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 = {'lat': 52.2, 'lon': 21.0};

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

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

Do

void example() {
  final map = {'lat': 52.2, 'lon': 21.0};

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

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

Configuration

This rule is in the recommended preset, so it is on with preset: recommended or preset: opinionated. Add it to preset: core with avoid_map_keys_contains: true.

To turn it off:

many_lints.yaml

rules:
  avoid_map_keys_contains: false

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

Related rules

• avoid_missing_enum_constant_in_map — Cover every enum constant in a map keyed by that enum.
• map_keys_ordering — Keep map literal keys in a configured order.
• avoid_accessing_collections_by_constant_index — Avoid accessing a collection by a constant index inside a loop.
• avoid_collection_equality_checks — Avoid comparing collections with == or != as it checks reference equality, not contents.