Using ESLint and Prettier in a TypeScript Project

Originally posted on my blog.

When it comes to linting TypeScript code, there are two major linting options to choose from: TSLint and ESLint. TSLint is a linter than can only be used for TypeScript, while ESLint supports both JavaScript and TypeScript.

In the TypeScript 2019 Roadmap, the TypeScript core team explains that ESLint has a more performant architecture than TSLint and that they will only be focusing on ESLint when providing editor linting integration for TypeScript. For that reason, I would recommend using ESLint for linting TypeScript projects.

---

Setting up ESLint to work with TypeScript

First, install all the required dev dependencies:

yarn add eslint @typescript-eslint/parser @typescript-eslint/eslint-plugin --dev

• eslint: The core ESLint linting library
• @typescript-eslint/parser: The parser that will allow ESLint to lint TypeScript code
• @typescript-eslint/eslint-plugin: A plugin that contains a bunch of ESLint rules that are TypeScript specific

Next, add an .eslintrc.js configuration file in the root project directory. Here is a sample configuration for a TypeScript project:

module.exports =  {
  parser:  '@typescript-eslint/parser',  // Specifies the ESLint parser
  extends:  [
    'plugin:@typescript-eslint/recommended',  // Uses the recommended rules from the @typescript-eslint/eslint-plugin
  ],
 parserOptions:  {
    ecmaVersion:  2018,  // Allows for the parsing of modern ECMAScript features
    sourceType:  'module',  // Allows for the use of imports
  },
  rules:  {
    // Place to specify ESLint rules. Can be used to overwrite rules specified from the extended configs
    // e.g. "@typescript-eslint/explicit-function-return-type": "off",
  },
};

Note: I prefer using a JavaScript file for the .eslintrc file (instead of a JSON file) as it supports comments that can be used to better describe rules.

If using TypeScript with React, the eslint-plugin-react dev dependency should be installed and the following configuration can be used:

module.exports =  {
  parser:  '@typescript-eslint/parser',  // Specifies the ESLint parser
  extends:  [
    'plugin:react/recommended',  // Uses the recommended rules from @eslint-plugin-react
    'plugin:@typescript-eslint/recommended',  // Uses the recommended rules from @typescript-eslint/eslint-plugin
  ],
  parserOptions:  {
  ecmaVersion:  2018,  // Allows for the parsing of modern ECMAScript features
  sourceType:  'module',  // Allows for the use of imports
  ecmaFeatures:  {
    jsx:  true,  // Allows for the parsing of JSX
  },
  },
  rules:  {
    // Place to specify ESLint rules. Can be used to overwrite rules specified from the extended configs
    // e.g. "@typescript-eslint/explicit-function-return-type": "off",
  },
  settings:  {
    react:  {
      version:  'detect',  // Tells eslint-plugin-react to automatically detect the version of React to use
    },
  },
};

Ultimately it's up to you to decide what rules you would like to extend from and which ones to use within the rules object in your .eslintrc.js file.

Adding Prettier to the mix

What works well along with ESLint is prettier, which does a great job at handling code formatting. Install the required dev dependencies to get prettier working with ESLint:

yarn add prettier eslint-config-prettier eslint-plugin-prettier --dev

• prettier: The core prettier library
• eslint-config-prettier: Disables ESLint rules that might conflict with prettier
• eslint-plugin-prettier: Runs prettier as an ESLint rule

In order to configure prettier, a .prettierrc.js file is required at the root project directory. Here is a sample .prettierrc.js file:

module.exports =  {
  semi:  true,
  trailingComma:  'all',
  singleQuote:  true,
  printWidth:  120,
  tabWidth:  4,
};

Next, the .eslintrc.js file needs to be updated:

module.exports =  {
  parser:  '@typescript-eslint/parser',  // Specifies the ESLint parser
  extends:  [
    'plugin:@typescript-eslint/recommended',  // Uses the recommended rules from the @typescript-eslint/eslint-plugin
    'prettier/@typescript-eslint',  // Uses eslint-config-prettier to disable ESLint rules from @typescript-eslint/eslint-plugin that would conflict with prettier
    'plugin:prettier/recommended',  // Enables eslint-plugin-prettier and displays prettier errors as ESLint errors. Make sure this is always the last configuration in the extends array.
  ],
  parserOptions:  {
    ecmaVersion:  2018,  // Allows for the parsing of modern ECMAScript features
    sourceType:  'module',  // Allows for the use of imports
  },
};

The advantage of having prettier setup as an ESLint rule using eslint-plugin-prettier is that code can automatically be fixed using ESLint's --fix option.

