ESLint

https://eslint.org/docs/latest
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ESLint is a pluggable linting utility for JavaScript and JSX, designed to identify and report on...
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### Initiate Config Inspector (bun)

Source: https://eslint.org/docs/latest/use/configure/debug

Use this command to start the ESLint config inspector, which helps determine if specific configuration objects in your configuration file match a given filename. It installs and starts the '@eslint/config-inspector' package.

```bash
bunx eslint --inspect-config 
```

--------------------------------

### Initiate Config Inspector (npm)

Source: https://eslint.org/docs/latest/use/configure/debug

Use this command to start the ESLint config inspector, which helps determine if specific configuration objects in your configuration file match a given filename. It installs and starts the '@eslint/config-inspector' package.

```bash
npx eslint --inspect-config 
```

--------------------------------

### Install an npm-installed formatter

Source: https://eslint.org/docs/latest/use/command-line-interface

Install a formatter package from npm to use it with ESLint. This example shows installing the `eslint-formatter-pretty` package.

```bash
npm install eslint-formatter-pretty
```

```bash
yarn add eslint-formatter-pretty
```

```bash
pnpm add eslint-formatter-pretty
```

```bash
bun add eslint-formatter-pretty
```

--------------------------------

### Initiate Config Inspector (pnpm)

Source: https://eslint.org/docs/latest/use/configure/debug

Use this command to start the ESLint config inspector, which helps determine if specific configuration objects in your configuration file match a given filename. It installs and starts the '@eslint/config-inspector' package.

```bash
pnpm dlx eslint --inspect-config 
```

--------------------------------

### Configuration example

Source: https://eslint.org/docs/latest/rules/template-tag-spacing

Example of how to configure the 'template-tag-spacing' rule in your ESLint configuration file.

```json
{
    "template-tag-spacing": ["error", "never"]
}
```

--------------------------------

### Install Dependencies with bun

Source: https://eslint.org/docs/latest/contribute/development-environment

After cloning the repository, install all necessary development dependencies using bun.

```bash
cd eslint
bun add
```

--------------------------------

### Configuration Example

Source: https://eslint.org/docs/latest/rules/capitalized-comments

This example shows how to configure the rule with options to ignore specific patterns and inline comments.

```json
{
    "capitalized-comments": [
        "error",
        "always",
        {
            "ignorePattern": "pragma|ignored",
            "ignoreInlineComments": true
        }
    ]
}
```

--------------------------------

### Install Dependencies with pnpm

Source: https://eslint.org/docs/latest/contribute/development-environment

After cloning the repository, install all necessary development dependencies using pnpm.

```bash
cd eslint
pnpm add
```

--------------------------------

### Initiate Config Inspector (yarn)

Source: https://eslint.org/docs/latest/use/configure/debug

Use this command to start the ESLint config inspector, which helps determine if specific configuration objects in your configuration file match a given filename. It installs and starts the '@eslint/config-inspector' package.

```bash
yarn dlx eslint --inspect-config 
```

--------------------------------

### Example Indentation Styles

Source: https://eslint.org/docs/latest/rules/indent-legacy

Illustrates common indentation scenarios for nested blocks and statements, aligning with various style guides.

```javascript
function hello(indentSize, type) {
    if (indentSize === 4 && type !== 'tab') {
        console.log('Each next indentation will increase on 4 spaces');
    }
}

1  
2  
3  
4  
5  


```

--------------------------------

### Install Dependencies with npm

Source: https://eslint.org/docs/latest/contribute/development-environment

After cloning the repository, install all necessary development dependencies using npm.

```bash
cd eslint
npm install
```

--------------------------------

### Install ESLint Configuration with Bun

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Use this command to install the Airbnb base configuration for ESLint when using Bun.

```bash
bun add --dev eslint-config-airbnb-base

1  


```

--------------------------------

### JSCS airbnb preset example

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Example of a JSCS configuration file using the 'airbnb' preset.

```json
{
	"preset": "airbnb"
}
```

--------------------------------

### Install Dependencies with yarn

Source: https://eslint.org/docs/latest/contribute/development-environment

After cloning the repository, install all necessary development dependencies using yarn.

```bash
cd eslint
yarn add
```

--------------------------------

### Install ESLint Configuration with pnpm

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Use this command to install the Airbnb base configuration for ESLint when using pnpm.

```bash
pnpm add --save-dev eslint-config-airbnb-base

1  


```

--------------------------------

### Install ESLint with bun

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Installs the ESLint package as a project dependency using bun.

```bash
bun add eslint

1  


```

--------------------------------

### Install @eslint/js with Bun

Source: https://eslint.org/docs/latest/use/configure/migration-guide

Install the @eslint/js package as a development dependency using Bun.

```bash
bun add --dev @eslint/js
```

--------------------------------

### Install ESLint Configuration with Yarn

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Use this command to install the Airbnb base configuration for ESLint when using Yarn.

```bash
yarn add --dev eslint-config-airbnb-base

1  


