Learn about segmenting express routes into different routers.

Learn Express Routers

Now that you've learned about nested route matching, let's refactor the rest of the `/expressions` routes into **expressions.js**.


Refactoring Expressions Routes

Generally, we will keep each router in its own file, and `require` them in the main application. This allows us to keep our code clean and our files short.

To do this with `monstersRouter`, we would create a new file **monsters.js** and move all code related to /monsters requests into it.
```js
// monsters.js
const express = require('express');
const monstersRouter = express.Router();

const monsters = {
  '1': {
    name: 'godzilla',
    age: 250000000
  },
  '2': {
    Name: 'manticore',
    age: 21
  }
}

monstersRouter.get('/:id', (req, res, next) => {
  const monster = monsters[req.params.id];
  if (monster) {
    res.send(monster);
  } else {
    res.status(404).send();
  }
});

module.exports = monstersRouter;
```

This code contains all the `monsters` specific code. In a more full-fledged API, this file would contain multiple routes. To use this router in another file, we use `module.exports` so that other files can access `monstersRouter`. The only other new line of code required is that Express must be `require`d in each file, since we'll need to create a router with `express.Router()`.

Our **main.js** file could then be refactored to import the `monstersRouter`:
```js
// main.js
const express = require('express');
const app = express();
const monstersRouter = require('./monsters.js');

app.use('/monsters', monstersRouter);
```
In this example, the `monstersRouter` is `require`d in **main.js** from **monsters.js** and used exactly as it was before.


Exercise: Using Multiple Router Files

Diagram representing 2 routes that an Express app uses: `/animals` and `/expressions` and how it locates the endpoint for `/expressions/1`. There are arrows to signify when a route matches a route in root, what the nested router file then does to locate the specific endpoint requested.

As you saw in the previous exercise, when using routers, it's important to remember that the full path of a request can be segmented.

In the provided diagram, you can see an Express application using two routers. A GET request arrives for `/expressions/1`. Because the beginning of the path does not match `/animals` in the first `app.use()`, the Express server moves on to the next `app.use()`, which matches `/expressions`.

Express's route matching algorithm then enters the `expressionsRouter` instance which is `require`d from **expressions.js**. Inside this router, the path matching changes. Even though the whole request path is `/expressions/1`, inside the `expressionsRouter`, all paths are matched from the parts of the path after `/expressions`, meaning that in this context, the router is trying to match the path `/1`.

Because the path is `/1`, the path does not match the first `.get()` method at `/`. The Express server moves on to the next route, which has a route parameter of `/:id`, so it matches! This route handles the necessary logic and sends the response.

Routers can be nested as many times as necessary for an application, so understanding nested route matching is important for creating complicated APIs.


Matching In Nested Routers

Your Expressions/Animals routes are all working well, and our machine is fully functional! Our **app.js** file, however, is getting quite long and hard to read. It's easy to imagine that as we add functionality to an application, this file would get long and cumbersome.

Luckily, Express provides functionality to alleviate this problem: Routers. Routers are mini versions of Express applications &mdash; they provide functionality for handling route matching, requests, and sending responses, but they do not start a separate server or listen on their own ports. Routers use all the `.get()`, `.put()`, `.post()`, and `.delete()` routes that you know and love.

In this lesson, we will use Routers to clean up our code and separate our application into a file to handle all `/expressions` routes, and another to handle all `/animals` routes.


This File Is Too Big!

Great job&mdash;you created a much more readable and clean code base using `Express.Router()`. Writing clean and readable code is one of the most important skills as a developer. This will make adding new features much easier as your APIs expand!


Routers Review

An Express router provides a subset of Express methods. To create an instance of one, we invoke the `.Router()` method on the top-level Express import.

To use a router, we _mount_ it at a certain path using `app.use()` and pass in the router as the second argument. This router will now be used for all paths that begin with that path segment. To create a router to handle all requests beginning with `/monsters`, the code would look like this:

```js
const express = require('express');
const app = express();

const monsters = {
  '1': {
    name: 'godzilla',
    age: 250000000
  },
  '2': {
    name: 'manticore',
    age: 21
  }
}

const monstersRouter = express.Router();

app.use('/monsters', monstersRouter);

monstersRouter.get('/:id', (req, res, next) => {
  const monster = monsters[req.params.id];
  if (monster) {
    res.send(monster);
  } else {
    res.status(404).send();
  }
});

```
Inside the `monstersRouter`, all matching routes are assumed to have `/monsters` prepended, as it is mounted at that path. `monstersRouter.get('/:id')` matches the full path `/monsters/:id`.

When a GET /monsters/1 request arrives, Express matches `/monsters` in `app.use()` because the beginning of the path (`'/monsters'`) matches. Express' route-matching algorithm enters the `monstersRouter`'s routes to search for full path matches. Since `monstersRouter.get('/:id`) is mounted at `/monsters`, the two paths together match the entire request path (`/monsters/1`), so the route matches and the callback is invoked. The `'godzilla'` `monster` is fetched from the `monsters` object and sent back.


Express.Router

Okay, training wheels off! Now refactor all your `/animals` routes to **animals.js**.