Dart cheatsheet

The Dart language is designed to be easy to learn for coders coming from other languages, but it has a few unique features. This tutorial walks you through the most important of these language features.

The embedded editors in this tutorial have partially completed code snippets. You can use these editors to test your knowledge by completing the code and clicking the Run button. The editors also contain thorough test code; don't edit the test code, but feel free to study it to learn about testing.

If you need help, expand the Solution for... dropdown beneath each DartPad for an explanation and the answer.

To put the value of an expression inside a string, use ${expression}. If the expression is an identifier, you can omit the {}.

Here are some examples of using string interpolation:

The following function takes two integers as parameters. Make it return a string containing both integers separated by a space. For example, stringify(2, 3) should return '2 3'.

String stringify(int x, int y) {
  return '$x $y';
}

Dart enforces sound null safety. This means values can't be null unless you say they can be. In other words, types default to non-nullable.

For example, consider the following code. With null safety, this code returns an error. A variable of type int can't have the value null:

int a = null; // INVALID.

When creating a variable, add ? to the type to indicate that the variable can be null:

int? a = null; // Valid.

You can simplify that code a bit because, in all versions of Dart, null is the default value for uninitialized variables:

int? a; // The initial value of a is null.

To learn more about null safety in Dart, read the sound null safety guide.

Declare two variables in this DartPad:

• A nullable String named name with the value 'Jane'.
• A nullable String named address with the value null.

Ignore all initial errors in the DartPad.

String? name = 'Jane';
String? address;

Dart offers some handy operators for dealing with values that might be null. One is the ??= assignment operator, which assigns a value to a variable only if that variable is currently null:

int? a; // = null
a ??= 3;
print(a); // <-- Prints 3.

a ??= 5;
print(a); // <-- Still prints 3.

Another null-aware operator is ??, which returns the expression on its left unless that expression's value is null, in which case it evaluates and returns the expression on its right:

print(1 ?? 3); // <-- Prints 1.
print(null ?? 12); // <-- Prints 12.

Try substituting in the ??= and ?? operators to implement the described behavior in the following snippet.

Ignore all initial errors in the DartPad.

// Substitute an operator that makes 'a string' be assigned to baz.
String? baz = foo ?? bar;

void updateSomeVars() {
  // Substitute an operator that makes 'a string' be assigned to bar.
  bar ??= 'a string';
}

To guard access to a property or method of an object that might be null, put a question mark (?) before the dot (.):

myObject?.someProperty

The preceding code is equivalent to the following:

(myObject != null) ? myObject.someProperty : null

You can chain multiple uses of ?. together in a single expression:

myObject?.someProperty?.someMethod()

The preceding code returns null (and never calls someMethod()) if either myObject or myObject.someProperty is null.

The following function takes a nullable string as a parameter. Try using conditional property access to make it return the uppercase version of str, or null if str is null.

String? upperCaseIt(String? str) {
  return str?.toUpperCase();
}

Dart has built-in support for lists, maps, and sets. You can create them using literals:

final aListOfStrings = ['one', 'two', 'three'];
final aSetOfStrings = {'one', 'two', 'three'};
final aMapOfStringsToInts = {'one': 1, 'two': 2, 'three': 3};

Dart's type inference can assign types to these variables for you. In this case, the inferred types are List<String>, Set<String>, and Map<String, int>.

Or you can specify the type yourself:

final aListOfInts = <int>[];
final aSetOfInts = <int>{};
final aMapOfIntToDouble = <int, double>{};

Specifying types is handy when you initialize a list with contents of a subtype, but still want the list to be List<BaseType>:

final aListOfBaseType = <BaseType>[SubType(), SubType()];

Try setting the following variables to the indicated values. Replace the existing null values.

// Assign this a list containing 'a', 'b', and 'c' in that order:
final aListOfStrings = ['a', 'b', 'c'];

// Assign this a set containing 3, 4, and 5:
final aSetOfInts = {3, 4, 5};

// Assign this a map of String to int so that aMapOfStringsToInts['myKey'] returns 12:
final aMapOfStringsToInts = {'myKey': 12};

// Assign this an empty List<double>:
final anEmptyListOfDouble = <double>[];

// Assign this an empty Set<String>:
final anEmptySetOfString = <String>{};

// Assign this an empty Map of double to int:
final anEmptyMapOfDoublesToInts = <double, int>{};

You might have seen the => symbol in Dart code. This arrow syntax is a way to define a function that executes the expression to its right and returns its value.

