)
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
String putIfAbsent(String key, String ifAbsent()) {
if (!containsKey(key)) this[key] = ifAbsent();
return this[key];
}