feat(eventually): custom error constructor with last result inspection

Author: mirek

src/eventually.test.ts

@@ -8,5 +8,5 @@ test('eventually', async () => {
 
 test('eventually fails', async () => {
   await expect($.eventually($.gt(2), async () => 1, { delay: 10 }))
-    .rejects.toThrow('Expected 1 to be greater than 2.')
+    .rejects.toThrow('Expected 1 to be greater than 2. Last result was 1.')
 })

src/eventually.ts

@@ -1,10 +1,18 @@
+const defaultError =
+  <T>(err: unknown, result: T) =>
+    new Error((err instanceof Error ? err.message : String(err)) + ' Last result was ' + JSON.stringify(result) + '.')
+
 const eventually =
   async <T, U>(assert: (result: T) => U, poll: () => Promise<T>, {
     retries = 10,
-    delay = 1000
+    delay = 1000,
+    error = defaultError
   }: {
     retries?: number,
-    delay?: number
+    delay?: number,
+
+    /** Error constructor based on thrown assertion error and last polled result. */
+    error?: (err: unknown, result: T) => unknown
   } = {}): Promise<U> => {
     for (let retry = 1; retry <= retries; retry++) {
       const before = Date.now()
@@ -13,7 +21,7 @@ const eventually =
         return assert(result)
       } catch (err) {
         if (retry === retries) {
-          throw err
+          throw error(err, result)
         }
       }
       const duration = Date.now() - before