Bun Mock Fetch

Mock fetch requests in Bun. Great for tests and debugging.

bun add @aryzing/bun-mock-fetch

Basic usage:

mockFetch(requestMatcher, response);

Request matcher examples:

// Simple string matching.
mockFetch("https://example.com", new Response());

// Using minimatch.
mockFetch("https://example.com/foo/**", new Response());

// Using regex.
mockFetch(/example/, new Response());

// Using a matcher function.
mockFetch((input, init) => {
  // Check input.
  if (typeof input === "string") return input.includes("example");
  if (input instanceof URL) return input.host.includes("example");
  if (input instanceof Request) return input.url.includes("example");

  // Check init.
  if (init) {
    // ...
  }
}, new Response());

// Using a detailed matcher object. All properties are optional.
mockFetch({
  // Must match this string, glob, or regex.
  url: "https://example.com",
  // Must match this method (case-insensitive).
  method: "POST",
  // Must include these headers (case-insensitive) and match their values.
  headers: {
    "Content-Type": "application/json",
  },
});

Response examples:

// Text response.
mockFetch(/example/, new Response("Hello, world!"));

// JSON response.
mockFetch(
  /example/,
  new Response('{"message": "Hello, world!"}', {
    headers: {
      "Content-Type": "application/json",
    },
  }),
);

// Response function.
mockFetch(/example/, async ({ mockedRequest, input, init }) => {
  console.log(mockedRequest); // The matched mock request config.
  console.log(input); // The 1st arg. to `fetch`.
  console.log(init); // The 2nd arg. to `fetch`.

  // Delayed response example, useful for spinners and timeouts.
  await delay(1000);

  // Configure response based on request.
  const name =
    (input instanceof Request ? input : init)?.body?.toString() || "guest";
  return new Response(`Hello ${name}!`);
});

Test example:

afterEach(() => {
  clearFetchMocks();
});

test("first test", async () => {
  mockFetch("https://api.example.com", new Response("first"));
  expect(await makeApiRequest()).toBe("first");
});

test("second test", async () => {
  mockFetch("https://api.example.com", new Response("second"));
  expect(await makeApiRequest()).toBe("second");
});

Each call to mockFetch defines a new mock. At most one mock is used, with each mock taking precendece over previously defined mocks.

By default, requests that aren't matched against any mock definitions are forwarded to the native built-in fetch. This behavior can be modified with setIsUsingBuiltInFetchFallback().

To clear the mocks and restore original built-in fetch,

clearFetchMocks();

Helpers

• setIsUsingBuiltInFetchFallback(value: boolean): Enable or disable using the built-in fetch for requests that haven't been matched against any mocks. Enabled by default.
• setIsVerbose(value: boolean): Enable or disable debugging logs. Disabled by default.