Build a scalable front-end with Rush monorepo and React — ESLint + Lint Staged

TL;DR

If you're interested in just see the code, you can find it here: https://github.com/abereghici/rush-monorepo-boilerplate

If you want to see an example with Rush used in a real, large project, you can look at [ITwin.js(https://github.com/imodeljs/imodeljs), an open-source project developed by Bentley Systems.

ESLint is a dominant tool for linting TypeScript and JavaScript code. We'll use it together with Lint Staged to achieve the goal "Enforced rules for code quality" we defined in Part 1.

ESLint works with a set of rules you define. If you already have a configuration for ESLint that you like, you can add it in our next setup. We'll be using AirBnB’s ESLint config, which is the most common rules list for JavaScript projects. As of mid-2021, it gets over 2.7 million downloads per week from NPM.

Build eslint-config package

Let's start by creating a folder named eslint-config in packages and creating package.json file.

mkdir packages/eslint-config

touch packages/eslint-config/package.json

Paste the following content to packages/eslint-config/package.json:

{
  "name": "@monorepo/eslint-config",
  "version": "1.0.0",
  "description": "Shared eslint rules",
  "main": "index.js",
  "scripts": {
    "build": ""
  },
  "dependencies": {
    "@babel/eslint-parser": "~7.14.4",
    "@babel/eslint-plugin": "~7.13.16",
    "@babel/preset-react": "~7.13.13",
    "@typescript-eslint/eslint-plugin": "^4.26.1",
    "@typescript-eslint/parser": "^4.26.1",
    "babel-eslint": "~10.1.0",
    "eslint-config-airbnb": "^18.2.1",
    "eslint-config-prettier": "^7.1.0",
    "eslint-config-react-app": "~6.0.0",
    "eslint-plugin-import": "^2.23.4",
    "eslint-plugin-flowtype": "^5.2.1",
    "eslint-plugin-jest": "^24.1.5",
    "eslint-plugin-jsx-a11y": "^6.4.1",
    "eslint-plugin-prettier": "^3.3.1",
    "eslint-plugin-react": "^7.24.0",
    "eslint-plugin-react-hooks": "^4.2.0",
    "eslint-plugin-testing-library": "^3.9.2"
  },
  "devDependencies": {
    "read-pkg-up": "7.0.1",
    "semver": "~7.3.5"
  },
  "peerDependencies": {
    "eslint": "^7.28.0",
    "typescript": "^4.3.5"
  },
  "peerDependenciesMeta": {
    "typescript": {
      "optional": true
    }
  }
}

Here we added all dependencies we need for our ESLint config.

Now, let's create a config.js file where we'll define ESLint configurations, non-related to rules.

const fs = require('fs');
const path = require('path');

const tsConfig = fs.existsSync('tsconfig.json')
  ? path.resolve('tsconfig.json')
  : undefined;

module.exports = {
  parser: '@babel/eslint-parser',
  parserOptions: {
    babelOptions: {
      presets: ['@babel/preset-react'],
    },
    requireConfigFile: false,
    ecmaVersion: 2021,
    sourceType: 'module',
    ecmaFeatures: {
      jsx: true,
    },
  },
  env: {
    es6: true,
    jest: true,
    browser: true,
  },
  globals: {
    globals: true,
    shallow: true,
    render: true,
    mount: true,
  },
  overrides: [
    {
      files: ['**/*.ts?(x)'],
      parser: '@typescript-eslint/parser',
      parserOptions: {
        ecmaVersion: 2021,
        sourceType: 'module',
        project: tsConfig,
        ecmaFeatures: {
          jsx: true,
        },
        warnOnUnsupportedTypeScriptVersion: true,
      },
    },
  ],
};

We'll split ESLint rules in multiple files. In base.js file we'll define the main rules that can be applied to all packages. In react.js will be the React-specific rules.
We might have packages that doesn't use React, so we'll use only the base rules.

