[#]: subject: "Code your first React UI app"
[#]: via: "https://opensource.com/article/22/7/code-first-react-app"
[#]: author: "Jessica Cherry https://opensource.com/users/cherrybomb"
[#]: collector: "lkxed"
[#]: translator: " "
[#]: reviewer: " "
[#]: publisher: " "
[#]: url: " "

Code your first React UI app
======
Learn to make back-end and front-end development work together with this JavaScript tutorial.

Who wants to create their first UI app? I do, and if you're reading this article, I assume you do, too. In today's example, I'll use some JavaScript and the [API with Express][2] I demonstrated in my previous article. First, let me explain some of the tech you're about to use.

### What is React?

React is a JavaScript library for building a user interface (UI). However, you need more than just the UI library for a functional UI. Here are the important components of the JavaScript web app you're about to create:

* npx: This package is for executing npm packages.
* axios: A promise-based HTTP client for the browser and node.js. A promise is a value that an API endpoint will provide.
* http-proxy-middleware: Configures proxy middleware with ease. A proxy is middleware that helps deal with messaging back and forth from the application endpoint to the requester.

### Preconfiguration

If you haven't already, look at my [previous article][3]. You'll use that code as part of this React app. In this case, you'll add a service to use as part of the app. As part of this application, you have to use the `npx` package to create the new folder structure and application:

```
$ npx create-react-app count-ui
npx: installed 67 in 7.295s

Creating a new React app in /Users/cherrybomb/count-ui.

Installing packages. This might take a couple of minutes.
Installing react, react-dom, and react-scripts with cra-template...
[...]
Installing template dependencies using npm...
+ @testing-library/jest-dom@5.16.4
+ @testing-library/user-event@13.5.0
+ web-vitals@2.1.4
+ @testing-library/react@13.3.0
added 52 packages from 109 contributors in 9.858s
[...]
Success! Created count-ui at /Users/cherrybomb/count-ui
[...]
We suggest that you begin by typing:

  cd count-ui
  npm start
```

As you can see, the `npx` command has created a new template with a folder structure, an awesome `README` file, and a Git repository. Here's the structure:

```
$ cd count-ui/
/Users/cherrybomb/count-ui

$ ls -A -1
.git
.gitignore
README.md
node_modules
package-lock.json
package.json
public
src
```

This process also initialized the Git repo and set the branch to master, which is a pretty cool trick. Next, install the `npm` packages:

```
$ npm install axios http-proxy-middleware
[...]
npm WARN @apideck/better-ajv-errors@0.3.4 requires a peer of ajv@>=8 but none is installed. You must install peer dependencies yourself.
+ http-proxy-middleware@2.0.6
+ axios@0.27.2
added 2 packages from 2 contributors, updated 1 package and audited 1449 packages in 5.886s
```

Now that those are set up, add your `services`, and `main.js` file:

```
$ mkdir src/services
src/services

$ touch src/services/main.js
```

Preconfiguration is now complete, so you can now work on coding.

### Code a UI from start to finish

Now that you have everything preconfigured, you can put together the service for your application. Add the following code to the `main.js` file:

```
import axios from 'axios';
const baseURL = 'http://localhost:5001/api';
export const get = async () => await axios.get(`${baseURL}/`);
export const increment = async () => await axios.post(`${baseURL}/`);
export default {
    get,
    increment
}
```

This process creates a JavaScript file that interacts with the API you created in my previous article.

### Set up the proxy

Next, you must set up a proxy middleware by creating a new file in the `src` directory.

```
$ touch src/setupProxy.js
```

Configure the proxy with this code in `setupProxy.js` :

```
const { createProxyMiddleware } = require('http-proxy-middleware');
module.exports = function(app) {
  app.use(
    '/api',
    createProxyMiddleware({
      target: 'http://localhost:5000',
      changeOrigin: true,
    })
  );
};
```

In this code, the `app.use` function specifies the service to use as `/api` when connecting to the existing API project. However, nothing defines `api` in the code. This is where a proxy comes in. With a proxy, you can define the `api` function on the proxy level to interact with your Express API. This middleware registers requests between both applications because the UI and API use the same host with different ports. They require a proxy to transfer internal traffic.

### JavaScript imports

In your base `src` directory, you see that the original template created an `App.js`, and you must add `main.js` (in the `services` directory) to your imports in the `App.js` file. You also need to import React on the very first line, as it is external to the project:

```
import React from 'react'
import main from './services/main';
```

### Add the rendering function

Now that you have your imports, you must add a render function. In the **App()** function of `App.js`, add the first section of definitions for **react** and **count** before the **return** section. This section gets the **count** from the API and puts it on the screen. In the **return** function, a button provides the ability to increment the count.

```
function App() {
const [count, setCount] = React.useState(0);
React.useEffect(()=>{
  async function fetchCount(){
    const newCount = (await main.get()).data.count;
    setCount(newCount);
  }
  fetchCount();
}, [setCount]);
return (  
    <div className="App">
      <header className="App-header">
        <h4>
          {count}
        </h4>
        <button onClick={async ()=>{
          setCount((await main.increment()).data.count);
        }}>
          Increment
        </button>
      </header>
    </div>
  );
}
```

To start and test the app, run `npm run start`. You should see the output below. Before running the application, confirm your API is running from the Express app by running node `./src/index.js`.

```
$ npm run start
> count-ui@0.1.0 start /Users/cherrybomb/count-ui
> react-scripts start

[HPM] Proxy created: /  -> http://localhost:5000
(node:71729) [DEP_WEBPACK_DEV_SERVER_ON_AFTER_SETUP_MIDDLEWARE] DeprecationWarning: 'onAfterSetupMiddleware' option is deprecated. Please use the 'setupMiddlewares' option.
(Use `node --trace-deprecation ...` to show where the warning was created)
(node:71729) [DEP_WEBPACK_DEV_SERVER_ON_BEFORE_SETUP_MIDDLEWARE] DeprecationWarning: 'onBeforeSetupMiddleware' option is deprecated. Please use the 'setupMiddlewares' option.
Starting the development server...
```

Once everything is running, open your browser to `localhost:5000` to see the front end has a nice, admittedly minimal, page with a button:

![Counter at zero][4]

Image by:

(Jessica Cherry, CC BY-SA 4.0)

What happens when you press the button? (Or, in my case, press the button several times.)

![Counter at four][5]

Image by:

(Jessica Cherry, CC BY-SA 4.0)

The counter goes up!

Congratulations, you now have a React app that uses your new API.

### Web apps and APIs

This exercise is a great way to learn how to make a back end and a front end work together. It's noteworthy to say that if you're using two hosts, you don't need the proxy section of this article. Either way, JavaScript and React are a quick, templated way to get a front end up and running with minimal steps. Hopefully, you enjoyed this walk-through. Tell us your thoughts on learning how to code in JavaScript.

--------------------------------------------------------------------------------

via: https://opensource.com/article/22/7/code-first-react-app

作者：[Jessica Cherry][a]
选题：[lkxed][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)

本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出

[a]: https://opensource.com/users/cherrybomb
[b]: https://github.com/lkxed
[1]: https://opensource.com/sites/default/files/lead-images/lenovo-thinkpad-laptop-window-focus.png
[2]: https://opensource.com/article/22/7/javascript-api-express
[3]: https://opensource.com/article/22/7/javascript-api-express
[4]: https://opensource.com/sites/default/files/2022-07/counter0.png
[5]: https://opensource.com/sites/default/files/2022-07/counter4.png