```

--------------------------------

### Install @eslint/eslintrc

Source: https://eslint.org/docs/latest/use/configure/migration-guide

Install the compatibility package required for translating legacy eslintrc configurations.

```bash
npm install --save-dev @eslint/eslintrc
```

```bash
yarn add --dev @eslint/eslintrc
```

```bash
pnpm add --save-dev @eslint/eslintrc
```

```bash
bun add --dev @eslint/eslintrc
```

--------------------------------

### Set up custom rule project

Source: https://eslint.org/docs/latest/extend/custom-rule-tutorial

Commands to create a new directory, initialize an npm project, and create the custom rule file.

```bash
mkdir eslint-custom-rule-example # create directory
cd eslint-custom-rule-example # enter the directory
npm init -y # init new npm project
touch enforce-foo-bar.js # create file enforce-foo-bar.js

1  
2  
3  
4  


```

--------------------------------

### Install ESLint with Yarn

Source: https://eslint.org/docs/latest/component-library/code-tabs

Use this command to install ESLint as a development dependency using Yarn.

```bash
yarn install

1  

```

--------------------------------

### Install ESLint Plugin with bun

Source: https://eslint.org/docs/latest/extend/custom-rule-tutorial

Install your published ESLint plugin as a development dependency using bun.

```bash
# Add your package name here
bun add --dev eslint-plugin-example

```

--------------------------------

### Run Configuration Initialization Wizard

Source: https://eslint.org/docs/latest/use/command-line-interface

Start the interactive wizard to help set up an ESLint configuration file. By default, this is disabled.

```bash
--init
```

--------------------------------

### Install ESLint with npm

Source: https://eslint.org/docs/latest/component-library/code-tabs

Use this command to install ESLint as a development dependency using npm.

```bash
npm install --save-dev

1  

```

--------------------------------

### Show Help Information

Source: https://eslint.org/docs/latest/use/command-line-interface

Display the help message, listing all available command-line options.

```bash
-h, --help
```

--------------------------------

### Install ESLint as Dev Dependency (bun)

Source: https://eslint.org/docs/latest/extend/custom-rule-tutorial

Install the `eslint` package as a development dependency using bun. This is required for running tests.

```bash
bun add --dev eslint


```

--------------------------------

### Initialize bun Project

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Initializes a new Node.js project with a package.json file using bun.

```bash
bun init -y

1  


```

--------------------------------

### Create Configuration File

Source: https://eslint.org/docs/latest/use/getting-started

Initialize the ESLint configuration file using the touch command.

```bash
touch eslint.config.js
```

--------------------------------

### Disallow Unicode BOM ('always' - incorrect)

Source: https://eslint.org/docs/latest/rules/unicode-bom

This example shows incorrect code when the 'always' option is used, as the file does not start with a BOM.

```javascript
/*eslint unicode-bom: ["error", "always"]*/

let abc;

1  
2  
3  

```

--------------------------------

### Initialize npm Project

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Initializes a new Node.js project with a package.json file using npm.

```bash
npm init -y

1  



```

--------------------------------

### Correct import with allowImportNamePattern

Source: https://eslint.org/docs/latest/rules/no-restricted-imports

This example demonstrates a correct import when `allowImportNamePattern` is configured to allow imports starting with 'is', and 'isEmpty' is imported.

```javascript
/*eslint no-restricted-imports: ["error", { patterns: [{ group: ["utils/*"], allowImportNamePattern: '^is' }]}]*/

import { isEmpty } from 'utils/collection-utils';
```

--------------------------------

### Incorrect import with allowImportNamePattern

Source: https://eslint.org/docs/latest/rules/no-restricted-imports

This example shows an incorrect import when `allowImportNamePattern` is configured to only allow imports starting with 'has', but 'isEmpty' is imported.

```javascript
/*eslint no-restricted-imports: ["error", { patterns: [{ group: ["utils/*"], allowImportNamePattern: '^has' }]}]*/

import { isEmpty } from 'utils/collection-utils';
```

--------------------------------

### Correct code with 'initializations' option

Source: https://eslint.org/docs/latest/rules/one-var-declaration-per-line

Examples of correct code when using the default 'initializations' option.

```javascript
/*eslint one-var-declaration-per-line: ["error", "initializations"]*/

var a, b;

let c,
    d;

let e,
    f = 0;

1  
2  
3  
4  
5  
6  
7  
8  
9  

```

--------------------------------

### Initial examples of function spacing

Source: https://eslint.org/docs/latest/rules/space-before-function-parentheses

These examples illustrate variations in spacing before function parentheses in named and anonymous functions.

```javascript
function withoutSpace(x) {
    // ...
}

function withSpace (x) {
    // ...
}

var anonymousWithoutSpace = function() {};

var anonymousWithSpace = function () {};
```

