Friday, 10 July 2020

Using ESLint

ESLint is one of most popular linters for JavaScript and TypeScript (via typescript-eslint).

To use, we need to install it first. It can be installed locally (per project) or globally. To install it globally:

$ npm install -g eslint

We can verify that package is installed:

$ npm -g list  | grep eslint
├─┬ eslint@7.4.0
│ ├─┬ eslint-scope@5.1.0
│ ├─┬ eslint-utils@2.1.0
│ │ └── eslint-visitor-keys@1.3.0 deduped
│ ├── eslint-visitor-keys@1.3.0
│ │ └── eslint-visitor-keys@1.3.0 deduped

...and also check its command line args:

$ eslint --help
eslint [options] file.js [file.js] [dir]

Basic configuration:
  --no-eslintrc                   Disable use of configuration from .eslintrc.*
  -c, --config path::String       Use this configuration, overriding .eslintrc.* config options if present
  --env [String]                  Specify environments
  --ext [String]                  Specify JavaScript file extensions
  --global [String]               Define global variables
  --parser String                 Specify the parser to be used
  --parser-options Object         Specify parser options
  --resolve-plugins-relative-to path::String  A folder where plugins should be resolved from, CWD by default

Specifying rules and plugins:
  --rulesdir [path::String]       Use additional rules from this directory
  --plugin [String]               Specify plugins
  --rule Object                   Specify rules

Fixing problems:
  --fix                           Automatically fix problems
  --fix-dry-run                   Automatically fix problems without saving the changes to the file system
  --fix-type Array                Specify the types of fixes to apply (problem, suggestion, layout)

Ignoring files:
  --ignore-path path::String      Specify path of ignore file
  --no-ignore                     Disable use of ignore files and patterns
  --ignore-pattern [String]       Pattern of files to ignore (in addition to those in .eslintignore)

Using stdin:
  --stdin                         Lint code provided on <STDIN> - default: false
  --stdin-filename String         Specify filename to process STDIN as

Handling warnings:
  --quiet                         Report errors only - default: false
  --max-warnings Int              Number of warnings to trigger nonzero exit code - default: -1

  -o, --output-file path::String  Specify file to write report to
  -f, --format String             Use a specific output format - default: stylish
  --color, --no-color             Force enabling/disabling of color

Inline configuration comments:
  --no-inline-config              Prevent comments from changing config or rules
  --report-unused-disable-directives  Adds reported errors for unused eslint-disable directives

  --cache                         Only check changed files - default: false
  --cache-file path::String       Path to the cache file. Deprecated: use --cache-location - default: .eslintcache
  --cache-location path::String   Path to the cache file or directory

  --init                          Run config initialization wizard - default: false
  --env-info                      Output execution environment information - default: false
  --no-error-on-unmatched-pattern  Prevent errors when pattern is unmatched
  --debug                         Output debugging information
  -h, --help                      Show help
  -v, --version                   Output the version number
  --print-config path::String     Print the configuration for the given file

To initialise and configure ESLint launch configuration wizard:

$ eslint --init

ESLint configuration will be saved in file .eslintrc.json (if you opt json file to be used). Example content:


    "env": {
        "browser": true,
        "es2020": true
    "extends": "eslint:recommended",
    "parserOptions": {
        "ecmaVersion": 11
    "rules": {
        "indent": [
        "quotes": [
        "semi": [

If you use VS Code, you can install ESLint plugin which will pick up this configuration and automatically lint your code and show warnings and error son the go.

ESLint should be added to Node project as a dev dependency.

package.json (created by $npm init):

  "devDependencies": {
    "eslint": "^7.4.0"

It is also possible to run this linter from the terminal. For example, to lint all files in the current project recursively with respect to .eslintrc.json:

$eslint .

  2:32009  error  Strings must use singlequote                                               quotes
  3:23     error  Strings must use singlequote                                               quotes
  3:42     error  Missing semicolon                                                          semi
  3:43     error  Strings must use singlequote                                               quotes
  3:70     error  Strings must use singlequote                                               quotes
  3:166    error  Strings must use singlequote                                               quotes
  3:207    error  Strings must use singlequote                                               quotes
  3:280    error  Strings must use singlequote                                               quotes
  3:338    error  Missing semicolon                                                          semi
  3:460    error  Missing semicolon                                                          semi
  3:555    error  Missing semicolon                                                          semi
  4:22578  error  Missing semicolon                                                          semi
  4:22601  error  Missing semicolon                                                          semi

✖ 11436 problems (11436 errors, 0 warnings)

To ignore rules in .eslintrc.json and run the linter only for JavaScript files and only to check one particular rule:

/my-project$ eslint . --ext .js --no-eslintrc --rule 'indent: ["error", 4, { "SwitchCase": 1 }]'

   28:1  error  Expected indentation of 12 spaces but found 16  indent
   46:1  error  Expected indentation of 12 spaces but found 16  indent
  186:1  error  Expected indentation of 4 spaces but found 8    indent
  187:1  error  Expected indentation of 4 spaces but found 8    indent
  188:1  error  Expected indentation of 4 spaces but found 8    indent
  189:1  error  Expected indentation of 4 spaces but found 8    indent

Rules used here was indent.


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