For example, consider this call to the List class's any() method:

bool hasEmpty = aListOfStrings.any((s) {
  return s.isEmpty;
});

Here's a simpler way to write that code:

bool hasEmpty = aListOfStrings.any((s) => s.isEmpty);

Try finishing the following statements, which use arrow syntax.

class MyClass {
  int value1 = 2;
  int value2 = 3;
  int value3 = 5;

  // Returns the product of the above values:
  int get product => value1 * value2 * value3;

  // Adds 1 to value1:
  void incrementValue1() => value1++;

  // Returns a string containing each item in the
  // list, separated by commas (e.g. 'a,b,c'):
  String joinWithCommas(List<String> strings) => strings.join(',');
}

To perform a sequence of operations on the same object, use cascades (..). We've all seen an expression like this:

myObject.someMethod()

It invokes someMethod() on myObject, and the result of the expression is the return value of someMethod().

Here's the same expression with a cascade:

myObject..someMethod()

Although it still invokes someMethod() on myObject, the result of the expression isn't the return value—it's a reference to myObject!

Using cascades, you can chain together operations that would otherwise require separate statements. For example, consider the following code, which uses the conditional member access operator (?.) to read properties of button if it isn't null:

final button = web.document.querySelector('#confirm');
button?.textContent = 'Confirm';
button?.classList.add('important');
button?.onClick.listen((e) => web.window.alert('Confirmed!'));
button?.scrollIntoView();

To instead use cascades, you can start with the null-shorting cascade (?..), which guarantees that none of the cascade operations are attempted on a null object. Using cascades shortens the code and makes the button variable unnecessary:

web.document.querySelector('#confirm')
  ?..textContent = 'Confirm'
  ..classList.add('important')
  ..onClick.listen((e) => web.window.alert('Confirmed!'))
  ..scrollIntoView();

Use cascades to create a single statement that sets the anInt, aString, and aList properties of a BigObject to 1, 'String!', and [3.0] (respectively) and then calls allDone().

BigObject fillBigObject(BigObject obj) {
  return obj
    ..anInt = 1
    ..aString = 'String!'
    ..aList.add(3)
    ..allDone();
}

You can define getters and setters whenever you need more control over a property than a simple field allows.

For example, you can make sure a property's value is valid:

class MyClass {
  int _aProperty = 0;

  int get aProperty => _aProperty;

  set aProperty(int value) {
    if (value >= 0) {
      _aProperty = value;
    }
  }
}

You can also use a getter to define a computed property:

class MyClass {
  final List<int> _values = [];

  void addValue(int value) {
    _values.add(value);
  }

  // A computed property.
  int get count {
    return _values.length;
  }
}

Imagine you have a shopping cart class that keeps a private List<double> of prices. Add the following:

• A getter called total that returns the sum of the prices
• A setter that replaces the list with a new one, as long as the new list doesn't contain any negative prices (in which case the setter should throw an InvalidPriceException).

Ignore all initial errors in the DartPad.

/// The total price of the shopping cart.
double get total => _prices.fold(0, (e, t) => e + t);

/// Set [prices] to the [value] list of item prices.
set prices(List<double> value) {
  if (value.any((p) => p < 0)) {
    throw InvalidPriceException();
  }

  _prices = value;
}

Dart has two kinds of function parameters: positional and named. Positional parameters are the kind you're likely familiar with:

int sumUp(int a, int b, int c) {
  return a + b + c;
}
  // ···
  int total = sumUp(1, 2, 3);

With Dart, you can make these positional parameters optional by wrapping them in brackets:

int sumUpToFive(int a, [int? b, int? c, int? d, int? e]) {
  int sum = a;
  if (b != null) sum += b;
  if (c != null) sum += c;
  if (d != null) sum += d;
  if (e != null) sum += e;
  return sum;
}
  // ···
  int total = sumUpToFive(1, 2);
  int otherTotal = sumUpToFive(1, 2, 3, 4, 5);

Optional positional parameters are always last in a function's parameter list. Their default value is null unless you provide another default value:

int sumUpToFive(int a, [int b = 2, int c = 3, int d = 4, int e = 5]) {
  // ···
}

void main() {
  int newTotal = sumUpToFive(1);
  print(newTotal); // <-- prints 15
}

Implement a function called joinWithCommas() that accepts one to five integers, then returns a string of those numbers separated by commas. Here are some examples of function calls and returned values:

String joinWithCommas(int a, [int? b, int? c, int? d, int? e]) {
  var total = '$a';
  if (b != null) total = '$total,$b';
  if (c != null) total = '$total,$c';
  if (d != null) total = '$total,$d';
  if (e != null) total = '$total,$e';
  return total;
}

Using a curly brace syntax at the end of the parameter list, you can define parameters that have names.

