eslint-config-prettier

What is eslint-config-prettier?

The eslint-config-prettier npm package is used to disable all formatting-related ESLint rules that might conflict with Prettier. This allows developers to use both ESLint and Prettier without having to deal with conflicting formatting rules. It ensures that ESLint focuses on code-quality rules, and Prettier takes care of code formatting.

What are eslint-config-prettier's main functionalities?

Disabling conflicting rules

By extending eslint-config-prettier in your ESLint configuration file, you can turn off all rules that are unnecessary or might conflict with Prettier.

{ "extends": ["prettier"] }

Integration with other ESLint configs

eslint-config-prettier can be easily integrated with other ESLint configurations like Airbnb by placing it last in the extends array, which ensures that it will override any other formatting rules that might conflict with Prettier.

{ "extends": ["airbnb", "prettier"] }

Support for plugins

eslint-config-prettier provides support for disabling conflicting rules from various ESLint plugins, such as React or TypeScript, ensuring compatibility across different codebases.

{ "extends": ["prettier/react", "prettier/@typescript-eslint"] }

Other packages similar to eslint-config-prettier

eslint-config-standard

eslint-config-standard is an ESLint Shareable Config for JavaScript Standard Style. It enforces a consistent style across your codebase. Unlike eslint-config-prettier, it does not turn off formatting rules and is not specifically designed to be used with Prettier.

eslint-config-airbnb

eslint-config-airbnb is one of the most popular ESLint configurations which enforces Airbnb's JavaScript style guide. It includes a set of rules for code quality and formatting. While it can be used with Prettier, it does not disable formatting rules by default like eslint-config-prettier does.

eslint-config-google

eslint-config-google is based on Google's JavaScript style guide. It provides a set of ESLint rules that align with Google's code style recommendations. Similar to eslint-config-airbnb, it is not tailored for use with Prettier and does not disable conflicting formatting rules.

README

eslint-config-prettier

Turns off all rules that are unnecessary or might conflict with Prettier.

This lets you use you favorite shareable config without letting its stylistic choices get in the way when using Prettier.

Installation

Tip: First, you might be interested in installing eslint-plugin-prettier. Follow the instructions over there. This is optional, though.

Install eslint-config-prettier:

$ npm install --save-dev eslint-config-prettier

Then, add eslint-config-prettier to the "extends" array in your .eslintrc.* file. Make sure to put it last, so it gets the chance to override other configs.

{
  "extends": [
    "prettier"
  ]
}

A few ESLint plugins are supported as well:

• eslint-plugin-flowtype
• eslint-plugin-react
• eslint-plugin-standard

Add extra exclusions for the plugins you use like so:

{
  "extends": [
    "prettier",
    "prettier/flowtype",
    "prettier/react",
    "prettier/standard"
  ]
}

CLI helper tool

eslint-config-prettier also ships with a little CLI tool to help you check if your configuration contains any rules that are unnecessary or conflict with Prettier.

First, add a script for it to package.json:

{
  "scripts": {
    "eslint-check": "eslint --print-config .eslintrc.js | eslint-config-prettier-check"
  }
}

Then run npm run eslint-check.

(Swap out .eslintrc.js with the path to your config if needed.)

Exit codes:

• 0: No problems found.
• 1: Unexpected error.
• 2: Conflicting rules found.

Example configuration

{
  "extends": [
    "standard",
    "plugin:flowtype/recommended",
    "plugin:react/recommended",
    "prettier",
    "prettier/flowtype",
    "prettier/react",
    "prettier/standard"
  ],
  "plugins": [
    "flowtype",
    "react",
    "prettier",
    "standard"
  ],
  "parserOptions": {
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    }
  },
  "env": {
    "es6": true,
    "node": true
  },
  "rules": {
    "prettier/prettier": "error"
  }
}

Special rules

There a few rules that eslint-config-prettier disables that actually can be enabled in some cases.

• Some require certain options. The CLI helper tool validates this.
• Some require special attention when writing code. The CLI helper tool warns you if any of those rules are enabled, but can’t tell if anything is problematic.

For maximum ease of use, the special rules are disabled by default. If you want them, you need to explicitly specify them in your ESLint config.

curly

This rule requires certain options.

