Functions

Dart is a true object-oriented language, so even functions are objects and have a type, Function. This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function. For details, see Callable objects.

Here's an example of implementing a function:

bool isNoble(int atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

Although Effective Dart recommends type annotations for public APIs, the function still works if you omit the types:

isNoble(atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

For functions that contain just one expression, you can use a shorthand syntax:

bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;

The => expr syntax is a shorthand for { return expr; }. The => notation is sometimes referred to as arrow syntax.

A function can have any number of required positional parameters. These can be followed either by named parameters or by optional positional parameters (but not both).

You can use trailing commas when you pass arguments to a function or when you define function parameters.

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

When defining a function, use {param1, param2, …} to specify named parameters. If you don't provide a default value or mark a named parameter as required, their types must be nullable as their default value will be null:

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool? bold, bool? hidden}) {
  ...
}

When calling a function, you can specify named arguments using paramName: value. For example:

enableFlags(bold: true, hidden: false);

To define a default value for a named parameter besides null, use = to specify a default value. The specified value must be a compile-time constant. For example:

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold = false, bool hidden = false}) {
  ...
}
​
// bold will be true; hidden will be false.
enableFlags(bold: true);

If you instead want a named parameter to be mandatory, requiring callers to provide a value for the parameter, annotate them with required:

const Scrollbar({super.key, required Widget child});

If someone tries to create a Scrollbar without specifying the child argument, then the analyzer reports an issue.

You might want to place positional arguments first, but Dart doesn't require it. Dart allows named arguments to be placed anywhere in the argument list when it suits your API:

repeat(times: 2, () {
  ...
});

Wrapping a set of function parameters in [] marks them as optional positional parameters. If you don't provide a default value, their types must be nullable as their default value will be null:

String say(String from, String msg, [String? device]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

Here's an example of calling this function without the optional parameter:

assert(say('Bob', 'Howdy') == 'Bob says Howdy');

And here's an example of calling this function with the third parameter:

assert(
  say('Bob', 'Howdy', 'smoke signal') ==
      'Bob says Howdy with a smoke signal',
);

To define a default value for an optional positional parameter besides null, use = to specify a default value. The specified value must be a compile-time constant. For example:

String say(String from, String msg, [String device = 'carrier pigeon']) {
  var result = '$from says $msg with a $device';
  return result;
}
​
assert(say('Bob', 'Howdy') == 'Bob says Howdy with a carrier pigeon');

In Dart 3.13 and later, you can't use modifiers like final or var for normal function parameters. These keywords are now reserved exclusively for primary constructors to declare instance fields.

If you wish to enforce immutability for function parameters, use lints like parameter_assignments instead of the final keyword in the signature.

Every app must have a top-level main() function, which serves as the entrypoint to the app. The main() function returns void and has an optional List<String> parameter for arguments.

Here's a simple main() function:

void main() {
  print('Hello, World!');
}

Here's an example of the main() function for a command-line app that takes arguments:

// Run the app like this: dart run args.dart 1 test
void main(List<String> arguments) {
  print(arguments);
​
  assert(arguments.length == 2);
  assert(int.parse(arguments[0]) == 1);
  assert(arguments[1] == 'test');
}

You can use the args library to define and parse command-line arguments.

You can pass a function as a parameter to another function. For example:

void printElement(int element) {
  print(element);
}
​
var list = [1, 2, 3];
​
// Pass printElement as a parameter.
list.forEach(printElement);

You can also assign a function to a variable, such as:

var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
assert(loudify('hello') == '!!! HELLO !!!');

This example uses an anonymous function. More about those in the next section.

You can specify the type of a function, which is known as a function type. A function type is obtained from a function declaration header by replacing the function name by the keyword Function. Moreover, you are allowed to omit the names of positional parameters, but the names of named parameters can't be omitted. For example:

void greet(String name, {String greeting = 'Hello'}) =>
    print('$greeting $name!');
​
// Store `greet` in a variable and call it.
void Function(String, {String greeting}) g = greet;
g('Dash', greeting: 'Howdy');

Though you name most functions, such as main() or printElement(), you can also create functions without names. These functions are called anonymous functions, lambdas, or closures.

An anonymous function resembles a named function as it has:

• Zero or more parameters, comma-separated
• Optional type annotations between parentheses.

The following code block contains the function's body:

([[Type] param1[, ...]]) {
  codeBlock;
}

The following example defines an anonymous function with an untyped parameter, item. The anonymous function passes it to the map function. The map function, invoked for each item in the list, converts each string to uppercase. Then, the anonymous function passed to forEach, prints each converted string with its length.

const list = ['apples', 'bananas', 'oranges'];
​
var uppercaseList = list.map((item) {
  return item.toUpperCase();
}).toList();
// Convert to list after mapping
​
for (var item in uppercaseList) {
  print('$item: ${item.length}');
}

Click Run to execute the code.

void main() {
  const list = ['apples', 'bananas', 'oranges'];

  var uppercaseList = list.map((item) {
    return item.toUpperCase();
  }).toList();
  // Convert to list after mapping

  for (var item in uppercaseList) {
    print('$item: ${item.length}');
  }
}

If the function contains only a single expression or return statement, you can shorten it using arrow notation. Paste the following line into DartPad and click Run to verify that it is functionally equivalent.

var uppercaseList = list.map((item) => item.toUpperCase()).toList();
uppercaseList.forEach((item) => print('$item: ${item.length}'));

Dart determines the scope of variables based on the layout of its code. A programming language with this feature is termed a lexically scoped language. You can "follow the curly braces outwards" to see if a variable is in scope.