Named parameters are optional unless they're explicitly marked as required.

void printName(String firstName, String lastName, {String? middleName}) {
  print('$firstName ${middleName ?? ''} $lastName');
}

void main() {
  printName('Dash', 'Dartisan');
  printName('John', 'Smith', middleName: 'Who');
  // Named arguments can be placed anywhere in the argument list.
  printName('John', middleName: 'Who', 'Smith');
}

As you might expect, the default value of a nullable named parameter is null, but you can provide a custom default value.

If the type of a parameter is non-nullable, then you must either provide a default value (as shown in the following code) or mark the parameter as required (as shown in the constructor section).

void printName(String firstName, String lastName, {String middleName = ''}) {
  print('$firstName $middleName $lastName');
}

A function can't have both optional positional and named parameters.

Add a copyWith() instance method to the MyDataObject class. It should take three named, nullable parameters:

• int? newInt
• String? newString
• double? newDouble

Your copyWith() method should return a new MyDataObject based on the current instance, with data from the preceding parameters (if any) copied into the object's properties. For example, if newInt is non-null, then copy its value into anInt.

Ignore all initial errors in the DartPad.

MyDataObject copyWith({int? newInt, String? newString, double? newDouble}) {
  return MyDataObject(
    anInt: newInt ?? this.anInt,
    aString: newString ?? this.aString,
    aDouble: newDouble ?? this.aDouble,
  );
}

Dart code can throw and catch exceptions. In contrast to Java, all of Dart's exceptions are unchecked. Methods don't declare which exceptions they might throw and you aren't required to catch any exceptions.

Dart provides Exception and Error types, but you're allowed to throw any non-null object:

throw Exception('Something bad happened.');
throw 'Waaaaaaah!';

Use the try, on, and catch keywords when handling exceptions:

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

The try keyword works as it does in most other languages. Use the on keyword to filter for specific exceptions by type, and the catch keyword to get a reference to the exception object.

If you can't completely handle the exception, use the rethrow keyword to propagate the exception:

try {
  breedMoreLlamas();
} catch (e) {
  print('I was just trying to breed llamas!');
  rethrow;
}

To execute code whether or not an exception is thrown, use finally:

try {
  breedMoreLlamas();
} catch (e) {
  // ... handle exception ...
} finally {
  // Always clean up, even if an exception is thrown.
  cleanLlamaStalls();
}

Implement tryFunction() below. It should execute an untrustworthy method and then do the following:

• If untrustworthy() throws an ExceptionWithMessage, call logger.logException with the exception type and message (try using on and catch).
• If untrustworthy() throws an Exception, call logger.logException with the exception type (try using on for this one).
• If untrustworthy() throws any other object, don't catch the exception.
• After everything's caught and handled, call logger.doneLogging (try using finally).

void tryFunction(VoidFunction untrustworthy, Logger logger) {
  try {
    untrustworthy();
  } on ExceptionWithMessage catch (e) {
    logger.logException(e.runtimeType, e.message);
  } on Exception {
    logger.logException(Exception);
  } finally {
    logger.doneLogging();
  }
}

Dart provides a handy shortcut for assigning values to properties in a constructor: use this.propertyName when declaring the constructor:

class MyColor {
  int red;
  int green;
  int blue;

  MyColor(this.red, this.green, this.blue);
}

final color = MyColor(80, 80, 128);

This technique works for named parameters, too. Property names become the names of the parameters:

class MyColor {
  // ...

  MyColor({required this.red, required this.green, required this.blue});
}

final color = MyColor(red: 80, green: 80, blue: 80);

In the preceding code, red, green, and blue are marked as required because these int values can't be null. If you add default values, you can omit required:

MyColor([this.red = 0, this.green = 0, this.blue = 0]);
// or
MyColor({this.red = 0, this.green = 0, this.blue = 0});

Add a one-line constructor to MyClass that uses this. syntax to receive and assign values for all three properties of the class.

Ignore all initial errors in the DartPad.