--------------------------------

### Example of new operator usage

Source: https://eslint.org/docs/latest/rules/new-cap

This example demonstrates the basic usage of the `new` operator with a constructor function.

```javascript
const friend = new Person();

1  



```

--------------------------------

### Complete ESLint Integration Example

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

A comprehensive example demonstrating ESLint integration, including instance creation, linting, fixing, and result output.

```javascript
const { ESLint } = require("eslint");

// Create an instance of ESLint with the configuration passed to the function
function createESLintInstance(overrideConfig) {
	return new ESLint({
		overrideConfigFile: true,
		overrideConfig,
		fix: true,
	});
}

// Lint the specified files and return the results
async function lintAndFix(eslint, filePaths) {
	const results = await eslint.lintFiles(filePaths);

	// Apply automatic fixes and output fixed code
	await ESLint.outputFixes(results);

	return results;
}

// Log results to console if there are any problems
function outputLintingResults(results) {
	// Identify the number of problems found
	const problems = results.reduce(
		(acc, result) => acc + result.errorCount + result.warningCount,
		0,
	);

	if (problems > 0) {
		console.log("Linting errors found!");
		console.log(results);
	} else {
		console.log("No linting errors found.");
	}
	return results;
}

// Put previous functions all together
async function lintFiles(filePaths) {
	// The ESLint configuration. Alternatively, you could load the configuration
	// from an eslint.config.js file or just use the default config.
	const overrideConfig = {
		languageOptions: {
			ecmaVersion: 2018,
			sourceType: "commonjs",
		},
		rules: {
			"no-console": "error",
			"no-unused-vars": "warn",
		},
	};

	const eslint = createESLintInstance(overrideConfig);
	const results = await lintAndFix(eslint, filePaths);
	return outputLintingResults(results);
}

// Export integration
module.exports = { lintFiles };
```

--------------------------------

### Run All ESLint Unit Tests

Source: https://eslint.org/docs/latest/contribute/tests

Execute all unit tests in the `tests` directory. This command is used after initial setup with `npm install`.

```bash
npm test
```

--------------------------------

### Require Unicode BOM ('never' - incorrect)

Source: https://eslint.org/docs/latest/rules/unicode-bom

This example shows incorrect code when the 'never' option is used, as the file incorrectly starts with a BOM.

```javascript
// U+FEFF at the beginning

/*eslint unicode-bom: ["error", "never"]*/

let abc;

1  
2  
3  
4  
5  

```

--------------------------------

### Using Migrated Flat Config in User Project

Source: https://eslint.org/docs/latest/extend/plugin-migration-flat-config

Example of how a user would import and use a plugin that has been migrated to the flat config format.

```javascript
import { defineConfig } from "eslint/config";
import example from "eslint-plugin-example";

export default defineConfig([
	// use recommended config and provide your own overrides
	{
		files: ["**/*.js", "**/*.cjs", "**/*.mjs"],
		plugins: {
			example,
		},
		extends: ["example/recommended"],
		rules: {
			"example/rule1": "warn",
		},
	},
]);

```

--------------------------------

### Correct variable sorting

Source: https://eslint.org/docs/latest/rules/sort-vars

Variables declared in the same block should be sorted alphabetically. This example shows correctly sorted variables, including those starting with underscores.

```javascript
/*eslint sort-vars: "error"*/

let a, b, c, d;

let _a = 10;
let _b = 20;

let E, e;

let G, f, h;
```

--------------------------------

### Install ESLint Packages

Source: https://eslint.org/docs/latest/use/getting-started

Install the latest ESLint and JS configuration packages using your preferred package manager.

```bash
npm install --save-dev eslint@latest @eslint/js@latest
```

```bash
yarn add --dev eslint@latest @eslint/js@latest
```

```bash
pnpm add --save-dev eslint@latest @eslint/js@latest
```

```bash
bun add --dev eslint@latest @eslint/js@latest
```

--------------------------------

### Incorrect import with importNamePattern (single pattern)

Source: https://eslint.org/docs/latest/rules/no-restricted-imports

This example shows an incorrect import when `importNamePattern` is set to '^is', restricting imports that do not start with 'is'.

```javascript
/*eslint no-restricted-imports: ["error", { patterns: [{ group: ["utils/*"], importNamePattern: '^is', message: "Use 'is*' functions from lodash instead." }]}]*/

import { isEmpty } from 'utils/collection-utils';
```

--------------------------------

### Correct code examples for newline-before-return

Source: https://eslint.org/docs/latest/rules/newline-before-return

Provides examples of code that adheres to the rule, with proper empty lines before return statements.

```javascript
/*eslint newline-before-return: "error"*/

function foo1() {
    return;
}

function foo2() {

    return;
}

function foo3(bar) {
    if (!bar) return;
}

function foo4(bar) {
    if (!bar) { return };
}

function foo5(bar) {
    if (!bar) {
        return;
    }
}

function foo6(bar) {
    if (!bar) {
        return;
    }

    return bar;
}

function foo7(bar) {
    if (!bar) {

        return;
    }
}

function foo8() {

    // comment
    return;
}
```

