Express is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications.

express

JavaScript® (often shortened to JS) is a lightweight, interpreted, object-oriented language with first-class functions, most known as the scripting language for Web pages, but used in many non-browser environments as well such as node.js or Apache CouchDB. It is a prototype-based, multi-paradigm scripting language that is dynamic, and supports object-oriented, imperative, and functional programming styles.

javascript

Node.js is a platform built on Chrome's JavaScript runtime for easily building fast, scalable network applications. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, perfect for data-intensive real-time applications that run across distributed devices.

Node.js

Learn how to build a simple REST API to store notes using the latest built-in node features including native ES modules, the fs/promises API, async/await, the nullish coalescing operator, and destructuring. No compilers, no build steps, pure code.

This collection of lessons is great for beginners to Express as long as they are familiar with JavaScript and the HTTP protocol. Those more familiar with node and express will appreciate a fast-paced, modern take on using these popular tools and will likely benefit most from the lessons on input validation, models (which emphasizes a lot of new syntax), using fs/promises to serialize a `Map`, and accessing asynchronous data with `async/await`.

Pre-requisites:

- [Install curl](https://curl.haxx.se/)
- [Install node 14](http://nodejs.org/)
- Be familiar with the terminal

View the source for this project in the [project's github repo](https://github.com/xjamundx/egghead-express).

**Notes**:

- I'm not very pedantic about REST and basically just mean an API over HTTP
- [Express 5](https://expressjs.com/en/guide/migrating-5.html) is still in alpha (and has been for over two years), but most of these lessons apply to Express 4 as long you use something like [express-async-errors](https://www.npmjs.com/package/express-async-errors) to add support for `async` routes and middleware

In this lesson we'll build a basic web server in 2 minutes using Express 5 and node 14's native ES module support. 

We'll start by creating a new project using `npm init` and then add `"type": "module"` to our package.json file to opt-in to ES Module support. From there we'll create an index.js file where we create an instance of the express module and add a single `GET` route that sends the response `"hello"`. Finally we listen on port 3000 and confirm that our route responds to requests using both curl and a web browser.

Pre-requisites:

- Install [node 14](http://nodejs.org/)
- Make sure you have [curl installed](https://curl.haxx.se/)
- Can use a terminal

---

**Express 5** Express 5 offers very few changes from version 4. The main one I'm interested in is built-in support for async/await, which greatly simplifies how we handle network and file system I/O calls in our web application. Read more at [Migrating to Express 5](https://expressjs.com/en/guide/migrating-5.html)

**async/await** Read more about these recent JavaScript language additions in [Getting Started with async/await](https://www.freecodecamp.org/news/getting-started-with-async-await-b66385983875/) on FreeCodeCamp.

Serve a Basic Route with Express 5 and node 14

This lesson demonstrates using the [morgan](https://github.com/expressjs/morgan) library to add basic logging to an express 5 application. We also log when the app starts up by passing a second argument to the `app.listen()` method. 

**morgan**

The [morgan logger](https://github.com/expressjs/morgan) has a whole list of [pre-defined logging formats](https://github.com/expressjs/morgan#predefined-formats) that you can use listed on their website. You can also build your own custom format. The format we chose for this lesson `dev` is a short-hand for the following `:method :url :status :response-time ms - :res[content-length]`.

**Middleware**

Middleware are tiny programs that inspect and manipulate incoming requests in an express application. They can run before, after, or even in-between your other routes. They are commonly used for logging, authentication, and form processing.

A thorough list of common middleware can be found on the express website here:
http://expressjs.com/en/resources/middleware.html

Middleware are typically added to your application with `app.use()`, but can also be added added to individual routes using `app.get("/", middleware, routeHandler)`. While not recommended, multiple middleware can be added that way: `app.get("/", middleware1, middleware2, routeHandler)`

While not covered in this lesson creating a middleware is easy and they have a lot in common with route handlers. Here is a simple logging middleware:

```
app.use(function(res, res, next) {
    console.log(req.path);
    next();
});
```


Add Basic Logging to an Express 5 App with morgan

In this lesson you'll learn how to add a custom 404 handler to your express app. You'll also learn how to define a general error handler that is called when any of your routes or middleware have errors.

Express provides an [excellent reference on error handling in express](https://expressjs.com/en/guide/error-handling.html) on the [Express website](https://expressjs.com/en/guide/error-handling.html).


**404 Handler**

A 404 error handler is basically a route handler defined after all of the others without a path associated with it. These handlers naturally become a catchall for any request that isn't handled by existing routes. You define it like this:

```
app.use(function(req, res) {
     res.status(404).send("Not Found");
});
```

Read more in [the express API doc](https://expressjs.com/en/5x/api.html#res.status).


**Error Handler**

Express allows you to register a general error handler that catches any errors thrown in the course of running your application* including both routes and middlewares. It's common to think of this as your `500` error handler, which is the HTTP status code for `Internal Server Error`. Think of this as something going wrong that is not the users fault.

To register the error handler call `app.use` with a function taking these 4 arguments: `err`, `req`, `res`, `next` as in this short example:

```
app.use(function(err, req, res, next) {
    console.error(err.stack); // log the error to the STDERR
    res.send(err.message); // send back a friendly message to the client
})
```

**Note:** express error handlers can't usually catch errors thrown in an async callback function such as a `setTimeout` call or `process.nextTick` function. Errors thrown in these situations will typically crash your server.

Add Basic Error Handling to an Express 5 App

This lesson will cover how to add & organize routes in an express app.

We'll create an instance of `express.Router()` in its own `routes/index.js` file and use `app.use(routes)` to include those in our main application. The route handlers will live in their own file `routes/notes.js`.

The lesson will cover CRUD routes (create, read, update, delete) and go over some of the HTTP verbs supported by the express router.

