options*: An optional object of options:
 
diff --git a/src/__tests__/dataloader.test.js b/src/__tests__/dataloader.test.js
index 47b2059..63ec745 100644
--- a/src/__tests__/dataloader.test.js
+++ b/src/__tests__/dataloader.test.js
@@ -91,6 +91,23 @@ describe('Primary API', () => {
     expect(values).toEqual([ 'a', 'b', new Error('Bad Key') ]);
   });
 
+  it('supports loading with resolved and rejected promises', async () => {
+    const identityLoader = new DataLoader(keys =>
+      Promise.resolve(
+        keys.map(key => (key === 'bad' ?
+          Promise.reject(new Error('Bad Key')) :
+          Promise.resolve(key))
+        )
+      )
+    );
+
+    const promiseAll = identityLoader.loadMany([ 'a', 'b', 'bad' ]);
+    expect(promiseAll).toBeInstanceOf(Promise);
+
+    const values = await promiseAll;
+    expect(values).toEqual([ 'a', 'b', new Error('Bad Key') ]);
+  });
+
   it('batches multiple requests', async () => {
     const [ identityLoader, loadCalls ] = idLoader<number>();
 
diff --git a/src/index.js b/src/index.js
index 6250df9..f48e26c 100644
--- a/src/index.js
+++ b/src/index.js
@@ -10,7 +10,7 @@
 // A Function, which when given an Array of keys, returns a Promise of an Array
 // of values or Errors.
 export type BatchLoadFn<K, V> =
-  (keys: $ReadOnlyArray<K>) => Promise<$ReadOnlyArray<V | Error>>;
+  (keys: $ReadOnlyArray<K>) => Promise<$ReadOnlyArray<V | Promise<V> | Error>>;
 
 // Optionally turn off batching or caching or provide a cache key function or a
 // custom cache instance.