Linting and Formatting

Deno includes built-in tools for linting and formatting your code, eliminating the need for external dependencies like ESLint or Prettier.

Formatting with deno fmt

Basic Usage

# Format all supported files in current directory
deno fmt

# Format specific files
deno fmt src/main.ts

# Format multiple files
deno fmt src/main.ts src/utils.ts

# Format directory
deno fmt src/

# Check formatting without making changes
deno fmt --check

Supported File Types

Deno’s formatter supports:

TypeScript (.ts, .tsx)
JavaScript (.js, .jsx, .mjs)
JSON (.json, .jsonc)
Markdown (.md, .markdown)

Configuration

Configure formatting in deno.json:

deno.json

{
  "fmt": {
    "useTabs": false,
    "lineWidth": 80,
    "indentWidth": 2,
    "semiColons": true,
    "singleQuote": false,
    "proseWrap": "always",
    "include": ["src/"],
    "exclude": ["src/generated/"]
  }
}

Formatting Options

Basic Options

useTabs - Use tabs instead of spaces (default: false)
lineWidth - Maximum line width (default: 80)
indentWidth - Number of spaces per indent (default: 2)
singleQuote - Use single quotes (default: false)
semiColons - Add semicolons (default: true)

Advanced Options

proseWrap - How to wrap prose in Markdown: "always", "never", "preserve" (default: "always")
quoteProps - When to quote object properties: "asNeeded", "consistent", "preserve" (default: "preserve")
newLineKind - Newline character: "auto", "crlf", "lf", "system" (default: "lf")

Brace Styles

useBraces - Use braces for single-line blocks: "maintain", "whenNotSingleLine", "always", "preferNone" (default: "whenNotSingleLine")
bracePosition - Opening brace position: "maintain", "sameLine", "nextLine", "sameLineUnlessHanging" (default: "sameLine")
singleBodyPosition - Single body position: "maintain", "sameLine", "nextLine", "sameLineUnlessHanging" (default: "sameLineUnlessHanging")
nextControlFlowPosition - Next control flow position: "maintain", "sameLine", "nextLine" (default: "sameLine")

JSX Options

jsx.bracketPosition - JSX bracket position: "maintain", "sameLine", "nextLine" (default: "nextLine")
jsx.forceNewLineSurroundingContent - Force newlines around JSX content (default: false)
jsx.multiLineParens - Wrap multi-line JSX in parentheses: "never", "prefer", "always" (default: "prefer")

Other Options

operatorPosition - Binary operator position: "maintain", "sameLine", "nextLine" (default: "sameLine")
trailingCommas - Add trailing commas: "never", "always", "onlyMultiLine" (default: "onlyMultiLine")
spaceAround - Spaces around enclosed expressions (default: false)
spaceSurroundingProperties - Spaces in object literals (default: true)

Include/Exclude Files

deno.json

{
  "fmt": {
    "include": [
      "src/**/*.ts",
      "tests/**/*.ts"
    ],
    "exclude": [
      "src/generated/",
      "dist/",
      "coverage/"
    ]
  }
}

Ignore Formatting

Use comments to ignore specific code:

// deno-fmt-ignore
const matrix = [
  [1,  2,  3],
  [4,  5,  6],
  [7,  8,  9],
];

// deno-fmt-ignore-file
// Entire file will not be formatted

Linting with deno lint

Basic Usage

# Lint all files in current directory
deno lint

# Lint specific files
deno lint src/main.ts

# Lint directory
deno lint src/

# Show rule documentation
deno lint --rules

Configuration

deno.json

{
  "lint": {
    "include": ["src/"],
    "exclude": ["src/generated/"],
    "rules": {
      "tags": ["recommended"],
      "include": ["ban-untagged-todo"],
      "exclude": ["no-unused-vars"]
    },
    "report": "pretty"
  }
}

Rule Configuration

Using Tags
Include Specific Rules
Exclude Rules

deno.json

{
  "lint": {
    "rules": {
      "tags": ["recommended"]
    }
  }
}

Available tags:

recommended - Recommended rules for most projects
fresh - Rules for Fresh framework

deno.json

{
  "lint": {
    "rules": {
      "tags": ["recommended"],
      "include": [
        "ban-untagged-todo",
        "no-await-in-sync-fn",
        "explicit-function-return-type"
      ]
    }
  }
}

deno.json

{
  "lint": {
    "rules": {
      "tags": ["recommended"],
      "exclude": [
        "no-unused-vars",
        "no-explicit-any"
      ]
    }
  }
}

Available Rules

View all available rules:

deno lint --rules

Common rules:

ban-ts-comment - Disallow @ts-ignore and similar comments
ban-untagged-todo - Require TODO comments to have an issue number
no-unused-vars - Disallow unused variables
no-explicit-any - Disallow explicit any type
no-await-in-sync-fn - Disallow await in non-async functions
eqeqeq - Require === and !==
no-const-assign - Disallow reassigning const variables
no-debugger - Disallow debugger statements

Report Formats

# Pretty format (default)
deno lint

# JSON format
deno lint --json

# Compact format
deno lint --compact

Configure in deno.json:

deno.json

{
  "lint": {
    "report": "pretty"
  }
}

Ignore Linting

Use comments to ignore specific rules:

// deno-lint-ignore no-explicit-any
function process(data: any) {
  return data;
}

// Multiple rules
// deno-lint-ignore no-explicit-any no-unused-vars
function example(data: any, unused: string) {
  return data;
}

