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The Dart formatter (dart format) automatically formats Dart code according to the official Dart style guide, ensuring consistent code style across your project.

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Overview

The formatter applies a consistent, opinionated style to Dart code:
  • Standard indentation (2 spaces)
  • Line length limit (80 characters by default)
  • Proper spacing and line breaks
  • Consistent brace placement

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Usage

dart format [options] <files or directories>

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Basic Examples

# Format a single file
dart format lib/main.dart

# Format multiple files
dart format lib/main.dart test/main_test.dart

# Format all Dart files in a directory
dart format lib/

# Format entire project
dart format .

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Command-Line Options

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Output Options

Control how formatted code is displayed: 
# Write formatted code to files (default)
dart format lib/

# Show formatted code on stdout
dart format --output=show lib/main.dart

# Don't output formatted code
dart format --output=none .

# Write formatted code to a JSON file
dart format --output=json --show=changed .

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Line Length

Set the maximum line length: 
# Use default line length (80 characters)
dart format lib/

# Set custom line length
dart format --line-length=120 lib/

# Common lengths:
dart format --line-length=100 lib/  # Wider screens
dart format --line-length=80 lib/   # Default

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Indentation

Configure indentation: 
# Use 2-space indentation (default)
dart format lib/

# Set custom indentation
dart format --indent=4 lib/

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File Selection

# Follow symlinks when formatting
dart format --follow-links lib/

# Process stdin
cat lib/main.dart | dart format

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Exit Codes

Control exit behavior: 
# Exit with code 1 if any files would change (useful for CI)
dart format --output=none --set-exit-if-changed .

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Formatting Behavior

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Before and After

class Person{
String name;int age;
Person(this.name,this.age);
void greet(){print("Hello, I'm $name and I'm $age years old.");}
}

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Long Lines

The formatter automatically wraps long lines: 
void myFunction() {
  var message = "This is a very long string that exceeds the line length limit and should be wrapped.";
}

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Method Chains

final result = myList.where((x) => x > 0).map((x) => x * 2).toList();

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Collections

final numbers = [1, 2, 3, 4, 5];
final map = {'a': 1, 'b': 2};

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Controlling Formatting

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Disable Formatting

Disable formatting for specific code: 
// dart format off
final matrix = [
  [1,  0,  0],
  [0,  1,  0],
  [0,  0,  1],
];
// dart format on

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Format Control Comments

class MyClass {
  // This code will be formatted normally
  void normalMethod() {
    print('formatted');
  }

  // dart format off
  void   specialMethod()   {
      print(  'not formatted'  );
  }
  // dart format on
}

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CI/CD Integration

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GitHub Actions

.github/workflows/format.yml
name: Format Check

on: [push, pull_request]

jobs:
  format:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - uses: dart-lang/setup-dart@v1
      
      - name: Check formatting
        run: dart format --output=none --set-exit-if-changed .

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Pre-commit Hook

.git/hooks/pre-commit
#!/bin/bash

# Format staged Dart files
STAGED_FILES=$(git diff --cached --name-only --diff-filter=ACM | grep "\.dart$")

if [ -n "$STAGED_FILES" ]; then
  echo "Formatting Dart files..."
  dart format $STAGED_FILES
  
  # Add formatted files back to staging
  git add $STAGED_FILES
fi

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Makefile Integration

Makefile
.PHONY: format format-check

format:
	@echo "Formatting Dart code..."
	dart format .

format-check:
	@echo "Checking Dart code formatting..."
	dart format --output=none --set-exit-if-changed .

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IDE Integration

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VS Code

Enable format on save:
.vscode/settings.json
{
  "editor.formatOnSave": true,
  "[dart]": {
    "editor.formatOnSave": true,
    "editor.defaultFormatter": "Dart-Code.dart-code",
    "editor.rulers": [80]
  },
  "dart.lineLength": 80
}

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IntelliJ IDEA / Android Studio

  1. Go to Preferences → Languages & Frameworks → Dart
  2. Check Format code on save
  3. Set Right margin (columns) to 80

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Command-line Formatting

Format before committing: 
# Format all files
dart format .

# Check formatting
dart format --output=none --set-exit-if-changed .
if [ $? -ne 0 ]; then
  echo "Some files are not formatted. Run 'dart format .' to fix."
  exit 1
fi

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Output Formats

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Default Output

$ dart format lib/
Formatted lib/main.dart
Formatted lib/utils.dart
Formatted 2 files (2 changed) in 0.5 seconds.

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JSON Output

$ dart format --output=json --show=changed .

{
  "files": [
    {
      "path": "lib/main.dart",
      "changed": true,
      "source": "class MyClass {\n  void method() {\n    print('formatted');\n  }\n}\n"
    },
    {
      "path": "lib/utils.dart",
      "changed": false
    }
  ]
}

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Machine-Readable Output

$ dart format --output=none --show=changed .
lib/main.dart
lib/utils.dart

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Exit Codes

  • 0 - Success (all files formatted or already formatted)
  • 1 - Files need formatting (when using --set-exit-if-changed)
  • 2 - Usage error (invalid arguments)

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Configuration

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Project-Level Configuration

While the formatter doesn’t have a configuration file, you can document your preferences:
.dart_format
# This file documents our formatting preferences
line-length: 80
indent: 2
Then use a script to apply these settings:
scripts/format.sh
#!/bin/bash
dart format --line-length=80 --indent=2 "$@"

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Makefile Helper

Makefile
LINE_LENGTH ?= 80

format:
	dart format --line-length=$(LINE_LENGTH) .

format-check:
	dart format --line-length=$(LINE_LENGTH) --output=none --set-exit-if-changed .

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Best Practices

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1. Format Early and Often

Format code before committing: 
git add .
dart format .
git commit -m "Your message"

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2. Automate Formatting

Use pre-commit hooks or IDE integration to format automatically.

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3. Consistent Line Length

Choose a line length and stick with it across your project: 
# Add to your Makefile or scripts/format.sh
dart format --line-length=80 .

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4. CI/CD Enforcement

Fail builds if code isn’t formatted: 
dart format --output=none --set-exit-if-changed .
if [ $? -ne 0 ]; then
  echo "ERROR: Code is not formatted. Run 'dart format .' to fix."
  exit 1
fi

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5. Team Agreement

Document your formatting preferences in your project README.

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Examples

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Format Entire Project

# Format all Dart files
dart format .

# Output:
# Formatted lib/main.dart
# Formatted lib/models/user.dart
# Formatted test/main_test.dart
# Formatted 3 files (3 changed) in 0.3 seconds.

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Verify Formatting in CI

ci_check.sh
#!/bin/bash
set -e

echo "Checking Dart code formatting..."
dart format --output=none --set-exit-if-changed .

if [ $? -eq 0 ]; then
  echo "✓ All files are properly formatted"
else
  echo "✗ Some files need formatting"
  echo "Run 'dart format .' to fix"
  exit 1
fi

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Format Specific Files

# Format specific files only
dart format lib/main.dart lib/utils.dart

# Format all files in lib/models/
dart format lib/models/

# Format all test files
dart format test/

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Troubleshooting

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Formatter Not Found

Ensure you have Dart SDK installed: 
dart --version

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Files Not Being Formatted

Check that files have .dart extension and are valid Dart code.

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Unexpected Formatting

The formatter follows the Dart style guide. If you find unexpected behavior, it’s usually intentional based on style guidelines.

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See Also

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