In this article we will look at the top 10 utility methods you should know about the Map type in Dart. This is a follow up to the earlier “Top 10” I did on List/Array types, so let’s jump right into it.
1. addAll()
This allows you to merge all key/value pairs of the provided map to the current one.
var user = {
"firstName": "Tom",
"age": 25,
};
user.addAll({
"lastName": "Smith",
"age": 26,
});
print(user);
// => {"firstName": "Tom", "age": 26, "lastName": "Smith"}
If a key already exists, its value will be replaced. This works similarly to Object.assign in JavaScript.
2. containsKey()
Checks whether the given key exists.
print(user.containsKey("age")); // => true
print(user.containsKey("accessToken")); // => false
3. containsValue()
Checks whether the given value exists.
print(user.containsValue("Smith")); // => true
print(user.containsValue(40)); // => false
4. forEach()
Runs the given function over each key/value pair.
user.forEach((key, value) => print('Key: $key, Value: $value'));
// => "Key: firstName, Value: Tom" "Key: age, Value: 26" "Key: lastName, Value: Smith"
5. putIfAbsent()
Adds a key/value pair if non-existent. If key already exists, a value will be set if there isn’t one.
user.putIfAbsent("accessToken", () => "abf329jklr90rnlk2...");
// => {"firstName": "Tom", "age": 26, "lastName": "Smith", "accessToken": "abf329jklr90rnlk2..."}
6. remove()
Removes the provided key and its associated value.
user.remove("accessToken"); // => abf329jklr90rnlk2...
This will return the value that was removed.
7. removeWhere()
Removes the key/value pair if the given condition is true.
user.removeWhere((key, value) => key == "lastName");
8. clear()
Removes every key/value pair in the map.
user.clear();
print(user); // => {}
9. update()
Updates the value for the given key.
user["age"] = 25;
user.update("age", (dynamic val) => ++val); // => 26
This also returns the new value. To prevent an error being thrown should the key not exist, there’s a third parameter:
user.update("name", (dynamic val) => "Jim", ifAbsent: () => "Jane");
print(user); // => {"age": 26, "name": "Jane"};
In most cases you could update using array bracket notation:
user["name"] = "Mary";
10. Map.from()
This technically is a constructor, but still useful as a utility. It creates and returns a copy of the provided map:
var userCopy = Map.from(user);
Conclusion
I hope this was insightful and if this is your first exposure to Dart, read my first steps tutorial to grasp the basics. The code snippets for this article are available on DartPad.
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