Create a base.js file and add:

module.exports = {
  extends: ['airbnb', 'prettier'],
  plugins: ['prettier'],
  rules: {
    camelcase: 'error',
    semi: ['error', 'always'],
    quotes: [
      'error',
      'single',
      {
        allowTemplateLiterals: true,
        avoidEscape: true,
      },
    ],
  },
  overrides: [
    {
      files: ['**/*.ts?(x)'],
      extends: [
        'prettier/@typescript-eslint',
        'plugin:@typescript-eslint/recommended',
      ],
      rules: {},
    },
  ],
};

Here we're extending airbnb and prettier configurations. Here you can include other base rules you would like to use.

In react.js add the following:

const readPkgUp = require('read-pkg-up');
const semver = require('semver');

let oldestSupportedReactVersion = '17.0.1';

// Get react version from package.json and used it in lint configuration
try {
  const pkg = readPkgUp.sync({ normalize: true });
  const allDeps = Object.assign(
    { react: '17.0.1' },
    pkg.peerDependencies,
    pkg.devDependencies,
    pkg.dependencies
  );

  oldestSupportedReactVersion = semver
    .validRange(allDeps.react)
    .replace(/[>=<|]/g, ' ')
    .split(' ')
    .filter(Boolean)
    .sort(semver.compare)[0];
} catch (error) {
  // ignore error
}

module.exports = {
  extends: [
    'react-app',
    'react-app/jest',
    'prettier/react',
    'plugin:testing-library/recommended',
    'plugin:testing-library/react',
  ],
  plugins: ['react', 'react-hooks', 'testing-library', 'prettier'],
  settings: {
    react: {
      version: oldestSupportedReactVersion,
    },
  },
  rules: {
    'react/jsx-fragments': ['error', 'element'],
    'react-hooks/rules-of-hooks': 'error',
  },
  overrides: [
    {
      files: ['**/*.ts?(x)'],
      rules: {
        'react/jsx-filename-extension': [
          1,
          {
            extensions: ['.js', '.jsx', '.ts', '.tsx'],
          },
        ],
      },
    },
  ],
};

We have to provide a react version to react-app configuration. Instead of hardcoding it we'll use read-pkg-up to get the version from package.json. semver is used to help us picking the right version.

Last step is to define the entry-point of our configurations. Create a index.js file and add:

module.exports = {
  extends: ['./config.js', './base.js'],
};

Add lint command to react-scripts

ESLint can be used in a variety of ways. You can install it on every package or create a lint script that runs ESLint bin for you. I'm feeling more comfortable with the second approach. We can control the ESLint version in one place which makes the upgrade process easier.

We'll need few util functions for lint script, so create an index.js file inside of
packages/react-scripts/scripts/utils and add the following:

const fs = require('fs');
const path = require('path');
const which = require('which');
const readPkgUp = require('read-pkg-up');

const { path: pkgPath } = readPkgUp.sync({
  cwd: fs.realpathSync(process.cwd()),
});

const appDirectory = path.dirname(pkgPath);

const fromRoot = (...p) => path.join(appDirectory, ...p);

function resolveBin(
  modName,
  { executable = modName, cwd = process.cwd() } = {}
) {
  let pathFromWhich;
  try {
    pathFromWhich = fs.realpathSync(which.sync(executable));
    if (pathFromWhich && pathFromWhich.includes('.CMD')) return pathFromWhich;
  } catch (_error) {
    // ignore _error
  }
  try {
    const modPkgPath = require.resolve(`${modName}/package.json`);
    const modPkgDir = path.dirname(modPkgPath);
    const { bin } = require(modPkgPath);
    const binPath = typeof bin === 'string' ? bin : bin[executable];
    const fullPathToBin = path.join(modPkgDir, binPath);
    if (fullPathToBin === pathFromWhich) {
      return executable;
    }
    return fullPathToBin.replace(cwd, '.');
  } catch (error) {
    if (pathFromWhich) {
      return executable;
    }
    throw error;
  }
}

module.exports = {
  resolveBin,
  fromRoot,
  appDirectory,
};

The most important function here is resolveBin that will try to resolve the binary for a given module.

