deepFreeze function

Object? deepFreeze(Object? value)

Returns a deeply-unmodifiable copy of value.

Maps, lists, and sets are rebuilt as unmodifiable collections with every nested collection frozen too; scalars (num, String, bool, null) are returned as-is. Any attempt to mutate the result — at any depth — throws UnsupportedError.

This is a copy, not a wrapper: later mutations to the ORIGINAL nested collections do not show through, because each level is rebuilt. Non- collection objects are passed through by reference and are only as immutable as their own type.

Example:

final frozen = deepFreeze({'a': [1, 2], 'b': {'c': 3}});
(frozen as Map)['a']; // unmodifiable List [1, 2]
(frozen['a'] as List).add(9); // throws UnsupportedError

Audited: 2026-06-12 11:26 EDT

Implementation

Object? deepFreeze(Object? value) {
  if (value is Map) {
    return Map<Object?, Object?>.unmodifiable(
      <Object?, Object?>{
        for (final MapEntry<Object?, Object?> e in value.entries) e.key: deepFreeze(e.value),
      },
    );
  }
  if (value is List) {
    return List<Object?>.unmodifiable(
      <Object?>[
        for (final Object? element in value) deepFreeze(element),
      ],
    );
  }
  // Set checked after List/Map; a Set is not a List, so order is irrelevant
  // here, but keeping scalars last avoids needless is-checks on the common case.
  if (value is Set) {
    return Set<Object?>.unmodifiable(
      <Object?>{
        for (final Object? element in value) deepFreeze(element),
      },
    );
  }
  return value;
}