Private Helpers Are Top-Level Functions

Private Helpers Are Top-Level Functions

Write a private helper that does not touch instance state as a top-level function in the file, not a static member, because library privacy already scopes it.

A private helper that does not touch instance state belongs at the top level of the file, not inside the class as static.

Dart's library privacy already scopes _name to the file. Putting it inside the class adds a qualifier without adding protection, and it pushes the class body further from the thing readers came to read.

Bad

class InvoiceRenderer {
  String render(Invoice invoice) {
    return _formatLines(invoice.lines).join('\n');
  }

  static List<String> _formatLines(List<Line> lines) {
    return lines.map((it) => '${it.label}: ${it.amount}').toList();
  }
}

Good

class InvoiceRenderer {
  String render(Invoice invoice) {
    return _formatLines(invoice.lines).join('\n');
  }
}

List<String> _formatLines(List<Line> lines) {
  return lines.map((it) => '${it.label}: ${it.amount}').toList();
}

The class body is only the public surface. The helper sits below it, private to the file, reachable from tests in the same library if needed.

Exceptions

Keep it inside the class when it genuinely belongs to the type:

• Factory-like constructors and named constructors.
• A static const that is part of the type's vocabulary, such as Duration.zero.
• A helper that reads the class's own static state.

Before adding a top-level helper at all, check extract-only-stable-concepts. A single-use helper whose name restates its body should be inlined, not relocated.