Thibaud’s blog

Notes, thoughts, and open-source software

Upgrading to a stricter ESLint config

Updating a codebase from one ESLint config to another can be daunting, especially if switching to something more strict. This post proposes a methodology to do those config overhauls step by step, and get the benefits of a new config without putting in all of the effort upfront.

As an example of such a switch, consider moving from Create React App’s ESLint config to something more opinionated like airbnb/javascript, my own eslint-plugin-cookbook, or Torchbox’s eslint-config-torchbox.

Retrofitting strict rules on existing projects

First, let’s remove the project’s existing ESLint-related dependencies (inspect the package.json to determine what they are for your project), and upgrade ESLint to the latest version.

npm uninstall --save-dev eslint-config-prettier eslint-plugin-react
npm install --save-dev eslint@latest

Then, install the config as described in its documentation and replace the project’s config file with one that just extends the installed config. Once you’re done, now is a good time to commit the work so far. Here is what this would look like with eslint-config-torchbox:

npx install-peerdeps --dev eslint-config-torchbox@latest
module.exports = {
  // See https://github.com/torchbox/eslint-config-torchbox for rules.
  extends: "torchbox",
};

In all likelihood, this configuration is more opinionated / checks more aspects of the code than what the project was using before, and ESLint now reports a lot of errors. This is good! It means that the project will benefit from having a newer, stricter config in place, and as a developer you get to learn about more JavaScript anti-patterns.

For ESLint and the new config to be useful, we need them to be put to use as soon as possible, and to fail CI builds whenever issues are reported – it’s not good to work on a project where ESLint reports countless issues. So, our goal is to get to 0 errors and warnings reported by ESLint.

Initial cleanup

Autofix

Let’s start by doing what ESLint suggests in its output – using its auto-fixing capabilities!

# At the root of the project to automatically apply ESLint’s fixes:
npx eslint . --fix
# If your project uses Prettier, also run it afterwards,
# to guarantee that ESLint’s changes still match your formatting.
npm run format
# Or if your project has Prettier set up differently,
npx prettier --write '<where your JS files are>/**/*.js'
# Or if you want to reformat **all** of your project’s JS files.
npx prettier --write '**/?(.)*.{js,ts,jsx,tsx}'

Autofixes are aware of the syntax, and generally can be considered highly reliable – review the changes, commit them, and let’s move on to the next phase.

Disable problematic rules

Now, let’s look at the remaining problems, and extract a list of the rules they relate to, using some command-line magic:

$ npx eslint . --format tap | grep ruleId | cut -d ':' -f 2 | cut -c 2- | sort | uniq
class-methods-use-this
consistent-return
[...]
react/prop-types

Let’s add this list to our .eslintrc.js config, disabling all of the rules that were reporting problems:

const legacyCode = {
  "class-methods-use-this": 0,
  "consistent-return": 0,
  // [...]
  "react/prop-types": 0,
};

module.exports = {
  // See https://github.com/torchbox/eslint-config-torchbox for rules.
  extends: "torchbox",
  rules: {
    ...legacyCode,
  },
};

Note how we put the rules in a “legacyCode” bucket, so we can clearly distinguish between rules we choose to configure differently from this project, and the ones that are enforced by the new config but we’re not ready to turn on yet.

Running ESLint again, there shouldn’t be any errors reported 🎉.

Calling it a day

Now is a good time to commit – and potentially stop there. We’ve upgraded the config, disabled all of the rules and got our linting back to 0 problems reported. 🌈 We might not be leveraging all of the checks that the new config comes with, but we don’t necessarily need to do this right away. And your project can already benefit from the work done so far!

Extra cleanup - group disabled rules

🚧 To proceed, you will need a good amount of knowledge of ESLint rules, or time set aside to investigate rules one by one.

Now, if there is time, the ideal next step is to re-enable all of the disabled rules to ensure we make the most of the linting. Proceeding with this is generally very time-consuming, so we’ll go through it gradually – the first step is for us to assess how big of an investment is ahead of us, and what the benefits may be. Let’s assess the rules we’ve just disabled, and group them into meaningful chunks.

Here are starting groups I would recommend:

// Rules which should ideally be re-enabled, that are about consistent code style more than anything.
const legacyConventions = {
  "class-methods-use-this": 0,
  "react/prefer-stateless-function": 0,
  "react/sort-comp": 0,
};

// Rules which are not necessarily issues on their own but can make the code confusing or hard to troubleshoot, leading to bugs.
const codeSmells = {
  "import/no-named-as-default": 0,
  "prefer-const": 0,
  "prefer-template": 0,
};

// Rules which point out patterns that are commonly sources of bugs.
const sourcesOfBugs = {
  eqeqeq: 0,
  "react/button-has-type": 0,
  radix: 0,
};

module.exports = {
  parser: "babel-eslint",
  // See https://github.com/torchbox/eslint-config-torchbox for rules.
  extends: "torchbox",
  rules: {
    ...legacyConventions,
    ...codeSmells,
    ...sourcesOfBugs,
  },
};

When deciding in what group a given rule belongs, it’s useful to think of:

  • The effort it would take to fix all of the violations of the rules,
    • Is the code’s behavior going to be different, or identical? If identical this should be a safe change.
    • Can the change be done semi-automatically, e.g. with a search-and-replace?
    • How many places in the codebase are there that violate this rule?
    • Does the project have tests, types, or another form of static analysis that would make it easy to tell if something broke because of the changes?
  • The impact of fixing the violations to the rule,
    • Does enforcing the rule protect us from a certain type of bugs?
    • Would using a more consistent code style make it easier for people to understand the code?