--------------------------------

### Allow non-warning comments (default)

Source: https://eslint.org/docs/latest/rules/no-warning-comments

This example demonstrates comments that are permitted by the default rule settings, as they do not contain the predefined warning terms at the start.

```javascript
/*eslint no-warning-comments: "error"*/

function callback(err, results) {
  if (err) {
    console.error(err);
    return;
  }
  // NOT READY FOR PRIME TIME
  // but too bad, it is not a predefined warning term
}
```

--------------------------------

### Initialize ESLint configuration with npm

Source: https://eslint.org/docs/latest/use/command-line-interface

Use this command with npm to start the ESLint configuration initialization wizard, which helps create an eslint.config.js file.

```bash
npx eslint --init 

1  

```

--------------------------------

### Disallow warning comments (default)

Source: https://eslint.org/docs/latest/rules/no-warning-comments

This example shows comments that violate the default rule settings, which flag 'TODO' and 'FIXME' at the start of comments.

```javascript
/*eslint no-warning-comments: "error"*/

/*
FIXME
*/
function callback(err, results) {
  if (err) {
    console.error(err);
    return;
  }
  // TODO
}
```

--------------------------------

### Correct code examples for 'smart' option

Source: https://eslint.org/docs/latest/rules/eqeqeq

These examples show correct usage when the `eqeqeq` rule is configured with the `smart` option, demonstrating allowed exceptions for literals, typeof, and null comparisons.

```javascript
/*eslint eqeqeq: ["error", "smart"]*/

typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null
```

--------------------------------

### Correct code with ignored decoration characters

Source: https://eslint.org/docs/latest/rules/no-warning-comments

This example shows comments that are considered correct when specific decoration characters are configured to be ignored at the start of comments.

```javascript
/*eslint no-warning-comments: ["error", { "decoration": ["/", "*"] }]*/

//!TODO preceded by non-decoration character
/**
 *!TODO preceded by non-decoration character in a block comment
 */
```

--------------------------------

### Correct code with 'always' padding

Source: https://eslint.org/docs/latest/rules/padded-blocks

This example demonstrates correct code when the 'always' option is used, showing blocks with required empty lines at the start and end.

```javascript
/*eslint padded-blocks: ["error", "always"]*/

if (a) {

    b();

}

if (a)
{

    b();

}

if (a) {

    // comment
    b();

}

class C {

    static {

        a();

    }

}
```

--------------------------------

### Correct code examples for space-return-throw-case

Source: https://eslint.org/docs/latest/rules/space-return-throw-case

These examples demonstrate code that adheres to the `space-return-throw-case` rule, with proper spacing after `throw`, `return`, and `case`.

```javascript
/*eslint space-return-throw-case: "error"*/

throw {a: 0};

function f(){ return -a; }

switch(a){ case 'a': break; }

1  
2  
3  
4  
5  
6  
7  


```

--------------------------------

### Ignore multiple decoration characters

Source: https://eslint.org/docs/latest/rules/no-warning-comments

This example demonstrates ignoring multiple types of decoration characters (like '/' and '*') at the start of comments when searching for warning terms.

```javascript
/*eslint no-warning-comments: ["error", { "decoration": ["/", "*"] }]*/

////// TODO decorative slashes and whitespace are ignored //////
//***** todo decorative asterisks are also ignored *****//
/**
 * TODO new lines are also ignored in block comments.
 */
```

--------------------------------

### Initialize ESLint configuration with bun

Source: https://eslint.org/docs/latest/use/command-line-interface

Use this command with bun to start the ESLint configuration initialization wizard, which helps create an eslint.config.js file.

```bash
bunx eslint --init 

1  

```

--------------------------------

### Correct usage of `require` and instantiation

Source: https://eslint.org/docs/latest/rules/no-new-require

This code demonstrates the recommended pattern: first, require the module, then create a new instance of the exported constructor.

```javascript
/*eslint no-new-require: "error"*/

var AppHeader = require('app-header');
var appHeader = new AppHeader();
```

--------------------------------

### Disallow /=foo/ regex

Source: https://eslint.org/docs/latest/rules/no-div-regex

This example shows incorrect usage where a regular expression starts with '/=', which can be mistaken for a division assignment operator. This pattern is flagged by the rule.

```javascript
function bar() { return /=foo/; }
```

--------------------------------

### Using capIsNewExceptionPattern with ^Foo prefix

Source: https://eslint.org/docs/latest/rules/new-cap

This example illustrates using `capIsNewExceptionPattern` to allow calls that start with `Foo`, such as `Foo(42)`, `Foobar(42)`, and `Foo.Bar(42)`.

```javascript
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^Foo" }]*/

const x = Foo(42);

const y = Foobar(42);

const z = Foo.Bar(42);

1  
2  
3  
4  
5  
6  
7  

