Add typings and test for priming with promise

src/__tests__/dataloader.test.js

@@ -380,6 +380,22 @@ describe('Primary API', () => {
     expect(loadCalls).toEqual([ [ 'B' ] ]);
   });
 
+  it('allows priming the cache with a promise', async () => {
+    const [ identityLoader, loadCalls ] = idLoader<string>();
+
+    identityLoader.prime('A', Promise.resolve('A'));
+
+    const [ a, b ] = await Promise.all([
+      identityLoader.load('A'),
+      identityLoader.load('B')
+    ]);
+
+    expect(a).toBe('A');
+    expect(b).toBe('B');
+
+    expect(loadCalls).toEqual([ [ 'B' ] ]);
+  });
+
 });
 
 describe('Represents Errors', () => {

src/index.d.ts

@@ -56,7 +56,7 @@ declare class DataLoader<K, V, C = K> {
    * Adds the provied key and value to the cache. If the key already exists, no
    * change is made. Returns itself for method chaining.
    */
-  prime(key: K, value: V | Error): this;
+  prime(key: K, value: V | PromiseLike<V> | Error): this;
 }
 
 declare namespace DataLoader {

src/index.js

@@ -178,7 +178,7 @@ class DataLoader<K, V, C = K> {
    *
    * To prime the cache with an error at a key, provide an Error instance.
    */
-  prime(key: K, value: V | Error): this {
+  prime(key: K, value: V | Promise<V> | Error): this {
     var cacheMap = this._cacheMap;
     if (cacheMap) {
       var cacheKey = this._cacheKeyFn(key);