Create lint.js file inside of packages/react-scripts/scripts and add the following:

const spawn = require('react-dev-utils/crossSpawn');
const yargsParser = require('yargs-parser');
const { resolveBin, fromRoot, appDirectory } = require('./utils');

let args = process.argv.slice(2);
const parsedArgs = yargsParser(args);

const cache = args.includes('--no-cache')
  ? []
  : [
      '--cache',
      '--cache-location',
      fromRoot('node_modules/.cache/.eslintcache'),
    ];

const files = parsedArgs._;

const relativeEslintNodeModules = 'node_modules/@monorepo/eslint-config';
const pluginsDirectory = `${appDirectory}/${relativeEslintNodeModules}`;

const resolvePluginsRelativeTo = [
  '--resolve-plugins-relative-to',
  pluginsDirectory,
];

const result = spawn.sync(
  resolveBin('eslint'),
  [
    ...cache,
    ...files,
    ...resolvePluginsRelativeTo,
    '--no-error-on-unmatched-pattern',
  ],
  { stdio: 'inherit' }
);

process.exit(result.status);

In packages/react-scripts/bin/react-scripts.js register the lint command:

. . .
 const scriptIndex = args.findIndex(
  x => x === 'build' || x === 'start' || x === 'lint' || x === 'test'
);
. . .

. . .
if (['build', 'start', 'lint', 'test'].includes(script)) {
. . .

Now, add our new dependencies in packages/react-scripts/package.json:

. . .
    "which": "~2.0.2",
    "read-pkg-up": "7.0.1",
    "yargs-parser": "~20.2.7",
    "eslint": "^7.28.0"
. . .

Lint script in action

Our lint script is ready, now let's run it in react-app project.

Create a new file named .eslintrc.js and add the following:

module.exports = {
  extends: ['@monorepo/eslint-config', '@monorepo/eslint-config/react'],
};

Inside package.json add eslint-config as dependency:

. . .
 "@monorepo/eslint-config": "1.0.0"
 . . .

In scripts section add lint command:

...
"lint": "react-scripts lint src"
...

Run rush update following by rushx lint. At this point you should see a bunch of ESLint errors. As an exercise, you can try to fix them by enabling / disabling some rules in eslint-config or modify react-app project to make it pass the linting.

Add lint-staged command to react-scripts

We'll follow the same approach as we did with lint script. Create lint-staged.js file inside of packages/react-scripts/scripts and add the following:

const spawn = require('react-dev-utils/crossSpawn');
const { resolveBin } = require('./utils');

const args = process.argv.slice(2);

result = spawn.sync(resolveBin('lint-staged'), [...args], {
  stdio: 'inherit',
});

process.exit(result.status);

Add lint-staged as dependency in package.json:

...
 "lint-staged": "~11.0.0"
...

Open packages/react-scripts/bin/react-scripts.js and register lint-staged command.

Next step is to register a lint-staged rush command in common/config/command-line.json, as we did with prettier command in Part 1.

{
  "name": "lint-staged",
  "commandKind": "bulk",
  "summary": "Run lint-staged on each package",
  "description": "Iterates through each package in the monorepo and runs the 'lint-staged' script",
  "enableParallelism": false,
  "ignoreMissingScript": true,
  "ignoreDependencyOrder": true,
  "allowWarningsInSuccessfulBuild": true
},

Now, let's run lint-staged command on git pre-commit hook. Open common/git-hooks/pre-commit and add the append to the end of the file:

node common/scripts/install-run-rush.js lint-staged || exit $?

Lint staged in action

Let's define what tasks we want lint-staged to run for react-app project.
Open package.json of react-app and add the configuration for lint-staged:

"lint-staged": {
    "src/**/*.{ts,tsx}": [
      "react-scripts lint --fix --",
      "react-scripts test --findRelatedTests --watchAll=false --silent"
    ],
  },

Also in package.json add the new lint-staged script:

"lint-staged": "react-scripts lint-staged"

Now, on each commit lint-staged will lint our files and will run tests for related files.

Run rush install to register our command, then rush update and let's commit our changes to see everything in action.

If you encountered any issues during the process, you can see the code related to this post here.