learning as we go

MirageJS to increase developer productivity

October 10, 2019

Code

A few months ago I wrote about how mocked apis can help in the real world where we frequently build frontends for with APIs that are not ready yet.

In ember, ember-cli-mirage was the solution, but outside the ember world, there was no go to solution to develop frontends without a finished API.

Funny thing was that the ember-cli-mirage team was also thinking about something similar:

They were starting to extract the core of ember-cli-mirage to @miragejs/server!

At the time I replied to this tweet showing my excitment. Ended up having a few chats with Sam because he wanted to understand what were people’s painpoints and how could Mirage help solve them.

I ended up helping them with the extraction to @miragejs/server, learned a lot and had a very nice opportunity to work with Sam and Ryan, and they are awesome 🙏. They were always very keen to help and discuss whatever topics I needed, even reviewing this post that you’re reading!

A few months latter @miragejs/server is in beta! v0.1.25 is out, as well as the new website!

Back to the problem

Developing frontends without a finished API… That’s a pain right? And why?

The Miragejs website explains it better than I ever could:

Have you ever worked on a React or Vue app that needed to talk to a backend API before it was ready?

If so, how’d you handle it?

Maybe you created some mock data directly in your app just to keep you moving:

export function App() {
  let [users, setUsers] = useState([])

  useEffect(() => {
    // API not ready
    // fetch('/api/users')
    //   .then(response => response.json())
    //   .then(json => setUsers(json.data));

    // Use dummy data for now
    setUsers([
      { id: "1", name: "Luke" },
      { id: "2", name: "Leah" },
      { id: "3", name: "Anakin" },
    ])
  }, [])

  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  )
}

Seems harmless enough.

Weeks later, the server’s ready and you wire up your app — but nothing works like it did during development.

Some screens flash with missing data, others are broken entirely, and worst of all, you have no idea how much of your code needs to be rewritten.

What went wrong?

You ignored the network for too long.

Networking code isn’t the kind of thing you can just tack onto the end of a sprint.

Think about everything that comes along with making network requests: loading and error states, fetching partial data, caching… not to mention the fact that asynchronous APIs like network requests add tons of new states to each one of your app’s existing user flows.

If you don’t grapple with this crucial part of your application up front, you’ll have to rewrite a ton of code when it comes time to deploy your app.

You’re not doing yourself any favors by writing code that pretends the network doesn’t exist. You’re just poking holes in reality. And code that ignores reality isn’t ready for production.

(source: https://miragejs.com/)

Mocked APIs v2 - A much better version

I’ll be showing how to do the same thing (and a little more) than I my last blog post about mocked APIs but now with @miragejs/server.

If you prefer to be looking at the code while following, here you have it


Imagine you want to build a page that lists the posts in your blog. You don’t have an API but you know how the contract will look like, and you write your fetching code.

const [posts, setPosts] = useState([])

useEffect(() => {
  fetch("https://alexandrempsantos.com/api/v1/posts")
    .then(response => response.json())
    .then(json => setPosts(json))
}, [])

Then, on Mirage side, to mock that specific URL:

import { Server } from "@miragejs/server"

// Create a new server instance - Intercepts the requests
const server = new Server({
  urlPrefix: "https://alexandrempsantos.com/",
  namespace: "api",
  routes() {
    this.get("/posts", () => [
      {
        id: 1,
        title: "Mocking an API with axios",
        author: "asantos00",
        createdAt: 1557937282,
        body: "Lorem ipsum dolor sit amet, consectetur.",
      },
      {
        id: 2,
        title: "Forget axios interceptors. @miragejs/server",
        author: "asantos00",
        createdAt: 758851200,
        body: "Lorem ipsum dolor sit amet, consectetur.",
      },
    ])
  },
})

With just this code Mirage will intercept your requests and start answering with the defined response.

What about the other endpoints?

If you’re not starting an app from scratch and you want to use @miragejs/server, you will most likely not be able to write all the mirage routes at once. To help you that and to cover the case that you have routes that you don’t want to mock, mirage has passthrough.

// mirage route definitions

this.passthrough()

And with this, all the requests for the current urlPrefix and namespace that don’t have a Mirage route will be mocked.

A common use case is to passthrough calls to external services, or authentication.

this.passthrough("https://authentication-server.com")

What if I want to do more?

I’ve just demonstrated the simplest use case possible. Let’s take a little more advantage of what Mirage provides us.

import { Server } from "@miragejs/server"

const server = new Server({
  urlPrefix: "https://alexandrempsantos.com/",
  namespace: "api",
  seeds({ db }) {
    db.loadData({
      posts: [
        {
          id: 1,
          title: "Mocking an API with axios",
          author: "asantos00",
          createdAt: 1557937282,
          body: "Lorem ipsum dolor sit amet, consectetur.",
        },
        {
          id: 2,
          title: "Forget axios interceptors. @miragejs/server",
          author: "asantos00",
          createdAt: 758851200,
          body: "Lorem ipsum dolor sit amet, consectetur.",
        },
      ],
    })
  },
  routes() {
    this.get("/posts", schema => schema.db.posts)
  },
})

By doing this, Mirage stores posts in a database that you can then access and modify later.

Talking about modifying stuff… Now that we have posts persisted, let’s add the endpoint that enables to edit them:

this.put("/posts/:id", (schema, request) => {
  schema.db.posts.update(request.params.id, {
    title: request.requestBody.title,
  })
})

After we do a PUT /posts/1 with the body { "title": "test-edit" }, our post will be edited.

Now if we do our GET /posts here’s how the post with id: 1 is going to look like.

{
  id: 1,
  title: "test-edit",
  author: "asantos00",
  createdAt: 1557937282,
  body: "Lorem ipsum dolor sit amet, consectetur.",
}

// ... rest of the posts

By having the posts stored into a database, we can now manipulate them in the route handlers, for instance to create a delete and a creation route.

Useful features

Mirage offers lots of features, since serializers to models (you can check in the docs). Besides those complex ones, there are a couple of simple features that end up being very useful daily:

  • Custom responses - Useful for things like developing error scenarios or returning the right code after creation/edition.
import { Response } from "@miragejs/server"

// ...

this.get("/posts", () => {
  return new Response(
    400,
    { "Content-Type": "application/json" },
    { message: "Title not valid" }
  )
})
  • API latency - Useful to test how your app deals with loading
const server = new Server({
  timing: 2000, // applies to all routes
})

this.get("/posts", handlePosts, { timing: 3000 }) // only applies to single route

Another great use of @miragejs/server is testing.

You can start the server before the tests with the provided data and then assert that the endpoints where called and that the right data was mutated (more on this on a next blogpost).

Conclusion

Now that Mirage is out there is no more reason to be mocking data locally in your application or to spin up your whole infrastructure just to develop a single page.

Mirage enables you to develop your frontend with the same exact concerns you would have if you would be developing against a server, but it makes it easier to simulate states.

More important than that you’re not ignoring the network.

Have you tried @miragejs/server? Are you interested?

I would love to hear what you have to say and answer any question that may arise, either about this blogpost or Mirage.

Feel free to reach out to me!


Alexandre Portela dos Santos

Alexandre Portela dos Santos - Lisbon, Portugal
Software developer (trying to) help businesses | Learner | Writer
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