Automatically Fixing Code (VS Code)

For a good developer experience, it's useful to setup your editor to automatically run ESLint's automatic fix command (i.e. eslint --fix) whenever a file is saved. Since i'm using VS Code, here is the config required in the settings.json file in VS Code to get automatic fixing whenever saving a file:

"eslint.autoFixOnSave":  true,
"eslint.validate":  [
  "javascript",
  "javascriptreact",
  {"language":  "typescript",  "autoFix":  true  },
  {"language":  "typescriptreact",  "autoFix":  true  }
],

If you've also set the editor.formatOnSave option to true in your settings.json, you'll need to add the following config to prevent running 2 formatting commands on save for JavaScript and TypeScript files:

"editor.formatOnSave":  true,
"[javascript]":  {
  "editor.formatOnSave":  false,
},
"[javascriptreact]":  {
  "editor.formatOnSave":  false,
},
"[typescript]":  {
  "editor.formatOnSave":  false,
},
"[typescriptreact]":  {
  "editor.formatOnSave":  false,
},

---

And there you have it. That's how you can lint a TypeScript project using ESLint. If you want to make sure all the files you commit to git pass the ESLint checks, take a look at lint-staged, which can run ESLint on files being commited.

Comments

[Sung M. Kim]

Thank you, Robert. This was quite educational.

Especially because I've recently moved to TSLint without knowing about the the TypeScript 2019 roadmap and being a TypeScript newbie :).

[Robert Cooper]

Ya, I was using TSLint before recently switching to ESLint for TypeScript. TSLint works well, but I found myself missing certain rules that were available in ESLint, but not TSLint.

[Gyandeep Singh]

Just curious about your opinion on if you need to use a linter to check code if you use typescript in strict mode. For styles, for sure using prettier makes sens to me.

Coding with TypeScript, Prettier and ?

Gyandeep Singh ・ Mar 9 '18 ・ 1 min read

#typescript #javascript #node #discuss

[Robert Cooper]

Good question. Using a linter allows you to check on a lot of stuff that isn't covered by the TypeScript compiler. For example, if you take a look at some of these ESLint rules for TypeScript code, you'll find a lot of rules related to code style that can be enforced with a linter.

Here are a few examples:

• @typescript-eslint/explicit-function-return-type: Require explicit return types on functions and class methods
• @typescript-eslint/generic-type-naming: Enforces naming of generic type variables
• @typescript-eslint/no-array-constructor: Disallow generic Array constructors
• @typescript-eslint/no-unused-vars: Disallow unused variables

[Gyandeep Singh]

all of the rules above are again kinda of stylistic (last one is already covered directly by typescript). If something is wrong, typescript will tell u during compile regardless whether you specify function return type explicitly or not.. I also depend on computer types in typescript (maybe thats why i think like this for linter with strict mode)

[Robert Cooper]

Well if you don't mind having the code written in a project not conform to code style rules, then a linter is of no value. You just might have a large variance in the way code is structured/ordered, variables are named, and other code style related things.

When I'm working on a project, I prefer to look at the code base and have it look like it was all written by the same person. A linter helps achieve this in my opinion.

[Bernardo Raposo]

And there's still things like no explicit any that can be enforced by the linter.

[Corey Alix]

This is also a tsconfig option. What use is eslint really? Tsconfig.json and .prettierrc cover most cases as far as I can tell. Is there one compelling use case for eslint?

[Robert Cooper]

There are many eslint rules that aren’t covered by tsconfig options.

[bitten]

In VSCode I get a ts error like this photo (thepracticaldev.s3.amazonaws.com/i...) but if I run eslint by itself it does not report on it.

pwd$ ./node_modules/.bin/eslint --fix src/
pwd$

So my pre commit hooks do not bail out correctly as they always seem to pass. Surely I must be missing something?

[Robert Cooper]

ESlint doesn't check for TypeScript errors, so if you want to check for TypeScript errors in your pre commit hook, you'll need to run the tsc --noEmit command in addition to the ESLint command.

Here's a pre commit command that you could use:

"husky": {
    "hooks": {
        "pre-commit": "tsc --noEmit && eslint --fix src/"
    }
},

However, I would suggest using lint-staged so you don't have to run the eslint command on all the files in your src/, but rather only run it on files that are staged to be committed. This will save you time.

"husky": {
    "hooks": {
        "pre-commit": "tsc --noEmit && lint-staged"
    }
},
"lint-staged": {
    "*.{js,ts,tsx,jsx}": [
        "eslint . --fix",
        "git add"
    ]
}

[bitten]

Oh well that makes total sense. I though eslint with ts was doing tslint, and I thought tslint would do these kind of checks.

Many thanks, and yeah, I went with the section option:

    "lint-staged": {
        "src/**/*.{tsx,js}": [
            "prettier --single-quote --write",
            "eslint --fix",
            "git add"
        ]
    },

[Sung M. Kim]

Thank you Robert for the easy-to-follow article & bitten for adding the prettier option.

I was able to get the eslint & prettier set up for my pet project after following this post & the comments.

cshooks / hooks

List of all React hooks using data structures and algorithms

cshooks

A collection of React Hooks using Computer Science (CS) data structures & algorithms.

Purpose

Mainly to learn CS data structure & algorithms
(and also implement'em in TypeScript).

Hopefully some of the arcane data structures & algorithms help you as well.

