How to use Jest to test Express middleware or a function which consumes a callback?

Posted on Edited on Disqus:

Jest’s document says that you could use done as a parameter to test a function which takes a callback function as a parameter, but is it really the answer to every case? Sometimes, errors in your assertion in your callback will yield a timeout with meaningless call stack. I happened to solve a problem with a different approach. The solution here is not only apply to express middleware, but also to any function which consumes a callback.

1. A simple middleware which generate a JSON Web Token.

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const issueJWT = (req, res, next) => {
  const signUser = {
    id: res.locals.user._id.toString()
  };

  jwt.sign(
    signUser,
    "HS256KEY",
    { expiresIn: "120days", algorithm: "HS256" },
    (err, token) => {
      if (err) {
        return next(err);
      }
      res.locals.token = token;
      next();
    }
  );
};

The middleware is simple, just uses id and some key to generate a JWT, invoke error handler if there is an error.

2. This is the test.

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test("Should return a JWT with proper value if nothing wrong happened", done => {
  const callback = err => {
    const JWT = response.locals.token;
    const tokenPayload = jwt.decode(JWT, { complete: true }).payload;

    expect(err).toBeFalsy();
    expect(tokenPayload).toHaveProperty("iat");
    expect(tokenPayload).toHaveProperty("exp");
    expect(tokenPayload).toHaveProperty("id");
    expect(tokenPayload).toHaveProperty("iss");
    done();
  };

  issueJWT(request, response, callback);
});

Is it pass? Yes, of course, and I actually use breakpoint to check it indeed invoked the callback function. And this is the way which follows the official documents said.

3. So, it works, what’s the problem?

The problem is, when you have an error, you won’t see a clear answer. The error message is like this:

● Test issueJWT › Should return a JWT with proper value if nothing wrong happened

Timeout - Async callback was not invoked within timeout specified by jasmine.DEFAULT_TIMEOUT_INTERVAL.

 at pTimeout (node_modules/jest-jasmine2/build/queueRunner.js:53:21)
 at Timeout.callback [as _onTimeout] (node_modules/jsdom/lib/jsdom/browser/Window.js:523:19)
 at ontimeout (timers.js:380:14)
 at tryOnTimeout (timers.js:244:5)
 at Timer.listOnTimeout (timers.js:214:5)

OMG, this is something we don’t want.

4. The solution.

The final solution is to wrap the logic inside a Promise.

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test("Should return a JWT with proper value if nothing wrong happened", () => {
  return new Promise(resolve => {
    issueJWT(request, response, err => {
      if (!err) {
        resolve(response.locals.token);
      }
    });
  }).then(result => {
    const tokenPayload = jwt.decode(result, { complete: true }).payload;

    expect(tokenPayload).toHaveProperty("iat");
    expect(tokenPayload).toHaveProperty("exp");
    expect(tokenPayload).toHaveProperty("id");
    expect(tokenPayload).toHaveProperty("iss");
  });
});

This time, when there is an error. You will get a meaningful stack and message.

● Test issueJWT › Should return a JWT with proper value if nothing wrong happened

expect(object).toHaveProperty(path)

Expected the object:
{“exp”: 1507424033, “iat”: 1497056033, “id”: “5935240e5d8761601b691137”}

To have a nested property:
“iat1”

 at tests/backend/unit/fblogin/issueJWT.test.js:48:34
 at tryCatcher (node_modules/bluebird/js/release/util.js:16:23)

5. Use async / await to make it clear

The above solution may seems too heavy. But you can use async / await to make it a little bit concise.

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test("Should return a JWT with proper value if nothing wrong happened", async () => {
  const result = await new Promise(resolve => {
    issueJWT(request, response, err => {
      if (!err) {
        resolve(response.locals.token);
      }
    });
  });

  const tokenPayload = jwt.decode(result, { complete: true }).payload;

  expect(tokenPayload).toHaveProperty("iat");
  expect(tokenPayload).toHaveProperty("exp");
  expect(tokenPayload).toHaveProperty("id");
  expect(tokenPayload).toHaveProperty("iss");
});

6. End of story.

A rule of thumb is, whenever your test pass, try to make it fail then check. :)

Thanks for reading!

Follow me (albertgao) on twitter, if you want to hear more about my interesting ideas.