```

--------------------------------

### Initialize pnpm Project

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Initializes a new Node.js project with a package.json file using pnpm.

```bash
pnpm init -y

1  


```

--------------------------------

### As Needed - Correct Code Examples

Source: https://eslint.org/docs/latest/rules/arrow-parens

Examples of code that adheres to the 'as-needed' rule, demonstrating correct usage of parentheses only when required.

```javascript
/*eslint arrow-parens: ["error", "as-needed"]*/

() => {};
a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => { if (true) {} });
(a, b, c) => a;
(a = 10) => a;
([a, b]) => a;
({a, b}) => a;
const f = (/** @type {number} */a) => a + a;
const g = (/* comment */ a) => a + a;
const h = (a /* comment */) => a + a;
```

--------------------------------

### Using capIsNewExceptionPattern with person.*

Source: https://eslint.org/docs/latest/rules/new-cap

This example demonstrates how to use the `capIsNewExceptionPattern` option to allow calls to `person.Acquaintance()` and `person.Friend()` even though they don't start with an uppercase letter.

```javascript
/*eslint new-cap: ["error", { "capIsNewExceptionPattern": "^person\\.." }]*/

const friend = person.Acquaintance();
const bestFriend = person.Friend();

1  
2  
3  
4  

```

--------------------------------

### Ignore Default Patterns

Source: https://eslint.org/docs/latest/rules/lines-around-comment

By default, comments starting with specific keywords like 'eslint', 'jshint', etc., are ignored. This example shows a comment that is ignored due to these defaults.

```javascript
/*eslint lines-around-comment: ["error"]*/

foo();
/* jshint mentioned in this comment */
bar();

```

--------------------------------

### Alphabetical list maintenance

Source: https://eslint.org/docs/latest/rules/sort-vars

The rule maintains an alphabetical list starting from the first variable, excluding problems. This example shows how multiple problems can be reported in a single declaration block.

```javascript
/*eslint sort-vars: "error"*/

let c, d, a, b;
```

--------------------------------

### Install Polyjuice via bun

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Install the Polyjuice utility globally using bun for converting JSCS configuration files to ESLint format.

```bash
bun add --global polyjuice
```

--------------------------------

### Examples of Function Calls

Source: https://eslint.org/docs/latest/rules/func-call-spacing

Demonstrates equivalent function calls with and without spacing before the parentheses.

```javascript
alert('Hello');
alert ('Hello');

console.log(42);
console.log (42);

new Date();
new Date ();
```

--------------------------------

### Ignoring Arguments by Pattern - Correct Code

Source: https://eslint.org/docs/latest/rules/no-unused-vars

This example demonstrates the `argsIgnorePattern` option, which allows ignoring arguments that match a specified regular expression. Here, arguments starting with an underscore are ignored.

```javascript
/*eslint no-unused-vars: ["error", { "argsIgnorePattern": "^_" }]*/

function foo(x, _y) {
    return x + 1;
}
foo();

1  
2  
3  
4  
5  
6  


```

--------------------------------

### Initialize yarn Project

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Initializes a new Node.js project with a package.json file using yarn.

```bash
yarn init -y

1  


```

--------------------------------

### Ignore Specific Comment Patterns

Source: https://eslint.org/docs/latest/rules/line-comment-position

Allows configuration of a custom regular expression to ignore specific line comments, in addition to default patterns. This example ignores comments starting with 'pragma'.

```javascript
/*eslint line-comment-position: ["error", { "ignorePattern": "pragma" }]*/
1 + 1; // pragma valid comment
```

```javascript
/*eslint line-comment-position: ["error", { "ignorePattern": "pragma" }]*/
1 + 1; // invalid comment
```

--------------------------------

### Incorrect Code Examples

Source: https://eslint.org/docs/latest/rules/prefer-template

Examples of string concatenation that will trigger the rule.

```javascript
/*eslint prefer-template: "error"*/

const str = "Hello, " + name + "!";
const str1 = "Time: " + (12 * 60 * 60 * 1000);
```

--------------------------------

### Correct code examples

Source: https://eslint.org/docs/latest/rules/no-multi-assign

Examples of code that comply with the no-multi-assign rule.

```javascript
/*eslint no-multi-assign: "error"*/

let a = 5;
let b = 5;
const c = 5;

const foo = "baz";
const bar = "baz";

let d = c;
let e = c;

class Foo {
    a = 10;
    b = 10;
}

a = "quux";
b = "quux";
```

--------------------------------

### Incorrect code examples

Source: https://eslint.org/docs/latest/rules/no-multi-assign

Examples of code that violate the no-multi-assign rule.

```javascript
/*eslint no-multi-assign: "error"*/

let a = b = c = 5;

const foo = bar = "baz";

let d =
    e =
    c;

class Foo {
    a = b = 10;
}

a = b = "quux";
```

