prefer_type_over_var

v0.3.0 Warning Fix Type Annotations

Flags variables declared with the var keyword instead of an explicit type annotation. Using var can make it harder to understand the type of a nickname, especially when the initializer is complex or the nullability is not obvious. This rule does not flag final or const declarations.

Conflicts with an SDK rule you probably have enabled

The SDK rule omit_local_variable_types mandates the opposite of this rule — it asks you to remove type annotations that inference can supply. It ships in package:lints/recommended.yaml, so most projects have it on by default.

With both enabled you get contradictory diagnostics on every var. Pick one philosophy and disable the other:

# analysis_options.yaml — if you keep prefer_type_over_var
linter:
  rules:
    omit_local_variable_types: false

If you want explicit types but find this rule too strict, the SDK offers softer alternatives: specify_nonobvious_local_variable_types flags only declarations whose type is not obvious from the initializer, while always_specify_types is the strict equivalent. Note that all three SDK rules are mutually incompatible with omit_local_variable_types.

Why use this rule

Explicit type annotations improve code readability and make the type system work for you. When a nickname is declared with var, readers must mentally resolve the initializer to understand the type, which slows down code review and increases the chance of subtle bugs around nullability or unexpected inference.

See also: Effective Dart - Type annotations

Don’t

var nickname = lookupNickname();
var anotherVar = 'string';
var number = 42;
var list = [1, 2, 3];

for (var i = 0; i < 10; i++) {
  print(i);
}

var cachedNickname = lookupNickname();

Do

String? nickname = lookupNickname();
String anotherVar = 'string';
int number = 42;
List<int> list = [1, 2, 3];

for (int i = 0; i < 10; i++) {
  print(i);
}

String? cachedNickname = lookupNickname();

// final and const are allowed:
final inferred = lookupNickname();
const text = 'hello';

Configuration

This rule is in the opinionated preset, so it is on with preset: opinionated, or by name:

many_lints.yaml

rules:
  prefer_type_over_var: true

To turn it off again:

many_lints.yaml

rules:
  prefer_type_over_var: false

To keep the rule on but skip certain paths, use per-rule exclude.

Related rules

• prefer_explicit_function_type — Prefer explicit function type annotations over the bare ‘Function’ type.
• prefer_explicit_type_arguments — Pin the type arguments of the APIs where inference surprises.
• prefer_async_callback — Use ‘AsyncCallback’ instead of ‘Future<void> Function()’.
• prefer_equatable_mixin — Prefer using EquatableMixin instead of extending Equatable.