Example: A series of nested functions with variables at each scope level:

bool topLevel = true;
​
void main() {
  var insideMain = true;
​
  void myFunction() {
    var insideFunction = true;
​
    void nestedFunction() {
      var insideNestedFunction = true;
​
      assert(topLevel);
      assert(insideMain);
      assert(insideFunction);
      assert(insideNestedFunction);
    }
  }
}

The nestedFunction() method can use variables from every level, all the way up to the top level.

A function object that can access variables in its lexical scope when the function sits outside that scope is called a closure.

Functions can close over variables defined in surrounding scopes. In the following example, makeAdder() captures the variable addBy. Wherever the returned function goes, it remembers addBy.

/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(int addBy) {
  return (int i) => addBy + i;
}
​
void main() {
  // Create a function that adds 2.
  var add2 = makeAdder(2);
​
  // Create a function that adds 4.
  var add4 = makeAdder(4);
​
  assert(add2(3) == 5);
  assert(add4(3) == 7);
}

When you refer to a function, method, or named constructor without parentheses, Dart creates a tear-off. This is a closure that takes the same parameters as the function and invokes the underlying function when you call it. If your code needs a closure that invokes a named function with the same parameters as the closure accepts, don't wrap the call in a lambda. Use a tear-off.

var charCodes = [68, 97, 114, 116];
var buffer = StringBuffer();

// Function tear-off
charCodes.forEach(print);
​
// Method tear-off
charCodes.forEach(buffer.write);

// Function lambda
charCodes.forEach((code) {
  print(code);
});
​
// Method lambda
charCodes.forEach((code) {
  buffer.write(code);
});

Here's an example of testing top-level functions, static methods, and instance methods for equality:

void foo() {} // A top-level function
​
class A {
  static void bar() {} // A static method
  void baz() {} // An instance method
}
​
void main() {
  Function x;
​
  // Comparing top-level functions.
  x = foo;
  assert(foo == x);
​
  // Comparing static methods.
  x = A.bar;
  assert(A.bar == x);
​
  // Comparing instance methods.
  var v = A(); // Instance #1 of A
  var w = A(); // Instance #2 of A
  var y = w;
  x = w.baz;
​
  // These closures refer to the same instance (#2),
  // so they're equal.
  assert(y.baz == x);
​
  // These closures refer to different instances,
  // so they're unequal.
  assert(v.baz != w.baz);
}

All functions return a value. If no return value is specified, the statement return null; is implicitly appended to the function body.

foo() {}
​
assert(foo() == null);

To return multiple values in a function, aggregate the values in a record.

(String, int) foo() {
  return ('something', 42);
}

Every property access (top-level, static, or instance) is an invocation of a getter or a setter. A variable implicitly creates a getter and, if it's mutable, a setter. This is why when you access a property, you're actually calling a small function in the background. Reading a property calls a getter function, and writing one calls a setter function, even in cases where the property is declared a variable.

However, you can also declare getters and setters explicitly with the get and set keywords respectively. This allows a property's value to be computed when it's read or written.

The purpose of using getters and setters is to create a clear separation between the client (the code that uses the property) and the provider (the class or library that defines it). The client asks for or sets a value without needing to know if that value is stored in a simple variable or calculated on the spot. This gives the provider the freedom to change how the property works.

For example, because the value of the property might not be stored anywhere, it could be computed each time the getter is called. Another example is that when a value is stored in a private variable, and public access is only allowed by calling a getter or a setter.

The following example showcases this, with the secret getter and setter providing indirect access to the private variable _secret with its own manipulations on the assigned and retrieved values.

// Defines a variable `_secret` that is private to the library since
// its identifier starts with an underscore (`_`).
String _secret = 'Hello';
​
// A public top-level getter that
// provides read access to [_secret].
String get secret {
  print('Getter was used!');
  return _secret.toUpperCase();
}
​
// A public top-level setter that
// provides write access to [_secret].
set secret(String newMessage) {
  print('Setter was used!');
  if (newMessage.isNotEmpty) {
    _secret = newMessage;
    print('New secret: "$newMessage"');
  }
}
​
void main() {
  // Reading the value calls the getter.
  print('Current message: $secret');
​
  /*
  Output:
  Getter was used!
  Current message: HELLO
  */
​
  // Assigning a value calls the setter.
  secret = 'Dart is fun';
​
  // Reading it again calls the getter to show the new computed value
  print('New message: $secret');
​
  /*
  Output:
  Setter was used! New secret: "Dart is fun"
  Getter was used!
  New message: DART IS FUN
  */
}

When you need to lazily produce a sequence of values, consider using a generator function. Dart has built-in support for two kinds of generator functions:

• Synchronous generator: Returns an Iterable object.
• Asynchronous generator: Returns a Stream object.

To implement a synchronous generator function, mark the function body as sync*, and use yield statements to deliver values:

Iterable<int> naturalsTo(int n) sync* {
  int k = 0;
  while (k < n) yield k++;
}

To implement an asynchronous generator function, mark the function body as async*, and use yield statements to deliver values:

Stream<int> asynchronousNaturalsTo(int n) async* {
  int k = 0;
  while (k < n) yield k++;
}

If your generator is recursive, you can improve its performance by using yield*:

Iterable<int> naturalsDownFrom(int n) sync* {
  if (n > 0) {
    yield n;
    yield* naturalsDownFrom(n - 1);
  }
}

An external function is a function whose body is implemented separately from its declaration. Include the external keyword before a function declaration, like so:

external void someFunc(int i);

An external function's implementation can come from another Dart library, or, more commonly, from another language. In interop contexts, external introduces type information for foreign functions or values, making them usable in Dart. Implementation and usage is heavily platform specific, so check out the interop docs on, for example, C or JavaScript to learn more.

External functions can be top-level functions, instance methods, getters or setters, or non-redirecting constructors. An instance variable can be external too, which is equivalent to an external getter and (if the variable is not final) an external setter.