MyClass(this.anInt, this.aString, this.aDouble);

Sometimes when you implement a constructor, you need to do some setup before the constructor body executes. For example, final fields must have values before the constructor body executes. Do this work in an initializer list, which goes between the constructor's signature and its body:

Point.fromJson(Map<String, double> json) : x = json['x']!, y = json['y']! {
  print('In Point.fromJson(): ($x, $y)');
}

The initializer list is also a handy place to put asserts, which run only during development:

NonNegativePoint(this.x, this.y) : assert(x >= 0), assert(y >= 0) {
  print('I just made a NonNegativePoint: ($x, $y)');
}

Complete the FirstTwoLetters constructor below. Use an initializer list to assign the first two characters in word to the letterOne and LetterTwo properties. For extra credit, add an assert to catch words of less than two characters.

Ignore all initial errors in the DartPad.

  FirstTwoLetters(String word)
      : assert(word.length >= 2),
        letterOne = word[0],
        letterTwo = word[1];

To allow classes to have multiple constructors, Dart supports named constructors:

class Point {
  double x, y;

  Point(this.x, this.y);

  Point.origin() : x = 0, y = 0;
}

To use a named constructor, invoke it using its full name:

final myPoint = Point.origin();

Give the Color class a constructor named Color.black that sets all three properties to zero.

Ignore all initial errors in the DartPad.

Color.black() : red = 0, green = 0, blue = 0;

Dart supports factory constructors, which can return subtypes or even null. To create a factory constructor, use the factory keyword:

class Square extends Shape {}

class Circle extends Shape {}

class Shape {
  Shape();

  factory Shape.fromTypeName(String typeName) {
    if (typeName == 'square') return Square();
    if (typeName == 'circle') return Circle();

    throw ArgumentError('Unrecognized $typeName');
  }
}

Replace the line TODO(); in the factory constructor named IntegerHolder.fromList to return the following:

• If the list has one value, create an IntegerSingle instance using that value.
• If the list has two values, create an IntegerDouble instance using the values in order.
• If the list has three values, create an IntegerTriple instance using the values in order.
• Otherwise, throw an Error.

If you succeed, the console should display Success!.

  switch (list.length) {
    case 1:
      return IntegerSingle(list[0]);
    case 2:
      return IntegerDouble(list[0], list[1]);
    case 3:
      return IntegerTriple(list[0], list[1], list[2]);
    default:
      throw ArgumentError("List must between 1 and 3 items. This list was ${list.length} items.");
  }

Sometimes a constructor's only purpose is to redirect to another constructor in the same class. A redirecting constructor's body is empty, with the constructor call appearing after a colon (:).

class Automobile {
  String make;
  String model;
  int mpg;

  // The main constructor for this class.
  Automobile(this.make, this.model, this.mpg);

  // Delegates to the main constructor.
  Automobile.hybrid(String make, String model) : this(make, model, 60);

  // Delegates to a named constructor
  Automobile.fancyHybrid() : this.hybrid('Futurecar', 'Mark 2');
}

Remember the Color class from above? Create a named constructor called black, but rather than manually assigning the properties, redirect it to the default constructor with zeros as the arguments.

Ignore all initial errors in the DartPad.

Color.black() : this(0, 0, 0);

If your class produces objects that never change, you can make these objects compile-time constants. To do this, define a const constructor and make sure that all instance variables are final.

class ImmutablePoint {
  static const ImmutablePoint origin = ImmutablePoint(0, 0);

  final int x;
  final int y;

  const ImmutablePoint(this.x, this.y);
}

Modify the Recipe class so its instances can be constants, and create a constant constructor that does the following:

• Has three parameters: ingredients, calories, and milligramsOfSodium (in that order).
• Uses this. syntax to automatically assign the parameter values to the object properties of the same name.
• Is constant, with the const keyword just before Recipe in the constructor declaration.

Ignore all initial errors in the DartPad.

class Recipe {
  final List<String> ingredients;
  final int calories;
  final double milligramsOfSodium;

  const Recipe(this.ingredients, this.calories, this.milligramsOfSodium);
}

We hope you enjoyed using this tutorial to learn or test your knowledge of some of the most interesting features of the Dart language.

What you can try next includes:

• Try other Dart tutorials.
• Read the Dart language tour.
• Play with DartPad.
• Get the Dart SDK.

Unless stated otherwise, the documentation on this site reflects Dart 3.7.3. Page last updated on 2025-04-21. View source or report an issue.