falcon

Rules / Complexity / avoid-unnecessary-type-assertions

avoid-unnecessary-type-assertions

lint/complexity/avoid-unnecessary-type-assertions
recommended
Same as avoid-unnecessary-type-assertions · Dart Code Metrics

Flags an is check whose result is already guaranteed by the declared type.

Testing a variable against a type it is already known to have is always true, so the check is dead code — drop it, or fix the declaration that makes it redundant. Without full type resolution the rule tracks the declared types of non-nullable local variables and class fields, and reports x is T when x's declared type and T name the same type with matching type arguments. It targets the affirmative is form; nullable declarations are skipped, since x is T there still meaningfully narrows away null.

Invalid

example.dartdart
// Test cases for avoid-unnecessary-type-assertions rule
// Flags 'is T' checks where the variable is already known to be T

void testExplicitIntType() {
  final int x = 5;
  if (x is int) {
    print("x is int");
  }
}

void testExplicitStringType() {
  final String name = "hello";
  if (name is String) {
    print("name is string");
  }
}

void testExplicitListType() {
  final List<String> items = [];
  if (items is List) {
    print("items is list");
  }
}

void testMultipleUnnecessaryAssertions() {
  final bool active = true;
  final double value = 3.14;

  if (active is bool) {
    print("active is bool");
  }

  if (value is double) {
    print("value is double");
  }
}

class MyClass {
  final int id = 42;

  void checkId() {
    if (id is int) {
      print("id is int");
    }
  }
}

void testInlineAssertion() {
  final String message = "test";
  final result = message is String ? "yes" : "no";
}
Unnecessary type assertion — variable is already known to be this type
5 final int x = 5;
6 if (x is int) {
Unnecessary type assertion — variable is already known to be this type
12 final String name = "hello";
13 if (name is String) {
Unnecessary type assertion — variable is already known to be this type
19 final List<String> items = [];
20 if (items is List) {
Unnecessary type assertion — variable is already known to be this type
28
29 if (active is bool) {
Unnecessary type assertion — variable is already known to be this type
32
33 if (value is double) {
Unnecessary type assertion — variable is already known to be this type
41 void checkId() {
42 if (id is int) {
Unnecessary type assertion — variable is already known to be this type
49 final String message = "test";
50 final result = message is String ? "yes" : "no";

Valid

example.dartdart
// Good examples for avoid-unnecessary-type-assertions rule
// Type checks where the type is not already known

void testDynamicTypeCheck() {
  dynamic value = 42;
  if (value is int) {
    print("value is int");
  }
}

void testObjectTypeCheck() {
  Object obj = "hello";
  if (obj is String) {
    print("obj is string");
  }
}

void testParameterTypeCheck() {
  void checkType(dynamic param) {
    if (param is bool) {
      print("param is bool");
    }
  }
}

void testNullableTypeCheck() {
  int? maybeInt = 5;
  if (maybeInt is int) {
    print("maybeInt is int");
  }
}

void testBroadTypeCheck() {
  List<dynamic> items = [];
  if (items is List<String>) {
    print("items is List<String>");
  }
}

void testInheritanceTypeCheck() {
  Object shape = Circle();
  if (shape is Circle) {
    print("shape is Circle");
  }
}

class Circle {}

How to configure

Set the severity of avoid-unnecessary-type-assertions in your falcon.json:

falcon.jsonjson
{
  "linter": {
    "rules": {
      "complexity": {
        "avoid-unnecessary-type-assertions": "error"
      }
    }
  }
}

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