The React Scripts Start Command – Create-React-App NPM scripts explained

If you‘re new to building React applications, getting all the necessary build tools and configurations set up can be daunting. Fortunately, Create React App (CRA) makes this process much easier by providing a pre-configured development environment with a single command.

When you run npx create-react-app my-app, CRA generates a basic project structure for your React app with some essential npm scripts already defined. Understanding what these scripts do and how they work under the hood can help you debug issues and customize your build process as needed.

In this article, we‘ll take an in-depth look at one of the most important CRA scripts – react-scripts start. We‘ll explain what it does, examine the key dependencies it uses, and compare it to other CRA scripts. By the end, you‘ll have a solid grasp of the react-scripts start command and how to leverage it effectively in your projects.

What is Create React App?

Before we dive into the npm scripts, let‘s briefly review what Create React App is and why it‘s so useful. CRA is an officially supported way to create single-page React applications. It offers a modern build setup with no configuration required.

Under the hood, CRA uses Babel to transpile your code, Webpack to bundle your assets, and a whole host of other tools to provide features like a development server, hot module reloading, linting, testing, and an optimized production build. But the key benefit is that CRA abstracts away all this complexity by providing sensible defaults and a simple, unified interface via npm scripts.

With CRA, you can go from nothing to a running React app with a single command:

npx create-react-app my-app
cd my-app
npm start

This creates a new React project in the my-app directory, installs all the necessary dependencies, and starts the development server. No manual configuration necessary!

The npm scripts included with Create React App

When you generate a new project with CRA, you‘ll find the following npm scripts defined in the package.json file:

"scripts": {
  "start": "react-scripts start",
  "build": "react-scripts build", 
  "test": "react-scripts test",
  "eject": "react-scripts eject"
}

Each of these scripts invokes the react-scripts CLI with a different command:

• start: Starts the development server
• build: Bundles the app for production
• test: Runs the test watcher in interactive mode
• eject: Removes the single build dependency and copies all config files and transitive dependencies into your project

We‘ll explore each of these in more detail later, but for now, let‘s focus on the start command.

What does `react-scripts start` do?

In short, running npm start (which executes react-scripts start under the hood) starts up a development server, compiles your code, and opens the app in your default web browser.

More specifically, here are the key things the start command does:

1. Sets the NODE_ENV environment variable to development. This enables useful development features like more detailed error messages.

2. Reads the environment variables from a .env file, if present. This allows you to set custom config options.

3. Checks for version mismatches in your installed dependencies vs the expected versions in react-scripts. This can help catch hard-to-debug issues caused by version incompatibilities.

4. Compiles your code using Babel, with support for modern JavaScript features, JSX syntax, and more. The transformed code is written to an in-memory filesystem.

5. Bundles your code and assets using Webpack into a single JavaScript file. This also applies code splitting to improve performance.

6. Starts up a Webpack development server on a local port (default 3000) and opens your app in the browser. The dev server provides features like live reloading and error overlays.

7. Watches your code for changes and automatically recompiles and refreshes the browser. This provides a fast feedback loop while developing.

That‘s a high-level overview, but what‘s really happening under the hood? Let‘s take a closer look at some of the key dependencies that make this possible.

Key dependencies used by react-scripts start

If you peek inside the package.json for react-scripts, you‘ll see it has a ton of dependencies – over 60! Here are a few of the most important ones used by the start command:

Babel

Babel is a JavaScript compiler that transforms your modern code (ES6+, JSX) into backwards-compatible JavaScript that can run in any browser or Node environment.

Some key Babel packages used:

• @babel/core – The core Babel compiler
• @babel/preset-env – Smart preset that allows you to use the latest JavaScript features with automatic polyfills
• @babel/preset-react – Preset for all React plugins, e.g. turning JSX into functions

Webpack

Webpack is a module bundler for JavaScript. It recursively builds a dependency graph of your code and assets, then packages them into optimized bundles.

Key Webpack packages:

• webpack – The core Webpack bundler
• webpack-dev-server – Serves your app in development and provides live reloading
• Various loaders for handling different file types (css-loader, babel-loader, etc.)

ESLint

ESLint is a pluggable JavaScript linter that statically analyzes your code to find problems and enforce style guidelines.

• eslint – The core ESLint linting utility
• eslint-config-react-app – Default ESLint configuration used by CRA

Jest

While not used directly by the start command, Jest is included by CRA for running your tests. It provides an interactive test watcher with intelligent defaults.

• jest – The core Jest test runner and assertion library
• jest-watch-typeahead – Provides a typeahead for filtering your tests

These are just a few of the key dependencies – there are many more that work behind the scenes. The magic of CRA is that it seamlessly integrates all these tools to provide a cohesive, batteries-included development experience.

Comparing react-scripts start to other CRA scripts

Now that we‘ve looked at react-scripts start in depth, how does it compare to the other CRA scripts?

react-scripts build

The build script is similar to start, but it creates an optimized production build of your app. Instead of using a development server, it outputs static files to a build folder, ready to be deployed to a hosting environment.

