add unit tests for withRetry()

src/retries.test.ts

@@ -0,0 +1,99 @@
+import { withRetry } from "./retries";
+
+const FAST_RETRY_CONFIG: Parameters<typeof withRetry>[1] = {
+  attempts: 6,
+  // Always retry almost immediately, even when using real timers.
+  backoffMs: () => 1,
+};
+
+test("successful function only executes once", async () => {
+  const fn = jest.fn(async () => "success!");
+  const result = await withRetry(fn, FAST_RETRY_CONFIG)();
+  expect(result).toEqual("success!");
+  expect(fn).toHaveBeenCalledTimes(1);
+});
+
+test("failing function rejects after correct number of attempts", async () => {
+  const fn = jest.fn(async () => {
+    throw "oops!";
+  });
+  await expect(withRetry(fn, FAST_RETRY_CONFIG)()).rejects.toEqual("oops!");
+  expect(fn).toHaveBeenCalledTimes(FAST_RETRY_CONFIG.attempts);
+});
+
+test("parameters are passed through", async () => {
+  const fn = async (str: string, n: number) => {
+    let result = "";
+    for (let i = 0; i < n; i += 1) {
+      result += str;
+    }
+    return result;
+  };
+  const retriable = withRetry(fn, FAST_RETRY_CONFIG);
+
+  // A bit of type system hackery to assert at build time that function
+  // signatures match.
+  /* eslint-disable @typescript-eslint/no-unused-vars */
+  type Assert<_T extends true> = void;
+  type _ = Assert<typeof retriable extends typeof fn ? true : false>;
+  /* eslint-enable @typescript-eslint/no-unused-vars */
+
+  await expect(retriable("hello;", 3)).resolves.toEqual("hello;hello;hello;");
+});
+
+test("retries back off with expected delays", async () => {
+  jest.useFakeTimers();
+
+  let n = 0;
+  const fn = jest.fn(async () => {
+    if (n < 5) {
+      n += 1;
+      throw "oops!";
+    } else {
+      // Fake timers don't play nicely with the `expect().rejects` API, so
+      // ensure that the last attempt succeeds.
+      return "success!";
+    }
+  });
+
+  const promise = withRetry(
+    fn,
+    {
+      attempts: 6,
+      backoffMs: (attempt) => 1000 * 2 ** attempt,
+    },
+  )();
+
+  expect(fn).toHaveBeenCalledTimes(1);
+  await jest.advanceTimersByTimeAsync(100);
+  expect(fn).toHaveBeenCalledTimes(1);
+
+  await jest.advanceTimersByTimeAsync(1000);
+  expect(fn).toHaveBeenCalledTimes(2);
+  await jest.advanceTimersByTimeAsync(100);
+  expect(fn).toHaveBeenCalledTimes(2);
+
+  await jest.advanceTimersByTimeAsync(2000);
+  expect(fn).toHaveBeenCalledTimes(3);
+  await jest.advanceTimersByTimeAsync(100);
+  expect(fn).toHaveBeenCalledTimes(3);
+
+  await jest.advanceTimersByTimeAsync(4000);
+  expect(fn).toHaveBeenCalledTimes(4);
+  await jest.advanceTimersByTimeAsync(100);
+  expect(fn).toHaveBeenCalledTimes(4);
+
+  await jest.advanceTimersByTimeAsync(8000);
+  expect(fn).toHaveBeenCalledTimes(5);
+  await jest.advanceTimersByTimeAsync(100);
+  expect(fn).toHaveBeenCalledTimes(5);
+
+  await jest.advanceTimersByTimeAsync(16000);
+  expect(fn).toHaveBeenCalledTimes(6);
+
+  await expect(promise).resolves.toEqual("success!")
+
+  // Fake timers don't play nicely with the `expect().rejects` API, so disable
+  // them again for other tests.
+  jest.useRealTimers();
+});