In terms of impact and effort, the three groups above generally fall into the following areas:

        ▲
        │                                    ┌───────────────┐
        │                            │       │               │
        │                                    │    Sources    │
        │                            │       │      of       │
 High   │                                    │     bugs      │
 impact │                            │       │               │
        │                                    └───────────────┘
        │                            │
        │
        │                            │
        │                ┌───────────────┐
        │                │               │
        │ ─ ─ ─ ─ ─ ─ ─ ─│               │─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
        │                │  Code smells  │
        │                │               │
        │                │               │    ┌───────────────┐
        │                └───────────────┘    │               │
        │                            │        │    Legacy     │
 Low    │                                     │  conventions  │
 impact │                            │        │               │
        │                                     │               │
        │                            │        └───────────────┘
        │
        │                            │
        │
        │                            │
        └────────────────────────────────────────────────────────▶
                   Low effort                High effort
  • Addressing sources of bugs generally has a high impact on the project, but generally involves changes that might alter the behavior of the programme (if there are actual bugs!), so takes a high effort.
  • Refactoring project conventions generally takes a high effort compared to the impact on the project (too many conventions / outdated conventions).
  • Code smells are a mixed bag, can generally be addressed safely but might take a lot of manual work.

The “High impact, low effort” area of our chart feels a bit empty – in practice there are a few rules in there, from all three categories, and that’s the rules we will want to identify next and have a look at fixing.


For now – this is a good time to commit. If you’re working on the project with other people, now is also a good time to make a pull request, and get it reviewed and merged. Further cleanup requires making changes to the code that go beyond what autofixing does, and will be trickier to review – so let’s get the config to be used first.

Going further

🚧 To proceed, you will need a good amount of knowledge of JavaScript.

If you have more time than strictly needed for the initial cleanup, let’s go further. Our goal from now on is to remove all legacy rule overrides we added during the initial setup, while making sure linting still passes. There are a few different ways to approach this depending on the rule:

  • Refactoring the code in a safe way
  • Disabling rules via inline code comments
  • Disabling rules with nested ESLint configs
  • Changing the code and its behavior
  • Doing nothing (😄)

Which of these is appropriate is for you to determine case-by-case. The general workflow from now on is:

  1. Run ESLint and get 0 problems reported.
  2. Comment out one of the rules overrides from our .eslintrc.js, for example // "consistent-return": 0,.
  3. Run ESLint and get problems from the commented-out rules reported.
  4. Decide which approach to take for what was reported.
  5. Apply your changes if any.
  6. Fully remove the commented-out rule override, or add back the line un-commented.
  7. Run ESLint again and get 0 problems reported.
  8. Commit your changes if any.

We can go through all of the rule overrides like this, or stop when time is up. Here is a one-liner to automatically watch files and re-run ESLint for us that will help speed up the process:

npm install -g nodemon@latest
nodemon -w '<path to our project’s JS files>/**/*.js' -w .eslintrc.js --exec 'npx eslint .'

Refactoring the code in a safe way

It’s not always possible to refactor code without altering its behaviour, and realising where this is an option requires a high level of JavaScript (and DOM) expertise. If in doubt, this isn’t a good option – it would be too time-consuming to re-test working software for each and every change.

Here’s an example of a change that could be considered a safe refactoring:

const params = getParams();

export const getCategoryURL = (category) => {
    const params = getParams();
    params.set('category', category);
    return getURL(params);
};

The params variable inside getCategoryURL raises a problem with no-shadow. We can remove the shadowing by renaming the local variable:

const params = getParams();

export const getCategoryURL = (category) => {
    const p = getParams();
    p.set('category', category);
    return getURL(p);
};

Disabling rules via code comments

This is the next best option – if a rule doesn’t raise too many problems around the codebase, adding eslint-disable comments where relevant makes it possible to only disable the rule for a few lines of legacy code, rather than the whole codebase.

This is always completely safe, and the only drawback is the manual effort involved. Here is an example:

// eslint-disable-next-line eqeqeq
if (window.innerWidth != this.windowWidth) {
    // Update the window width for next time
    this.windowWidth = window.innerWidth;
}

Here, we’re not really interested in figuring out whether it’s safe to replace != with !== – we just want to move on and be able to benefit from eqeqeq elsewhere in the code.

Disabling rules for a group of files

ESLint supports overrides for only some of the files in the project: Disallowing Rules Only for a Group of Files. Generally I would recommend using inline comments instead of config-level overrides. The overrides might seem a bit cleaner (we don’t have to change the code, and litter it with comments), but they are harder to keep track of since they are decoupled from the code they are for.

Changing the code and its behavior

If an option, this can be a great way to progress – but it requires good knowledge of the project, and time to test. Here is an example:

const itemLevel = parseInt(navItem.dataset.navLevel);

Here, ESLint expects parseInt to be provided with a radix parameter instead of relying on the default behavior, which can be confusing or a source of bug. While it might seem harmless enough to add a radix of 10 as a parameter, we can’t really tell from the code alone whether navLevel is indeed in base 10 – so we would effectively be changing the code in a way that could be very significant.

Doing nothing (😄)

Again, if none of those options seem appropriate – it’s also perfectly fine to leave the newly updated config with its overrides and move on. Updating the config, even if only partially, already goes towards addressing technical debt and raising quality standards.

Wrapping up

Beyond updating the config, here are additional steps you should consider to make the most of ESLint on your project:

  • Use Prettier, if you aren’t already. With Prettier’s great support for many languages, in 2020 formatting code manually is a complete waste of time.
  • Make sure you and your team have editor / IDE extensions for ESLint. This will make it much easier for everyone to understand the new config and address problems as they arise.
  • Consider using pre-commit hooks on your project. While these might seem annoying at first, they can save you a lot of back-and-forth with CI (and code reviews!).