If a block (for example after if, else, for or while) contains only one statement, JavaScript allows omitting the curly braces around that statement. This rule enforces if or when those optional curly braces should be omitted.

If you use the "multi-line" or "multi-or-nest" option, the rule can conflict with Prettier.

For example, the "multi-line" option allows this line:

if (cart.items && cart.items[0] && cart.items[0].quantity === 0) updateCart(cart);

However, Prettier might consider the line too long and turn it into the following, which the "multi-line" option does not allow:

if (cart.items && cart.items[0] && cart.items[0].quantity === 0)
  updateCart(cart);

If you like this rule, it can be used just fine with Prettier as long as you don’t use the "multi-line" or "multi-or-nest" option.

Example configuration:

{
  "rules": {
    "curly": ["error", "all"]
  }
}

lines-around-comment

This rule can be used with certain options.

This rule requires empty lines before and/or after comments. Prettier preserves blank lines, with two exceptions:

• Several blank lines in a row are collapsed into a single blank line. This is fine.
• Blank lines at the beginning and end of blocks, objects and arrays are always removed. This may lead to conflicts.

By default, ESLint requires a blank line above the comment is this case:

if (result) {

  /* comment */
  return result;
}

However, Prettier removes the blank line:

if (result) {
  /* comment */
  return result;
}

If you like this rule, it can be used just fine with Prettier as long as add extra configuration to allow comments at the start and end of blocks, objects and arrays.

Example configuration:

{
  "rules": {
    "lines-around-comment": [
      "error",
      {
        "beforeBlockComment": true,
        "afterBlockComment": true,
        "beforeLineComment": true,
        "afterLineComment": true,
        "allowBlockStart": true,
        "allowBlockEnd": true,
        "allowObjectStart": true,
        "allowObjectEnd": true,
        "allowArrayStart": true,
        "allowArrayEnd": true
      }
    ]
  }
}

max-len

This rule requires special attention when writing code.

Usually, Prettier takes care of following a maximum line length automatically. However, there are cases where Prettier can’t do anything, such as for long strings, regular expressions and comments. Those need to be split up by a human.

If you’d like to enforce an even stricter maximum line length policy than Prettier can provide automatically, you can enable this rule. Just remember to keep max-len’s options and Prettier’s printWidth option in sync.

Keep in mind that you might have to refactor code slightly if Prettier formats lines in a way that the max-len rule does not approve of.

Example configuration:

{
  "rules": {
    "max-len": ["error", {"code": 80, "ignoreUrls": true}]
  }
}

no-confusing-arrow

This rule requires certain options.

For example, the rule could warn about this line:

var x = a => 1 ? 2 : 3;

By default, ESLint suggests switching to an explicit return:

var x = a => { return 1 ? 2 : 3; };

That causes no problems with Prettier.

With {allowParens: true}, adding parentheses is also considered a valid way to avoid the arrow confusion:

var x = a => (1 ? 2 : 3);

While Prettier keeps thoses parentheses, it removes them if the line is long enough to introduce a line break:

EnterpriseCalculator.prototype.calculateImportantNumbers = inputNumber =>
  1 ? 2 : 3;

If you like this rule, it can be used just fine with Prettier as long as the allowParens option is off.

Example configuration:

{
  "rules": {
    "no-confusing-arrow": "error"
  }
}

no-mixed-operators

This rule requires special attention when writing code.

This rule forbids mixing certain operators, such as && and ||.

For example, the rule could warn about this line:

var foo = a + b * c;

The rule suggests adding parentheses, like this:

var foo = a + (b * c);

However, Prettier removes many “unnecessary” parentheses, turning it back to:

var foo = a + b * c;

If you want to use this rule with Prettier, you need to split the expression into another variable:

var bar = b * c;
var foo = a + bar;

Keep in mind that Prettier prints some “unnecessary” parentheses, though:

var foo = (a && b) || c;

Example configuration:

{
  "rules": {
    "no-mixed-operators": "error"
  }
}

no-tabs

This rule requires certain Prettier options.

This rule disallows the use of tab characters at all. It can be used just fine with Prettier as long as you don’t configure Prettier to indent using tabs.