// Entire file
// deno-lint-ignore-file

// Specific rule for entire file
// deno-lint-ignore-file no-explicit-any

Lint Plugins

Load custom lint plugins (unstable):

deno.json

{
  "lint": {
    "plugins": [
      "./custom-lint-rules.ts",
      "jsr:@custom/lint-rules"
    ]
  }
}

CI/CD Integration

GitHub Actions

.github/workflows/lint.yml

name: Lint and Format

on:
  pull_request:
  push:
    branches: [main]

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: denoland/setup-deno@v1
      
      - name: Check formatting
        run: deno fmt --check
      
      - name: Lint code
        run: deno lint

Pre-commit Hook

Create .git/hooks/pre-commit:

#!/bin/sh

# Format staged files
deno fmt

# Lint staged files
deno lint

# Add formatted files
git add -u

Make it executable:

chmod +x .git/hooks/pre-commit

VS Code Integration

The Deno VS Code extension automatically:

Formats on save
Shows lint errors inline
Provides quick fixes

Configure in .vscode/settings.json:

.vscode/settings.json

{
  "deno.enable": true,
  "deno.lint": true,
  "editor.formatOnSave": true,
  "[typescript]": {
    "editor.defaultFormatter": "denoland.vscode-deno"
  },
  "[javascript]": {
    "editor.defaultFormatter": "denoland.vscode-deno"
  }
}

Task Configuration

Add tasks to deno.json:

deno.json

{
  "tasks": {
    "fmt": "deno fmt",
    "fmt:check": "deno fmt --check",
    "lint": "deno lint",
    "check": "deno fmt --check && deno lint"
  }
}

Run tasks:

deno task fmt
deno task lint
deno task check

Best Practices

Format before committing

Always run deno fmt before committing code

Use recommended rules

Start with recommended lint rules and customize as needed

Automate in CI

Add formatting and linting checks to your CI pipeline

Configure editor

Set up format-on-save in your editor

Document exceptions

Add comments explaining why rules are disabled

Keep config minimal

Only override defaults when necessary

Example Configuration

deno.json

{
  "fmt": {
    "useTabs": false,
    "lineWidth": 100,
    "indentWidth": 2,
    "semiColons": true,
    "singleQuote": false,
    "proseWrap": "preserve",
    "include": ["src/", "tests/"],
    "exclude": ["src/generated/", "dist/", "coverage/"]
  },
  "lint": {
    "include": ["src/", "tests/"],
    "exclude": ["src/generated/"],
    "rules": {
      "tags": ["recommended"],
      "include": ["ban-untagged-todo"],
      "exclude": ["no-explicit-any"]
    },
    "report": "pretty"
  },
  "tasks": {
    "fmt": "deno fmt",
    "fmt:check": "deno fmt --check",
    "lint": "deno lint",
    "check": "deno fmt --check && deno lint && deno check **/*.ts"
  }
}

Migration from Other Tools

From Prettier

Deno’s formatter is similar to Prettier with sensible defaults:

deno.json

{
  "fmt": {
    "lineWidth": 80,        // printWidth
    "indentWidth": 2,       // tabWidth
    "useTabs": false,       // useTabs
    "semiColons": true,     // semi
    "singleQuote": false,   // singleQuote
    "trailingCommas": "onlyMultiLine"  // trailingComma: "es5"
  }
}

From ESLint

Map common ESLint rules to Deno lint:

no-unused-vars → no-unused-vars
@typescript-eslint/no-explicit-any → no-explicit-any
eqeqeq → eqeqeq
no-debugger → no-debugger
prefer-const → prefer-const

Getting Started

Core Concepts

Command Reference

Guides

Standard Library

Linting and Formatting

Formatting with deno fmt

Basic Usage

Supported File Types

Configuration

Formatting Options

Include/Exclude Files

Ignore Formatting

Linting with deno lint

Basic Usage

Configuration

Rule Configuration

Available Rules

Report Formats

Ignore Linting

Lint Plugins

CI/CD Integration

GitHub Actions

Pre-commit Hook

VS Code Integration

Task Configuration

Best Practices

Format before committing

Use recommended rules

Automate in CI

Configure editor

Document exceptions

Keep config minimal

Example Configuration

Migration from Other Tools

From Prettier

From ESLint

Build docs developers (and LLMs) love

Getting Started

Core Concepts

Command Reference

Guides

Standard Library

​Formatting with deno fmt

​Basic Usage

​Supported File Types

​Configuration

​Formatting Options

​Include/Exclude Files

​Ignore Formatting

​Linting with deno lint

​Basic Usage

​Configuration

​Rule Configuration

​Available Rules

​Report Formats

​Ignore Linting

​Lint Plugins

​CI/CD Integration

​GitHub Actions

​Pre-commit Hook

​VS Code Integration

​Task Configuration

​Best Practices

Format before committing

Use recommended rules

Automate in CI

Configure editor

Document exceptions

Keep config minimal

​Example Configuration

​Migration from Other Tools

​From Prettier

​From ESLint

Build docs developers (and LLMs) love

Formatting with deno fmt

Basic Usage

Supported File Types

Configuration

Formatting Options

Include/Exclude Files

Ignore Formatting

Linting with deno lint

Basic Usage

Configuration

Rule Configuration

Available Rules

Report Formats

Ignore Linting

Lint Plugins

CI/CD Integration

GitHub Actions

Pre-commit Hook

VS Code Integration

Task Configuration

Best Practices

Example Configuration

Migration from Other Tools

From Prettier

From ESLint