Implemented

useTrie

Returns a Trie data structure, which is used to save a list of words to search in memory efficient manner & provide a type-ahead (not yet implemented) functionality.

Contributors

Thanks goes to these wonderful people (emoji key):

Sung M. Kim 🚇 🔧 ⚠️ 📖 🤔 💻

This project follows the all-contributors specification. Contributions of any kind welcome!

View on GitHub

[Robert Cooper]

Just took a peak at your article. Great write up 👍🏼

[Andrew Byrd 🐦]

I'm glad this was linked because the article was missing the crucial step of adding --ext js,jsx,ts,tsx to the npm run script.

[Tom Yeoman]

Thanks for the article I've just converted my TS node project to this setup!

[Sebastien Chartier]

Hi Robert, this was very instructive.

Thank you :)

[Robert Cooper]

Thanks Seb :D

[Eloi Simard-Quesnel]

Thanks Robert that was really helpful! Greetings from Montreal!

[Robert Cooper]

✌🏻

[Gustavo Cabral]

Thanks Robert! You nailed it 👌

[Osama Qarem]

Very helpful! I've used typescript but I'm a newbie to eslint and prettier. I understand them much better now. Thanks!

[bitten]

I ran a problem today with this setup. If I run prettier --single-quote --write then eslint --fix, it will fail because of this line:

                    {state === PENDING
                        ? Array(10)
                              .fill(true)

prettier wants the above, but eslint wants:

                    {state === PENDING
                        ? Array(10)
                            .fill(true)

(so 28 spaces instead of 30).

Any tips? I am kinda stuck.

[Robert Cooper]

You need to run prettier through ESLint, otherwise you'll get these kind of conflicts.

[bitten]

It seems that vscode is running prettier when formatting on save, rather than eslint! I will look into it on Monday when I back with my laptop :P

[Robert Cooper]

You need to make sure to turn off the formatOnSave in VSCode for your JavaScript and Typescript files. Here are the settings:

"editor.formatOnSave": true,
"[javascript]": {
    "editor.formatOnSave": false,
  },
  "[javascriptreact]": {
    "editor.formatOnSave": false,
  },
  "[typescript]": {
    "editor.formatOnSave": false,
  },
  "[typescriptreact]": {
    "editor.formatOnSave": false,
  },

Then to make sure you get ESLint to fix problems on save, make sure you have the following settings in VS Code:

"eslint.autoFixOnSave": true,
  "eslint.validate": [
    "javascript",
    "javascriptreact",
    {"language": "typescript", "autoFix": true },
    {"language": "typescriptreact", "autoFix": true }
  ],

[bitten]

Thanks! But there is still an issue with it :( It seems like a conflict between eslint(prettier/prettier) and eslint(@typescript-eslint/indent). I also feel like eslint is running twice or something. For now I have set:

    rules: {
        '@typescript-eslint/indent': 'off',
    }

Here's my full eslint config file:

module.exports = {
    parser: '@typescript-eslint/parser',
    extends: [
        'plugin:@typescript-eslint/recommended',
        'plugin:prettier/recommended',
        'plugin:react/recommended',
    ],
    parserOptions: {
        ecmaVersion: 2018,
        sourceType: 'module',
        ecmaFeatures: {
            jsx: true,
        },
        project: './tsconfig.json',
        tsconfigRootDir: './',
    },
    settings: {
        react: {
            version: 'detect',
        },
    },
    rules: {
        '@typescript-eslint/interface-name-prefix': 'always',
        '@typescript-eslint/no-explicit-any': 'always',
        '@typescript-eslint/explicit-function-return-type': 'off',
        '@typescript-eslint/no-non-null-assertion': 'off',
        '@typescript-eslint/no-use-before-define': 'off',
        '@typescript-eslint/member-delimiter-style': {
            delimiter: 'none',
            requireLast: true,
        },
    }
}

VSCode User settings: hastebin.com/jucodoheqi.json
VSCode Workspace settings: hastebin.com/anenanupuh.json

[Robert Cooper]

I think your extends array should be the following (I've added comments explaining things):

    extends: [
        'plugin:@typescript-eslint/recommended', // uses typescript-specific linting rules
        'plugin:react/recommended', // uses react-specific linting rules
        'plugin:prettier/recommended', // enables eslint-plugin-prettier and eslint-config-prettier
        'prettier/react', // disables react-specific linting rules that conflict with prettier
    ],

[bitten]

Still didn't seem to fix the issue! Although I also ran into this last week, which I have not been able to solve. Something must be wrong with my setup somewhere haha. Check it out:

Prettier wants this:

export default function Heading(
    props: HTMLProps<HTMLHeadingElement> & {
        as?: AsTags
        withComponent?: HeadingTags
    },
) /* .... */

But typescript fails to compile because of the missing comma after as?: AsTags as it wants as?: AsTags, but prettier removes it ._.

Thanks for providing the snippets though. If you can shed any light on this one I would appreciate it a ton :3