--------------------------------

### Correct Code Examples for Default Configuration

Source: https://eslint.org/docs/latest/rules/semi-spacing

Demonstrates code that adheres to the default semi-spacing rule (no space before, space after semicolon).

```javascript
/*eslint semi-spacing: "error"*/

var foo;
var foo; var bar;
throw new Error("error");
while (a) { break; }
for (i = 0; i < 10; i++) {}
for (;;) {}
if (true) {;}
;foo();

1  
2  
3  
4  
5  
6  
7  
8  
9  
10  


```

--------------------------------

### Incorrect Code Examples

Source: https://eslint.org/docs/latest/rules/no-constant-binary-expression

Examples of code that trigger the no-constant-binary-expression rule.

```javascript
/*eslint no-constant-binary-expression: "error"*/

const value1 = +x == null;

const value2 = condition ? x : {} || DEFAULT;

const value3 = !foo == null;

const value4 = new Boolean(foo) === true;

const objIsEmpty = someObj === {};

const arrIsEmpty = someArr === [];

const shortCircuit1 = condition1 && false && condition2;

const shortCircuit2 = condition1 || true || condition2;

const shortCircuit3 = condition1 ?? "non-nullish" ?? condition2;
```

--------------------------------

### DoWhileStatement

Source: https://eslint.org/docs/latest/extend/code-path-analysis

Demonstrates code path branching in a do-while loop.

```javascript
do {
	foo();
	bar();
} while (a);

1  
2  
3  
4  



```

--------------------------------

### Create ESLint Integration File

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Creates a new JavaScript file named 'example-eslint-integration.js' in the project root.

```bash
touch example-eslint-integration.js

1  


```

--------------------------------

### Correct Code Examples

Source: https://eslint.org/docs/latest/rules/no-constant-binary-expression

Examples of code that avoid constant binary expressions.

```javascript
/*eslint no-constant-binary-expression: "error"*/

const value1 = x == null;

const value2 = (condition ? x : {}) || DEFAULT;

const value3 = !(foo == null);

const value4 = Boolean(foo) === true;

const objIsEmpty = Object.keys(someObj).length === 0;

const arrIsEmpty = someArr.length === 0;
```

--------------------------------

### Correct Usage with Configuration File

Source: https://eslint.org/docs/latest/rules/no-process-env

This snippet shows the recommended approach by importing configuration from a separate file, thus avoiding direct `process.env` usage. This promotes better maintainability and avoids global dependencies.

```javascript
/*eslint no-process-env: "error"*/

var config = require("./config");

if(config.env === "development") {
    //...
}

```

--------------------------------

### Correct Code Examples for Space Before, No Space After

Source: https://eslint.org/docs/latest/rules/semi-spacing

Shows code that correctly follows the configuration enforcing space before and disallowing space after semicolons.

```javascript
/*eslint semi-spacing: ["error", { "before": true, "after": false }]*/

var foo ;
var foo ;var bar ;
throw new Error("error") ;
while (a) {break ;}
for (i = 0 ;i < 10 ;i++) {}

1  
2  
3  
4  
5  
6  
7  


```

--------------------------------

### Initialize ESLint Configuration

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Use `eslint --init` and select 'Inspect your JavaScript file(s)' to create a configuration based on your existing code, similar to JSCS's `--auto-configure`.

```bash
eslint --init
? How would you like to configure ESLint? (Use arrow keys)
> Answer questions about your style
  Use a popular style guide
  Inspect your JavaScript file(s)

1  
2  
3  
4  
5  


```

--------------------------------

### Install ESLint with pnpm

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Installs the ESLint package as a project dependency using pnpm.

```bash
pnpm add eslint

1  


```

--------------------------------

### Correct code: Allowed modules

Source: https://eslint.org/docs/latest/rules/no-restricted-modules

This example shows correct usage of the `crypto` module when `fs` and `cluster` are restricted.

```javascript
/*eslint no-restricted-modules: ["error", "fs", "cluster"]*/

const crypto = require('crypto');
```

--------------------------------

### Install ESLint with yarn

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Installs the ESLint package as a project dependency using yarn.

```bash
yarn add eslint

1  


```

--------------------------------

### Correct code: Allowed modules with exceptions

Source: https://eslint.org/docs/latest/rules/no-restricted-modules

This example demonstrates correct usage of `crypto` and `lodash/pick` when `fs` and `cluster` are restricted, and `lodash/*` is restricted except for `lodash/pick`.

```javascript
/*eslint no-restricted-modules: ["error", {
    "paths": ["fs", "cluster"],
    "patterns": ["lodash/*", "!lodash/pick"]
}]*/

const crypto = require('crypto');
const pick = require('lodash/pick');
```

--------------------------------

### Install ESLint with npm

Source: https://eslint.org/docs/latest/integrate/integration-tutorial

Installs the ESLint package as a project dependency using npm.

```bash
npm install eslint

1  


