How to get Map keys by values in Dart?

var usdKey = curr.keys.firstWhere(
    (k) => curr[k] == 'USD', orElse: () => null);

There is another one method (similar to Günter Zöchbauer answer):

void main() {

  Map currencies = {
     "01": "USD",
     "17": "GBP",
     "33": "EUR"
  };

  MapEntry entry = currencies.entries.firstWhere((element) => element.value=='GBP', orElse: () => null);

  if(entry != null){
    print('key = ${entry.key}');
    print('value = ${entry.value}');
  }

}

In this code, you get MapEntry, which contains key and value, instead only key in a separate variable. It can be useful in some code.

If you will be doing this more than a few times on the same data, you should create an inverse map so that you can perform simple key lookup instead of repeated linear searches. Here's an example of reversing a map (there may be easier ways to do this):

main() {
  var orig = {"01": "USD", "17": "GBP", "33": "EUR"};
  var reversed = Map.fromEntries(orig.entries.map((e) => MapEntry(e.value, e.key)));
  for (var kv in reversed.entries) {
    print(kv);
  }
}

Edit: yes, reversing a map can simply be:

var reversed = orig.map((k, v) => MapEntry(v, k));

Tip of the hat to Joe Conway on gitter. Thanks.