Key differences from start:

• Builds for production (minification, sourcemaps, etc)
• Doesn‘t include a dev server or support hot reloading
• Outputs static files vs running in-memory

react-scripts test

react-scripts test runs your Jest tests in watch mode. It automatically finds all test files associated with your code and re-runs them whenever you make changes.

While not directly related to the start command, running your tests is usually an important part of development. CRA provides a testing setup out of the box with no configuration.

react-scripts eject

The eject script is a one-way operation that allows you to "eject" your project from the CRA setup and expose all the underlying configuration files.

After ejecting, you‘ll have full control over the Babel, Webpack, and ESLint configs, but you‘ll lose the ability to upgrade react-scripts and will have to maintain the configs yourself. This is useful for advanced customization but not recommended for most users.

Using react-scripts start in your project

Using react-scripts start in your project is easy – simply navigate to your project directory and run:

npm start

This will fire up the development server and open your app in a browser window. Any changes you make to the code will be instantly reflected in the browser.

Some tips and tricks:

• The dev server usually runs on port 3000, but if that port is already in use, it will automatically use the next available port. You can specify a custom port with the PORT environment variable:

  PORT=4000 npm start

• By default, the dev server only listens on localhost. To use it on another device on your network, set the HOST env variable:

  HOST=0.0.0.0 npm start

• If you‘re using a custom proxy setup to avoid CORS issues in development, you can add a proxy field to your package.json:

  "proxy": "http://localhost:4000" 

  This will forward any requests that the dev server doesn‘t recognize to the specified URL.

Troubleshooting common issues

While CRA generally provides a smooth development experience, you may occasionally run into issues. Here are a few common problems and how to resolve them:

Error: `EADDRINUSE: address already in use`

This means the port that the dev server is trying to use is already occupied by another process. You can either stop that process or specify a different port for the dev server with the PORT variable:

PORT=3001 npm start

Error: `Invalid Host Header`

This usually happens when you‘re trying to access the dev server from a remote device or your Host header doesn‘t match the origin. Make sure you‘ve set the HOST variable to allow remote access:

HOST=0.0.0.0 npm start

You can also allow specific hosts by adding a DANGEROUSLY_DISABLE_HOST_CHECK variable:

DANGEROUSLY_DISABLE_HOST_CHECK=true npm start

Be careful with this though, as it disables an important security check!

Changes aren‘t reflecting in the browser

If you‘ve made changes to your code but they aren‘t showing up in the browser after a refresh, a few things could be happening:

• Make sure you‘ve saved the file you‘re editing. The dev server only recompiles when it detects changes.
• Check the console for lint or syntax errors – a compile error could be preventing the changes from being applied.
• Rarely, the dev server may get stuck in a bad state. Try stopping it with Ctrl+C and restarting.

If you‘re still having trouble, check the official CRA docs for more troubleshooting tips and common issues.

Advanced configuration options

For most projects, the default setup provided by react-scripts start is more than sufficient. But if you need more advanced customization, there are a few options available:

Environment-specific settings

You can provide different configuration settings for development, test, and production environments by adding .env files to your project root:

• .env: Default settings, always applied
• .env.local: Local overrides, always applied (ignored by git)
• .env.development, .env.test, .env.production: Environment-specific settings
• .env.development.local, .env.test.local, .env.production.local: Local overrides of environment-specific settings

To access these settings in your code, use the process.env object:

console.log(process.env.MY_CUSTOM_VARIABLE);

Proxying API requests

If your React app needs to make API requests to a backend server in development, you can avoid CORS issues by proxying those requests through the dev server. To do this, add a proxy field to your package.json:

"proxy": "http://localhost:4000"

This will forward any requests that don‘t match a static file to the specified URL. You can also get more fine-grained control with a src/setupProxy.js file:

const { createProxyMiddleware } = require(‘http-proxy-middleware‘);

module.exports = function(app) {
  app.use(
    ‘/api‘,
    createProxyMiddleware({
      target: ‘http://localhost:4000‘,
      changeOrigin: true,
    })
  );
};

Ejecting from react-scripts

If you need complete control over your build setup, you can opt to "eject" from react-scripts. This copies all the configuration files and transitive dependencies into your project so you can customize everything.

To eject, run:

npm run eject

This is a permanent action, so only do it if you‘re sure you need that level of customization! In most cases, it‘s better to use the built-in customization options or find a community plugin that solves your specific need.

Conclusion

The react-scripts start command is a powerful tool that makes developing React applications a breeze. By abstracting away the complex configuration of Babel, Webpack, and other build tools, it allows you to focus on writing your application code.

In this article, we‘ve taken a deep dive into what react-scripts start does, how it works under the hood, and how it compares to the other CRA scripts. We‘ve also looked at some common troubleshooting scenarios and advanced configuration options.

Armed with this knowledge, you should be able to leverage react-scripts start to streamline your React development workflow. Happy coding!