Configuring Trailing Commas in Dart 3.7+

Problem: Automatic Trailing Comma Removal in Dart Formatting

Starting with Dart SDK 3.7, the formatter automatically removes trailing commas when it collapses multi-line collections into a single line. This behavior occurs even when editor.formatOnSave or editor.formatOnType is enabled in IDEs like VS Code or Android Studio. For example:

// Before formatting (with trailing comma)
final colors = [
  Colors.red,
  Colors.green,
];

// After formatting (comma removed)
final colors = [Colors.red, Colors.green];

Users reported this change feels disruptive compared to previous Dart versions, where trailing commas were preserved and influenced code formatting by creating new lines.

Solution 1: Preserve Trailing Commas (Dart 3.8+)

For Dart SDK 3.8 and later, add this configuration to your project's analysis_options.yaml:

analyzer:
  # Other analyzer settings...

formatter:
  trailing_commas: preserve

TIP

After modifying analysis_options.yaml:

1. Restart your IDE
2. Run dart format . in the terminal to apply changes
3. Ensure Flutter/Dart SDK is updated (flutter upgrade)

Solution 2: Backward-Compatible SDK Constraint

For projects using Dart 3.7 where upgrading isn't feasible, modify pubspec.yaml to use a broader SDK range:

environment:
  sdk: ">=3.6.0 <4.0.0"   # Replace "^3.7.0"

WARNING

This workaround reverts to pre-3.7 formatting behavior but may prevent accessing newer Dart features. Prefer Solution 1 if you can upgrade to Dart 3.8+.

Why This Happens

The Dart 3.7 formatter introduced logic to automatically manage trailing commas:

1. Collections spanning multiple lines get a trailing comma added
2. Collections collapsing to one line get the trailing comma removed

This change (based on formal proposal #1253) standardized collection formatting. Unlike other solutions, adjusting page_width in analysis_options.yaml does not affect trailing comma behavior. The preserve option (added in Dart 3.8) overrides this automation.

Key Takeaways

• Use trailing_commas: preserve for Dart 3.8+ (recommended solution)
• Use broader SDK constraints as a temporary workaround
• The behavior is intentional and matches official formatting guidelines
• Upgrade Flutter/Dart SDK for access to the latest configuration options