We will be stubbing the following routes needed to power a note-taking application:

```
GET /notes
GET /notes/:id
POST /notes
POST /notes/:id
DELETE /notes/:id
```

The routes won't themselves do anything but return sample data. This approach is often helpful when building a new API, instead of building out the full functionality, first define the shape of the URLs and structure of the responses. Mock data can be used to simulate the response and ensure the routes are going to serve the needs of your application without having to build it all.

---

[More on the verbs supported by HTTP here](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods)
[More on the express routes capabilities here](http://expressjs.com/en/guide/routing.html)





Add and Organize New Routes in an Express App

In this lesson we'll cover how to access route params (`/users/:id`) and then use `express.urlencoded()` to give express the ability to process [URL encoded form data](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST) that we'll need to add and update notes in our application. The POST body will be made available on `req.body`. A mention is also made about how to read query parameters from `req.query`.

---

**express.urlencoded()**

The `extended` option we pass into the `urlencoded` method adds the following capabilities according to the docs:

> This option allows to choose between parsing the URL-encoded data with the querystring library (when false)
> or the [qs library](https://github.com/ljharb/qs) (when true). The “extended” syntax allows for rich objects and arrays to be encoded into the URL-encoded format, allowing for a JSON-like experience with URL-encoded


**body-parser**

In earlier versions of express it was encouraged to use the `body-parser` middleware package which is published separately from express. The common functionality is now built-in however as `express.urlencoded()` and `express.json()`.

---

More info:

- https://expressjs.com/en/api.html#req.params
- https://expressjs.com/en/api.html#req.query
- https://github.com/expressjs/body-parser


Access Form Data and URL Parameters with Express

Whenever you're designing an API that receives input, there's a chance people are going to use it maliciously or incorrectly.

This lesson will look at some common problems that can occur when we assume input exists that doesn't and how to pro-actively handle it by sending status code `400 Bad Request` with custom response messages. A popular library for input validation is [Joi](https://github.com/hapijs/joi).

A few notes:

- Always assume query params and post bodies are `undefined`
- Set defaults or check that they exist
- When required values aren't found send a `400` status code and appropriate message
- When you use `urlencoded({ extended: true })` with express it tries to parse numbers and booleans into those primitives instead of strings, so be ready for that.
- Always validate each value that is sent as input into your API, don't assume it's safe to use it without checking (see below for an example).

---

One simple strategy for validating input is to put the correct values in an Array or Set:

```
const { sort = "desc" } = req.query;
const validSorts = new Set(["desc", "asc"]);
if (!validSorts.has(sort)) {
    return res.status(400).send("Invalid Sort Param");
}
```

---

One of the worst things you can do in your express app is forget to `return` when you call `res.send()`. It leads to this dreaded `Error: Can't set headers after they are sent to the client` error which can be very hard to trace.


---

Another important concept is to never ever ever blindly pass in the results of `req.query` or `req.body` to some other object. It's potentially a massive security vulnerability. Imagine a scenario like this:

** DO THIS **
```
const { squareFootage, numRooms} = req.body
const house = new House({ squareFootage, numRooms})
```

**DO NOT DO THIS**
```
const body = req.body // someone could pass in price: 0
const house = new House(body) // free house for me!!
```



Validating Input to an Express API

We'll create a `new Map()` data structure to store Notes that are passed into our API in memory. With that in place we'll create a model called `Note.js` that has the responsibility for saving and manipulating those notes and saving them back into our data store. After we hook up the our routes to call into our Note model and confirm that it all works.

To power our API we use some modern JavaScript syntax:

- [nullish coalescing operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator)
- [Map.prototype.values](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/values)
- [array and object spread](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax)
- [Array.from](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from)




Create a Model to Manage Data for an Express API

We have an API that allows us to return a list of notes at `/notes`. We want to add the ability to sort that list by passing in the `sort` query param, which we read off of `req.query`. Once we know the expected sort order we pass a function to `Array.prototype.sort` that compares two notes' `lastEdited` values to decide which item comes first.

A quick refresher about how [custom Array sort functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort) work in JavaScript. Sort takes a function with two params (which are items in the array). If the function returns true, the `a` item will be placed higher in the sorted array. If the function returns `false`, the `b` item will be placed higher.

View the source for this project on the [projects github repo](https://github.com/xjamundx/egghead-express/).

Sort and Return an Array of Objects with a Query Param in an Express API

We create a class called `MapStore` that serializes the [JavaScript Map object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map) to JSON and stores that on the filesystem using node 14's [fs/promises API](https://nodejs.org/api/fs.html#fs_fs_promises_api).

In future lessons we'll use this class to persist our in-memory notes.


View the source for [MapStore.js](https://github.com/xjamundx/egghead-express/blob/master/lib/mapstore.js) on the [projects github repo](https://github.com/xjamundx/egghead-express/).

Use the fs/promises node API to Serialize a Map to the Filesystem

This lesson will rely on the built-in support for async functions found in Express 5 to demonstrate how an API can easily work with data asynchronously. By the end of the lesson our API will have the ability to save its notes to disk as they're added and read them when the server starts up again.

First we'll update our Note model to persist data to the filesystem using the `MapStore` class created in a previous lesson. Then we'll access those updated methods on the Note model using the `await` keyword in our routes. We'll also briefly discuss the importance of catching errors that come from promises and async functions.

**Resources**

- To learn more about `async/await` read [Getting started with async/await](https://www.freecodecamp.org/news/getting-started-with-async-await-b66385983875/).
- You can learn a great deal more about error handling in express in their guide to [error handling in express](http://expressjs.com/en/guide/error-handling.html).

Build a REST API with Express 5 and node 14

Jamund Ferguson

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