V putIfAbsent(
K key,
V ifAbsent()
)

Look up the value of key, or add a new value if it isn't there.

Returns the value associated to key, if there is one. Otherwise calls ifAbsent to get a new value, associates key to that value, and then returns the new value.

Map<String, int> scores = {'Bob': 36}; for (var key in 'Bob', 'Rohan', 'Sophena') { scores.putIfAbsent(key, () => key.length); } scores'Bob'; // 36 scores'Rohan'; // 5 scores'Sophena'; // 7

Calling ifAbsent must not add or remove keys from the map.

Source

V putIfAbsent(K key, V ifAbsent()) {
  throw new UnsupportedError("Cannot modify unmodifiable map");
}