prefer_contains

v0.4.0 Warning Fix Collection & Type

Comparing .indexOf() to -1 is a common pattern for checking whether an element exists in a collection or a substring exists in a string. The .contains() method expresses this intent more clearly and directly.

Why use this rule

.contains() communicates “does this exist?” more clearly than .indexOf() != -1. It is also less error-prone since there is no magic number to get wrong. This rule catches both indexOf(x) == -1 and indexOf(x) != -1 patterns, including reversed operand order.

See also: Iterable.contains

Don’t

void example() {
  final list = [1, 2, 3];

  // Use .contains() instead of .indexOf() == -1
  final notFound = list.indexOf(1) == -1;

  // Use .contains() instead of .indexOf() != -1
  final found = list.indexOf(1) != -1;

  // Also reversed comparisons
  final reversed = -1 == list.indexOf(1);

  // Works on strings too
  final s = 'hello';
  final hasA = s.indexOf('a') != -1;
}

Do

void example() {
  final list = [1, 2, 3];

  final notFound = !list.contains(1);
  final found = list.contains(1);

  // Comparing to specific index positions is fine
  final isFirst = list.indexOf(1) == 0;
  final isThird = list.indexOf(1) == 2;

  // Using indexOf for its return value is fine
  final idx = list.indexOf(1);
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      prefer_contains: false