```

--------------------------------

### Incorrect Code Examples (Default)

Source: https://eslint.org/docs/latest/rules/no-useless-rename

Examples of code that violates the 'no-useless-rename' rule by default.

```javascript
/*eslint no-useless-rename: "error"*/

import { foo1 as foo1 } from "bar";
import { "foo2" as foo2 } from "bar";
export { foo1 as foo1 };
export { foo2 as "foo2" };
export { foo3 as foo3 } from "bar";
export { "foo4" as "foo4" } from "bar";
let { foo3: foo3 } = bar;
let { 'foo4': foo4 } = bar;
function foo({ bar: bar }) {}
({ foo: foo }) => {}
```

--------------------------------

### Initialize ESLint configuration with pnpm

Source: https://eslint.org/docs/latest/use/command-line-interface

Use this command with pnpm to start the ESLint configuration initialization wizard, which helps create an eslint.config.js file.

```bash
pnpm dlx eslint --init 

1  

```

--------------------------------

### Initialize SourceCode instance

Source: https://eslint.org/docs/latest/integrate/nodejs-api

Creates a new SourceCode instance using a text string and an ESTree-compliant AST.

```javascript
const SourceCode = require("eslint").SourceCode;

const code = new SourceCode("var foo = bar;", ast);
```

--------------------------------

### Correct code examples for max lines

Source: https://eslint.org/docs/latest/rules/max-lines-per-function

Examples that satisfy the max lines constraint.

```javascript
/*eslint max-lines-per-function: ["error", 3]*/
function foo() {
    const x = 0;
}
```

```javascript
/*eslint max-lines-per-function: ["error", 4]*/
function foo() {
    // a comment
    const x = 0;
}
```

```javascript
/*eslint max-lines-per-function: ["error", 5]*/
function foo() {
    // a comment followed by a blank line

    const x = 0;
}
```

--------------------------------

### Create ESLint Configuration File

Source: https://eslint.org/docs/latest/extend/custom-rule-tutorial

Initialize an ESLint configuration file using `eslint.config.js`. This file will define project-wide ESLint settings.

```bash
touch eslint.config.js

```

--------------------------------

### Incorrect code examples for max lines

Source: https://eslint.org/docs/latest/rules/max-lines-per-function

Examples that violate the max lines constraint.

```javascript
/*eslint max-lines-per-function: ["error", 2]*/
function foo() {
    const x = 0;
}
```

```javascript
/*eslint max-lines-per-function: ["error", 3]*/
function foo() {
    // a comment
    const x = 0;
}
```

```javascript
/*eslint max-lines-per-function: ["error", 4]*/
function foo() {
    // a comment followed by a blank line

    const x = 0;
}
```

--------------------------------

### Nested Callback Example

Source: https://eslint.org/docs/latest/rules/max-nested-callbacks

An example of deeply nested callbacks that may trigger the rule.

```javascript
foo(function () {
    bar(function () {
        baz(function() {
            qux(function () {

            });
        });
    });
});
```

--------------------------------

### Configure no-shadow Rule Options

Source: https://eslint.org/docs/latest/rules/no-shadow

This example shows how to configure the no-shadow rule with various options, including `builtinGlobals`, `hoist`, `allow`, and `ignoreOnInitialization`.

```json
{
    "no-shadow": ["error", { "builtinGlobals": false, "hoist": "functions", "allow": [], "ignoreOnInitialization": false }]
}
```

--------------------------------

### Configure ECMAScript 5 and Script Mode

Source: https://eslint.org/docs/latest/use/configure/language-options

Example configuration for linting legacy ECMAScript 5 code using script mode.

```javascript
// eslint.config.js
import { defineConfig } from "eslint/config";

export default defineConfig([
	{
		languageOptions: {
			ecmaVersion: 5,
			sourceType: "script",
		},
	},
]);
```

--------------------------------

### Top-level require() example

Source: https://eslint.org/docs/latest/rules/global-require

Demonstrates a standard `require()` call at the top level of a module.

```javascript
var fs = require("fs");
```

--------------------------------

### Run ESLint using bunx

Source: https://eslint.org/docs/latest/use/command-line-interface

Use bunx to execute ESLint from the command line. This is an alternative to npx for Bun users.

```bash
bunx eslint [options] [file|dir|glob]*
```

--------------------------------

### Print Configuration for a File

Source: https://eslint.org/docs/latest/use/command-line-interface

Output the effective configuration for a specified file, including inherited configurations and resolved rules.

```bash
--print-config path::String
```

--------------------------------

### Stylish Formatter Output Example

Source: https://eslint.org/docs/latest/use/formatters

Example of the default human-readable output generated by the stylish formatter.

```text
/var/lib/jenkins/workspace/eslint Release/eslint/fullOfProblems.js
  1:10  error    'addOne' is defined but never used                              no-unused-vars
  2:9   error    Use the isNaN function to compare with NaN                      use-isnan
  3:16  error    Unexpected space before unary operator '++'                     space-unary-ops
  3:16  error    The value assigned to 'i' is not used in subsequent statements  no-useless-assignment
  3:20  warning  Missing semicolon                                               semi
  4:12  warning  Unnecessary 'else' after 'return'                               no-else-return
  5:1   warning  Expected indentation of 8 spaces but found 6                    indent
  5:7   error    Function 'addOne' expected a return value                       consistent-return
  5:13  warning  Missing semicolon                                               semi

✖ 9 problems (5 errors, 4 warnings)
  1 error and 4 warnings potentially fixable with the `--fix` option.
```

