Linting JavaScript

Kea - High level abstraction between React and Redux - Interview with Marius Andra

Linting is one of those techniques that can help you make fewer mistakes while coding JavaScript. You can spot issues before they become actual problems. Modern editors and IDEs offer strong support for popular tools allowing you to detect possible issues as you are developing.

Despite this, it's a good idea to set them up with webpack or at least in a separate task that gets run regularly. That allows you to cancel a production build that is not up to your standards.

Brief Introduction to ESLint#

The linter that started it all for JavaScript is Douglas Crockford's JSLint. JSLint is known to be opinionated like Douglas himself. The next step in evolution was JSHint, which took the opinionated edge out of JSLint and allowed for more customization. ESLint is the newest tool in vogue, and it goes even further.

ESLint Is Customizable#

ESLint goes to the next level as it allows you to implement custom rules, parsers, and reporters. ESLint works with JSX syntax making it a good fit for React projects. You have to use a Babel specific parser with custom language features, although ESLint supports ES6 out of the box.

ESLint rules have been documented well, and you have full control over their severity. These features alone make it a powerful tool. Better yet, there is a significant number of rules and presets beyond the core as the community has built on top of it.

It's telling that a competing project, JSCS, decided to merge its efforts with ESLint. JSCS reached the end of its life with its 3.0.0 release, and the core team joined with ESLint.


eslint-config-airbnb is a good example of a popular preset. Often it's enough to find a preset you like, tweak it to your liking with local rules or by deriving a preset of your own based on it, and then using that. This way you don't have to worry so much about all the available functionality.

eslint-config-cleanjs is a good example of how you can use ESLint to restrict JavaScript to a purely functional subset.

Linting Is About More than Catching Issues#

Besides linting for issues, it can be valuable to manage the code style. Nothing is more annoying than having to work with source code that has mixed tabs and spaces. Stylistically consistent code reads better. When integrated with an Editor or IDE linting also points out your mistakes as you make them, making it easier to adjust your technique and avoid making more of the same errors.

Establishing strong linting can be beneficial, especially in a context where you need to collaborate with others. Even when working alone you benefit from linting as it can catch issues you could otherwise neglect. JavaScript as a language allows usage which, while valid, is not be the clearest to understand or may even be incorrect.

Linting does not replace proper testing, but it can complement testing approaches. It allows you to harden a codebase and make it more difficult to break. As the size of your project grows, and it becomes more challenging to manage, this becomes particularly important.

Setting Up ESLint#


ESLint is the most versatile linting solution for JavaScript. It builds on top of ideas presented by JSLint and JSHint. More importantly, it allows you to develop custom rules.

Since v1.4.0 ESLint supports autofixing. It allows you to perform certain rule fixes automatically. To activate it, pass the flag --fix to the tool. It's also possible to use this feature with webpack, although you should be careful with it. js-beautify can perform a similar operation.

Connecting ESlint with package.json#

To get started, install ESLint as a development dependency:

npm install eslint --save-dev

Next, you have to write configuration so you can run ESLint smoothly through npm. The lint namespace can be used to signify it's a linting related task. Caching should be enabled to improve performance on subsequent runs.

Add the following to lint the application files and configuration:


"scripts": {
"lint:js": "eslint app/ webpack.*.js --cache",
... },

The potential problem with using an include based approach is that you forget to lint source code. Using excludes solves this but then you have to be careful to update the exclude list as your project grows to avoid linting too much.

The exclusion approach can be achieved by pointing ESLint to the project root through eslint . and setting up a .eslintignore file for the excludes like for .gitignore. You can point to your Git ignores using --ignore-path .gitignore for maximum reuse. Individual patterns are supported through --ignore-pattern <pattern>.

Defining Linting Rules#

Given ESLint expects configuration to work, you need to define rules to describe what to lint and how to react if the rules aren't obeyed. The severity of an individual rule is defined by a number as follows:

  • 0 - The rule has been disabled.
  • 1 - The rule emits a warning.
  • 2 - The rule emits an error.

Rules, such as quotes, accept an array instead allowing you to pass extra parameters to them. Refer to the ESLint rules documentation for specifics.

Here's a starting point that works with the project:


module.exports = {
  env: {
    browser: true,
    commonjs: true,
    es6: true,
    node: true,
  extends: 'eslint:recommended',
  parserOptions: {
    sourceType: 'module',
  rules: {
    'comma-dangle': ['error', 'always-multiline'],
    indent: ['error', 2],
    'linebreak-style': ['error', 'unix'],
    quotes: ['error', 'single'],
    semi: ['error', 'always'],
    'no-unused-vars': ['warn'],
    'no-console': 0,

If you invoke npm run lint:js now, it should execute without any warnings or errors. If you see either, this is a good time to try ESLint autofixing. You can run it through npm run lint:js -- --fix. Running an npm script this way allows you to pass extra parameters to it.

Another alternative would be to push it behind a package.json script. Autofix is not able to repair each error, but it can fix a lot. And as time goes by and ESLint improves, it can perform more work.

Beyond vanilla JSON, ESLint supports other formats, such as JavaScript or YAML. I.e., .eslintrc.yaml would expect YAML. See the documentation for further details.

When ESLint gives errors, npm shows a long ELIFECYCLE error error block of its own. It's possible to disable that using the silent flag: npm run lint:js --silent or a shortcut npm run lint:js -s.

