Dart Modern Features

1. When to use this skill

Use this skill when:

• Writing or reviewing Dart code targeting Dart 3.0 or later.
• Refactoring legacy Dart code to use modern, concise, and safe features.
• Looking for idiomatic ways to handle multiple return values, deep data

extraction, or exhaustive checking.

Discovery

To find candidates for modernization:

Switch Expressions

Search for switch statements where every case assigns to the same variable or returns:

• Regex: switch\s\([^)]+\)\s\{\s*case

Pattern Matching Candidates

Search for manual map or JSON property extraction and type checking:

• Regex: containsKey\(['"][^'"]+['"]\)
• Regex: json\[['"][^'"]+['"]\]\s+is\s+

Null-Aware Elements

Search for collection if statements checking for null:

• Regex: if\s\(\w+\s!=\snull\)\s\w+

Digit Separators

Search for long numbers without separators:

• Regex: \b\d{6,}\b (Matches numbers with 6 or more digits).

2. Features

Records

Use records as anonymous, immutable, aggregate structures to bundle multiple objects without defining a custom class. Prefer them for returning multiple values from a function or grouping related data temporarily.

Avoid: Creating a dedicated class for simple multiple-value returns.

class UserResult {
  final String name;
  final int age;
  UserResult(this.name, this.age);
}

UserResult fetchUser() {
  return UserResult('Alice', 42);
}

Prefer: Using records to bundle types seamlessly on the fly.

(String, int) fetchUser() {
  return ('Alice', 42);
}

void main() {
  var user = fetchUser();
  print(user.$1); // Alice
}

Patterns and Pattern Matching

Use patterns to destructure complex data into local variables and match against specific shapes or values. Use them in switch, if-case, or variable declarations to unpack data directly.

Avoid: Manually checking types, nulls, and keys for data extraction.

void processJson(Map<String, dynamic> json) {
  if (json.containsKey('name') && json['name'] is String &&
      json.containsKey('age') && json['age'] is int) {
    String name = json['name'];
    int age = json['age'];
    print('$name is $age years old.');
  }
}

Prefer: Combining type-checking, validation, and assignment into a single statement.

void processJson(Map<String, dynamic> json) {
  if (json case {'name': String name, 'age': int age}) {
    print('$name is $age years old.');
  }
}

Switch Expressions

Use switch expressions to return a value directly, eliminating bulky case and break statements.

Avoid: Using switch statements where every branch simply returns or assigns a value.

String describeStatus(int code) {
  switch (code) {
    case 200:
      return 'Success';
    case 404:
      return 'Not Found';
    default:
      return 'Unknown';
  }
}

Prefer: Returning the evaluated expression directly using the => syntax.

String describeStatus(int code) => switch (code) {
  200 => 'Success',
  404 => 'Not Found',
  _ => 'Unknown',
};

Class Modifiers

Use class modifiers (sealed, final, base, interface) to restrict how classes can be used outside their defines library. Prefer sealed for defining closed families of subtypes to enable exhaustive checking.

Avoid: Using open abstract classes when the set of subclasses is known and fixed.

abstract class Result {}

class Success extends Result {}
class Failure extends Result {}

String handle(Result r) {
  if (r is Success) return 'OK';
  if (r is Failure) return 'Error';
  return 'Unknown';
}

Prefer: Using sealed to guarantee to the compiler that all cases are covered.

sealed class Result {}

class Success extends Result {}
class Failure extends Result {}

String handle(Result r) => switch(r) {
  Success() => 'OK',
  Failure() => 'Error',
};

Extension Types

Use extension types for a zero-cost wrapper around an existing type. Use them to restrict operations or add custom behavior without runtime overhead.

Avoid: Allocating new wrapper objects just for domain-specific logic or type safety.

class Id {
  final int value;
  Id(this.value);
  bool get isValid => value > 0;
}

Prefer: Using extension types which compile down to the underlying type at runtime.

extension type Id(int value) {
  bool get isValid => value > 0;
}

Digit Separators

Use underscores (_) in number literals strictly to improve visual readability of large numeric values.

Avoid: Long number literals that are difficult to read at a glance.

const int oneMillion = 1000000;

Prefer: Using underscores to separate thousands or other groupings.

const int oneMillion = 1_000_000;

Wildcard Variables

Use wildcards (_) as non-binding variables or parameters to explicitly signal that a value is intentionally unused.

Avoid: Inventing clunky, distinct variable names to avoid "unused variable" warnings.

void handleEvent(String ignoredName, int status) {
  print('Status: $status');
}

Prefer: Explicitly dropping the binding with an underscore.

void handleEvent(String _, int status) {
  print('Status: $status');
}

Null-Aware Elements

Use null-aware elements (?) inside collection literals to conditionally include items only if they evaluate to a non-null value.

Avoid: Using collection if statements for simple null checks.

var names = [
  'Alice',
  if (optionalName != null) optionalName,
  'Charlie'
];

Prefer: Using the ? prefix inline.

var names = ['Alice', ?optionalName, 'Charlie'];

Dot Shorthands

Use dot shorthands to omit the explicit type name when it can be confidently inferred from context, such as with enums or static fields.

Avoid: Fully qualifying type names when the type is obvious from the context.

LogLevel currentLevel = LogLevel.info;

Prefer: Reducing visual noise with inferred shorthand.

LogLevel currentLevel = .info;

Pragmatic Balance: When NOT to Over-Patternize

Pattern matching and switch expressions should simplify code, not add syntactic overhead.

1. Prefer is Type Promotion over if-case for Single Variables

If you only need to check a type or promote a variable, use standard is checks instead of if-case or case patterns that introduce shadow aliases.

Avoid:

// ❌ Anti-pattern: Introduces unnecessary alias variable `k`
for (final MapEntry(:key, :value) in map.entries) {
  if (key case final String k when value != null) {
    process(k, value);
  }
}

Prefer:

// ✅ Promotes `key` directly in-place without extra variables
for (final MapEntry(:key, :value) in map.entries) {
  if (key is String && value != null) {
    process(key, value);
  }
}

2. Consolidate Nullable Types in Switch Arms

When mapping or returning values in a switch expression where both null and a type T are valid and passed through, match the nullable type T? directly rather than creating redundant null arms.

Avoid:

// ❌ Redundant separate null arm
switch (value) {
  final String s => s,
  null => null,
  _ => throw FormatException(...),
}

Prefer:

// ✅ Clean nullable pattern match
switch (value) {
  final String? s => s,
  _ => throw FormatException(...),
}

Related Skills

• [dart-best-practices]: General code

style and foundational Dart idioms that predate or complement the modern syntax features.

[dart-best-practices]: https://github.com/kevmoo/dash_skills/blob/main/skills/dart-best-practices/SKILL.md