--------------------------------

### Correct Code Examples

Source: https://eslint.org/docs/latest/rules/prefer-template

Examples of valid code using template literals or non-concatenated strings.

```javascript
/*eslint prefer-template: "error"*/

const str = "Hello World!";
const str1 = `Hello, ${name}!`;
const str2 = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
const str4 = "Hello, " + "World!";
```

--------------------------------

### Install Polyjuice via yarn

Source: https://eslint.org/docs/latest/use/migrating-from-jscs

Install the Polyjuice utility globally using yarn for converting JSCS configuration files to ESLint format.

```bash
yarn global add polyjuice
```

--------------------------------

### Correct Code Examples (Default)

Source: https://eslint.org/docs/latest/rules/no-useless-rename

Examples of code that is considered correct by the 'no-useless-rename' rule by default.

```javascript
/*eslint no-useless-rename: "error"*/

import * as foo1 from "foo";
import { foo2 } from "bar";
import { foo as bar1 } from "baz";
import { "foo" as bar2 } from "baz";

export { foo };
export { foo as bar1 };
export { foo as "bar2" };
export { foo as bar3 } from "foo";
export { "foo" as "bar4" } from "foo";

let { foo } = bar;
let { foo: bar } = baz;
let { [qux]: qux } = bar;

function foo3({ bar }) {}
function foo4({ bar: baz }) {}

({ foo }) => {}
({ foo: bar }) => {}
```

--------------------------------

### Examples of correct code with default spacing

Source: https://eslint.org/docs/latest/rules/arrow-spacing

These examples show correct spacing before and after the arrow when the default configuration ({ "before": true, "after": true }) is used.

```javascript
/*eslint arrow-spacing: "error"*/

() => {};
(a) => {};
a => a;
() => {'\n'};
```

--------------------------------

### Example of unused label

Source: https://eslint.org/docs/latest/rules/no-unused-labels

This example demonstrates an unused label `OUTER_LOOP` which is likely a refactoring error.

```javascript
OUTER_LOOP:
for (const student of students) {
    if (checkScores(student.scores)) {
        continue;
    }
    doSomething(student);
}
```

--------------------------------

### Start ESLint MCP Server

Source: https://eslint.org/docs/latest/use/command-line-interface

This option starts the ESLint MCP server, which is intended for use with AI agents. It does not take any arguments.

```bash
npx eslint --mcp
```

```bash
yarn dlx eslint --mcp
```

```bash
pnpm dlx eslint --mcp
```

```bash
bunx eslint --mcp
```

--------------------------------

### Correct code with max: 2 and maxBOF: 1 (Alternative)

Source: https://eslint.org/docs/latest/rules/no-multiple-empty-lines

An alternative example of correct code with maxBOF: 1, demonstrating proper spacing at the beginning of the file.

```javascript

/*eslint no-multiple-empty-lines: ["error", { "max": 2, "maxBOF": 1}]*/


var foo = 5;


var bar = 3;

1  
2  
3  
4  
5  
6  


```

--------------------------------

### Install @eslint/js Package

Source: https://eslint.org/docs/latest/use/configure/migration-guide

Commands to install the @eslint/js package required for predefined configs in flat config.

```bash
npm install --save-dev @eslint/js
```

```bash
yarn add --dev @eslint/js
```

```bash
pnpm add --save-dev @eslint/js
```

--------------------------------

### Migrating Environment to Flat Config

Source: https://eslint.org/docs/latest/extend/plugin-migration-flat-config

Demonstrates migrating a legacy environment to a flat config format by adding it to the `plugin.configs` object.

```javascript
const plugin = {
	configs: {},
	rules: {},
	processors: {},
};

// assign configs here so we can reference `plugin`
Object.assign(plugin.configs, {
	mocha: {
		languageOptions: {
			globals: {
				it: "writeable",
				xit: "writeable",
				describe: "writeable",
				xdescribe: "writeable",
			},
		},
	},
});

// for ESM
export default plugin;

// OR for CommonJS
module.exports = plugin;

```

--------------------------------

### Configure MCP for local installation

Source: https://eslint.org/docs/latest/use/mcp

Update your MCP configuration to use the locally installed @eslint/mcp package.

```json
{
	"servers": {
		"ESLint": {
			"type": "stdio",
			"command": "npx",
			"args": ["@eslint/mcp"]
		}
	}
}
```

```json
{
	"mcpServers": {
		"eslint": {
			"command": "npx",
			"args": ["@eslint/mcp"]
		}
	}
}
```
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