Connecting ESLint with Webpack#

You can make webpack emit ESLint messages by using eslint-loader. As the first step execute

npm install eslint-loader --save-dev
eslint-loader uses a globally installed version of ESLint unless you have one included in the project itself. Make sure you have ESLint as a development dependency to avoid the strange behavior.

The loader needs wiring to work. Loaders are discussed in detail in the Loading part of this book, but the basic idea is fast to understand. A loader is connected to webpack through a rule that contains preconditions related to it and a reference to the loader itself.

In this case, you want to ensure that ESLint gets executed before anything else using the enforce field. It allows to guarantee that linting happens before any other processing. The idea is discussed in detail in the Loader Definitions chapter.

To add linting to the project, adjust the configuration as follows:


const developmentConfig = () => {
  const config = {
module: { rules: [ { test: /\.js$/, enforce: 'pre', loader: 'eslint-loader', options: { emitWarning: true, }, }, ], },
}; ... };

If you execute npm start now and break a linting rule while developing, you should see that in the terminal output.

Webpack configuration lints only the application code you refer. If you want to lint webpack configuration itself, execute npm run lint:js separately.
Attaching the linting process to Git through a prepush hook allows you to catch problems earlier. husky allows you to achieve this quickly. Doing this allows you to rebase your commits and fix possible problems early.

Enabling Error Overlay#

WDS provides an overlay for capturing warnings and errors:


const developmentConfig = () => {
  const config = {
    devServer: {
// overlay: true is equivalent overlay: { errors: true, warnings: true, },
}, }; ... };

Run the server now (npm start) and break the code to see an overlay in the browser:

Error overlay

ESLint Tips#

The great thing about ESLint is that you can shape it to your purposes. The community around it's active, and you can find good integration in other tooling as well. Consider the tips below.

Usability Tips#

  • It can make sense to rely on an existing preset or set up custom configuration. That's where --init can come in handy. You can run it from npm bin and end up with a call like node_modules/.bin/eslint --init
  • ESLint supports custom formatters through --format parameter. eslint-friendly-formatter is an example of a formatter that provides terminal-friendly output. This way you can jump conveniently straight to the warnings and errors from there.

Performance Tips#

  • Especially on bigger projects it's beneficial to run ESLint outside of webpack. That keeps code compilation fast while still giving the advantage of linting. Solutions like lint-staged and fastlint can make this even more quickly.
  • You can get more performance out of ESLint by running it through a daemon, such as eslint_d. Using it brings down the overhead, and it can bring down linting times considerably.

Extension Tips#

Configuring ESLint Further#

Since webpack 2, the configuration schema of webpack has become stricter, and it doesn't allow arbitrary fields at configuration root level anymore. To overcome this issue and to access all functionality of the eslint-loader, you have to use LoaderOptionsPlugin as below:

  plugins: [
    new webpack.LoaderOptionsPlugin({
      options: {
        eslint: {
          // Fail only on errors
          failOnWarning: false,
          failOnError: true,

          // Toggle autofix
          fix: false,

          // Output to Jenkins compatible XML
          outputReport: {
            filePath: 'checkstyle.xml',
            formatter: require('eslint/lib/formatters/checkstyle'),

There are more options, and eslint-loader documentation covers those in detail.

Webpack and JSHint#

No JSLint particular loader exists for webpack yet. Fortunately, there's one for JSHint. You could set it up on a legacy project quickly. The key is in configuring jshint-loader.

JSHint looks into specific rules to apply from .jshintrc. You can also define custom settings within a jshint object at your webpack configuration. Exact configuration options have been covered by the JSHint documentation in detail.


Prettier goes one step further and can format your code automatically according to your coding style. If you want to use Prettier with ESLint, you should use eslint-config-prettier to disable ESLint rules that conflict with Prettier. Be warned that Prettier is highly opinionated in how it thinks code should look and offers few formatting options.


Danger operates on a higher level than other tools discussed. For example, it can check that the project change log was updated before a release is pushed to the public. You can also force pull requests of your project to comply specific standards.


EditorConfig allows you to maintain a consistent coding style across different IDEs and editors. Also, you need to set up a file:


root = true

# General settings for whole project
indent_style = space
indent_size = 4
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true

# Format specific overrides
indent_style = space
indent_size = 2


Linting is one of those techniques that yields benefits over the long term. You can fix possible problems before they become actual issues.

To recap:

  • ESLint is the most versatile of the current options. You can expand it to fit your exact use case.
  • ESLint can be run through webpack. It can terminate your build and even prevent it from getting deployed if your build does not pass the linting rules.
  • To get a better development experience, consider enabling WDS overlay to capture errors and warnings emitted by webpack.
  • EditorConfig complements ESLint by allowing you to define a project-level coding style. Editors integrate with EditorConfig making it easier to keep a project consistent regardless of the development platform.
  • Prettier is a complimentary solution that can format your code automatically whilst Danger operates on repository level and can perform higher level tasks related to the development process.

Given the webpack configuration of the project is starting to get messy, it's a good time to discuss how to compose configuration and improve the situation.

This book is available through Leanpub (digital), Amazon (paperback), and Kindle (digital). By purchasing the book you support the development of further content. A part of profit (~30%) goes to Tobias Koppers, the author of webpack.

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