avoid_map_keys_contains

v0.4.0 Warning Fix Collection & Type

Using map.keys.contains(key) iterates through all keys to check for existence, while map.containsKey(key) performs a direct hash lookup. This rule catches the slower pattern and suggests the more efficient alternative.

Why use this rule

Map.keys returns an Iterable that must be traversed linearly to check for a key, making it O(n). Map.containsKey() uses the map’s hash table directly and runs in O(1). For large maps, the performance difference is significant.

See also: Map.containsKey

Don’t

void example() {
  final map = {'hello': 'world', 'foo': 'bar'};

  // Use containsKey() instead
  final exists = map.keys.contains('hello');

  // Also in conditions
  if (map.keys.contains('foo')) {
    print('found');
  }
}

Do

void example() {
  final map = {'hello': 'world', 'foo': 'bar'};

  final exists = map.containsKey('hello');

  if (map.containsKey('foo')) {
    print('found');
  }
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      avoid_map_keys_contains: false