use_existing_variable

v0.4.0 Warning Fix Pattern Matching

When an expression duplicates the initializer of an existing final or const variable in the same scope, you should reference the variable instead. Repeating the expression creates a maintenance risk where only one copy gets updated during refactoring.

Why use this rule

Duplicated expressions are easy to introduce and hard to spot during code review. If the expression changes, you need to update every occurrence. Using the existing variable avoids this inconsistency and makes refactoring safer.

See also: Dart patterns

Don’t

// Repeating an expression that is already stored in a variable
void badPropertyAccess(String value) {
  final isOdd = value.length.isOdd;
  print(value.length.isOdd);
}

// Repeating a method call
void badMethodCall(List<int> list) {
  final copy = list.toList();
  print(list.toList());
}

// Duplicate in a second variable initializer
void badSecondVariable(String value) {
  final a = value.length.isOdd;
  final b = value.length.isOdd;
  print(b);
}

Do

// Using the existing variable
void goodReuse(String value) {
  final isOdd = value.length.isOdd;
  print(isOdd);
}

// No variable exists for the expression (no lint)
void goodNoVariable(String value) {
  print(value.length.isOdd);
  print(value.length.isOdd);
}

// Different expression (isEven vs isOdd) — no lint
void goodDifferentExpression(String value) {
  final isOdd = value.length.isOdd;
  print(value.length.isEven);
}

// Non-final variable — value may have changed
void goodNonFinal(String value) {
  var isOdd = value.length.isOdd;
  print(value.length.isOdd);
  isOdd = false;
}

// Expression appears before the variable declaration
void goodBeforeDeclaration(String value) {
  print(value.length.isOdd);
  final isOdd = value.length.isOdd;
  print(isOdd);
}

// Inside a nested function (different execution context)
void goodNestedFunction(String value) {
  final isOdd = value.length.isOdd;
  void inner() {
    print(value.length.isOdd);
  }
  inner();
}

// Trivial expressions (literals, identifiers) are not flagged
void goodTrivial() {
  final x = 42;
  print(42);
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      use_existing_variable: false