Example configuration:

{
  "rules": {
    "no-tabs": "error"
  }
}

no-unexpected-multiline

This rule requires special attention when writing code.

This rule disallows confusing multiline expressions where a newline looks like it is ending a statement, but is not.

For example, the rule could warn about this:

var hello = "world"
[1, 2, 3].forEach(addNumber)

Prettier usually formats this in a way that makes it obvious that a semicolon was missing:

var hello = "world"[(1, 2, 3)].forEach(addNumber);

However, there are cases where Prettier breaks things into several lines such that the no-unexpected-multiline conflicts.

const value = text.trim().split("\n")[position].toLowerCase();

Prettier breaks it up into several lines, though, causing a conflict:

const value = text
  .trim()
  .split("\n")
  [position].toLowerCase();

If you like this rule, it can usually be used with Prettier without problems, but occasionally you might need to either temporarily disable the rule or refactor your code.

const value = text
  .trim()
  .split("\n")
  // eslint-disable-next-line no-unexpected-multiline
  [position].toLowerCase();

// Or:

const lines = text.trim().split("\n");
const value = lines[position].toLowerCase();

Note: If you do enable this rule, you have to run ESLint and Prettier as two separate steps (and ESLint first) in order to get any value out of it. Otherwise Prettier might reformat your code in such a way that ESLint never gets a chance to report anything (as seen in the first example).

Example configuration:

{
  "rules": {
    "no-unexpected-multiline": "error"
  }
}

quotes

This rule requires certain options.

If you’d like to enforce the use of backticks rather than single or double quotes for strings, you can enable this rule. Otherwise, there’s no need to. Just remember to enable the "backtick" option!

Example configuration:

{
  "rules": {
    "quotes": ["error", "backtick"]
  }
}

Contributing

eslint-config-prettier has been tested with:

• ESLint 4.10.0 (eslint-config-prettier 2.1.1 and older were tested with ESLint 3.x)
• prettier 1.7.4
• eslint-plugin-flowtype 2.39.1
• eslint-plugin-react 7.4.0
• eslint-plugin-standard 3.0.1

Have new rules been added since those versions? Have we missed any rules? Is there a plugin you would like to see exclusions for? Open an issue or a pull request!

If you’d like to add support for eslint-plugin-foobar, this is how you’d go about it:

First, create foobar.js:

"use strict";

module.exports = {
  rules: {
    "foobar/some-rule": "off"
  }
};

Then, create test-lint/foobar.js:

/* eslint-disable quotes */
"use strict";

// Prettier does not want spaces before the parentheses, but
// eslint-config-foobar wants one.
console.log ();

test-lint/foobar.js must fail when used with eslint-plugin-foobar and eslint-plugin-prettier at the same time – until "prettier/foobar" is added to the "extends" property of an ESLint config.

Finally, you need to mention the plugin in several places:

• Add "foobar.js" to the "files" field in package.json.
• Add eslint-plugin-foobar to the "devDependencies" field in package.json.
• Make sure that at least one rule from eslint-plugin-foobar gets used in .eslintrc.base.js.
• Add it to the list of supported plugins, to the example config and to Contributing section in README.md.

When you’re done, run npm test to verify that you got it all right. It runs several other npm scripts:

• "test:lint" makes sure that the files in test-lint/ pass ESLint when the exclusions from eslint-config-prettier are used. It also lints the code of eslint-config-prettier itself.
• "test:lint-verify-fail" is run by a test in test/lint-verify-fail.js.
• "test:lint-rules" is run by a test in test/rules.js.
• "test:ava" runs unit tests that check a number of things:
  • That eslint-plugin-foobar is mentioned in all the places shown above.
  • That no unknown rules are turned off. This helps catching typos, for example.
  • That the CLI works.
• "test:cli-sanity" is a sanity check for the CLI.

License

MIT.

Changelog

Version 2.7.0 (2017-11-01)

• Added: The [lines-around-comment] rule (as a [special rule][lines-around-comment-special]). Thanks to Maurice de Beijer (@mauricedb)!
• Added: The [no-unexpected-multiline] rule (as a [special rule][no-unexpected-multiline-special]). Thanks to Suhas Karanth (@sudo-suhas)!