No need to run npm run eject

Step 1

npm install react-app-rewired --save-dev

Step 2

Add config-overrides.js to the project root directory.(NOT ./src)

// config-overrides.js
module.exports = function override(config, env) {
    // New config, e.g. config.plugins.push...
    return config
}

Step 3

'Flip' the existing calls to react-scripts in npm scripts for start, build and test

/* package.json */
"scripts": {
    -   "start": "react-scripts start",
    +   "start": "react-app-rewired start",
    -   "build": "react-scripts build",
    +   "build": "react-app-rewired build",
    -   "test": "react-scripts test",
    +   "test": "react-app-rewired test",
        "eject": "react-scripts eject"
}

Step 4

Restart your app. Done

Option 1 - Eject your CRA

If you've just created your app using CRA, and haven't made big changes to it, you could use npm run eject - more about it here

Keep in mind that there is no going back (except by commits, of course) after doing this. This will basically provide you with webpack file and other files which are currently 'hidden' in CRA

Some critiques and second thoughts about this method here

Option 2 - React App Rewired

This might be the right choice for you. This allows you to extend your current webpack without ejecting, or messing up / making too many changes at your project as npm run eject will. Take a look at the package here

A great tutorial by Egghead.io using react-app-rewired here

I solved this problem by running a script between yarn install and yarn build. After yarn install the webpack.config.json file is generated, then immediately run a script on Node that modifies it, and then run the build.

My code:
custom.webpack.config.js

const fs = require('fs')

// WebPack.config File
const fileConfig = 'node_modules/react-scripts/config/webpack.config.js'

new Promise((resolve) => {
   fs.readFile(fileConfig, 'utf8', function (err, data) {
      if (err) {
        return console.log(err)
      }
      resolve(data)
   })
}).then((file) => {
    
    let CodeAsString = "Code as String to save"

    let regexCode = /YourCodeRegex}/g

    let result = file.replace(regexCode, CodeAsString)

    fs.writeFile(fileConfig, result, function (err) {
        if (err) return console.log(err)
        console.log('The webpack.config file was modifed!')
    })
})

So, now do you need to edit the package.json to include this code in the process:

"scripts": {
    "prestart": "node custom.webpack.config.js",
    "prebuild": "node custom.webpack.config.js",
    "start": "react-scripts start",
    "build": "react-scripts build"
}

Done! :)

Webpack configuration is being handled by react-scripts. I assume that you don't want to eject and just want to look at the config, you will find them in /node_modules/react-scripts/config

webpack.config.dev.js. //used by `npm start`
webpack.config.prod.js //used by `npm run build`

https://www.npmjs.com/package/react-app-rewired

Complete answer is :

How to rewire your create-react-app project

Create your app using create-react-app and then rewire it.

1. Install react-app-rewired For create-react-app 2.x with Webpack 4:

npm install react-app-rewired --save-dev

For create-react-app 1.x or react-scripts-ts with Webpack 3:

npm install react-app-rewired@1.6.2 --save-dev

2. Create a config-overrides.js file in the root directory

/* config-overrides.js */
module.exports = function override(config, env) {
  //do stuff with the webpack config...
  return config;
}

like this:

+-- your-project
|   +-- config-overrides.js
|   +-- node_modules
|   +-- package.json
|   +-- public
|   +-- README.md
|   +-- src

for example :

module.exports = function override(config, env) {
    // New config, e.g. config.plugins.push...

    config.module.rules = [...config.module.rules, 
        {
            test: /\.m?js/,
            resolve: {
              fullySpecified: false
            }
        }
      ]

    return config
}

3. 'Flip' the existing calls to react-scripts in npm scripts for start, build and test

from:

/* package.json */

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

To:

/* package.json */

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

Note: Do NOT flip the call for the eject script. That gets run only once for a project, after which you are given full control over the webpack configuration making react-app-rewired no longer required. There are no configuration options to rewire for the eject script.

4. Start the Dev Server

npm start

5. Build your app

npm run build

You can modify your webpack config or any other node_module using patch-package https://www.npmjs.com/package/patch-package

Install the version of react-scripts you want with npm i react-scripts@5.0.0

Then go into node_modules/react-scripts/webpack/webpack.config.js. Make the changes you need and then npx patch-package react-scripts

patch-package will generate a file in your project root like patches/react-scripts+5.0.0.patch

Add post-install script to package.json with

"scripts": {
    "postinstall": "patch-package",
    ...
}

Now whenever you run npm i you will automatically get this patch included with the installed library.

Here I found a simple solution without ejecting and we don't need to install other dependencies like react-app-rewired. Because If you want to add some loaders or update some configurations we need to update the webpack config of create-react-app.

How to do it ?

1. Go to node_modules/react-scripts/config/webpack.config.js.
2. Go to the line number 600.

Note:Here you'll see the following info

            // "file" loader makes sure those assets get served by WebpackDevServer.
            // When you `import` an asset, you get its (virtual) filename.
            // In production, they would get copied to the `build` folder.
            // This loader doesn't use a "test" so it will catch all modules
            // that fall through the other loaders.
            {
              loader: require.resolve('file-loader'),
              // Exclude `js` files to keep "css" loader working as it injects
              // its runtime that would otherwise be processed through "file" loader.
              // Also exclude `html` and `json` extensions so they get processed
              // by webpacks internal loaders.
              exclude: [/\.(js|mjs|jsx|ts|tsx)$/, /\.html$/, /\.json$/],
              options: {
                name: 'static/media/[name].[hash:8].[ext]',
              },
            },
            // ** STOP ** Are you adding a new loader?
            // Make sure to add the new loader(s) before the "file" loader.

3. Add the following line to the above file-loader.

         // #1 Example customer loader for handling svg images 
            {
              test: /\.svg$/,
              use: ['@svgr/webpack']
            },

That's it

Warning: Be careful to change it and put your configuration at the appropriate place that's very important.