dynamic putIfAbsent(
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

/**
 * 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.
 */
V putIfAbsent(K key, V ifAbsent());