Variables
Here's an example of creating a variable and initializing it:
var name = 'Bob';
Variables store references. The variable called
name
contains a
reference to a
String
object with a value of "Bob".
The type of the
name
variable is inferred to be
String
,
but you can change that type by specifying it.
If an object isn't restricted to a single type,
specify the
Object
type (or
dynamic
if necessary).
Object name = 'Bob';
Another option is to explicitly declare the type that would be inferred:
String name = 'Bob';
Null safety
#The Dart language enforces sound null safety.
Null safety prevents an error that results from unintentional access
of variables set to
null
. The error is called a null dereference error.
A null dereference error occurs when you access a property or call a method
on an expression that evaluates to
null
.
An exception to this rule is when
null
supports the property or method,
like
toString()
or
hashCode
. With null safety, the Dart compiler
detects these potential errors at compile time.
For example, say you want to find the absolute value of an
int
variable
i
.
If
i
is
null
, calling
i.abs()
causes a null dereference error.
In other languages, trying this could lead to a runtime error,
but Dart's compiler prohibits these actions.
Therefore, Dart apps can't cause runtime errors.
Null safety introduces three key changes:
-
When you specify a type for a variable, parameter, or another relevant component, you can control whether the type allows
null
. To enable nullability, you add a?
to the end of the type declaration.dartString? name // Nullable type. Can be `null` or string. String name // Non-nullable type. Cannot be `null` but can be string.
-
You must initialize variables before using them. Nullable variables default to
null
, so they are initialized by default. Dart doesn't set initial values to non-nullable types. It forces you to set an initial value. Dart doesn't allow you to observe an uninitialized variable. This prevents you from accessing properties or calling methods where the receiver's type can benull
butnull
doesn't support the method or property used. -
You can't access properties or call methods on an expression with a nullable type. The same exception applies where it's a property or method that
null
supports likehashCode
ortoString()
.
Sound null safety changes potential runtime errors into edit-time analysis errors. Null safety flags a non-null variable when it has been either:
- Not initialized with a non-null value.
- Assigned a
null
value.
This check allows you to fix these errors before deploying your app.
Default value
#
Uninitialized variables that have a nullable type
have an initial value of
null
.
Even variables with numeric types are initially null,
because numbers—like everything else in Dart—are objects.
int? lineCount;
assert(lineCount == null);
With null safety, you must initialize the values of non-nullable variables before you use them:
int lineCount = 0;
You don't have to initialize a local variable where it's declared,
but you do need to assign it a value before it's used.
For example, the following code is valid because
Dart can detect that
lineCount
is non-null by the time
it's passed to
print()
:
int lineCount;
if (weLikeToCount) {
lineCount = countLines();
} else {
lineCount = 0;
}
print(lineCount);
Top-level and class variables are lazily initialized; the initialization code runs the first time the variable is used.
Late variables
#The late
modifier has two use cases:
- Declaring a non-nullable variable that's initialized after its declaration.
- Lazily initializing a variable.
Often Dart's control flow analysis can detect when a non-nullable variable is set to a non-null value before it's used, but sometimes analysis fails. Two common cases are top-level variables and instance variables: Dart often can't determine whether they're set, so it doesn't try.
If you're sure that a variable is set before it's used,
but Dart disagrees,
you can fix the error by marking the variable as
late
:
late String description;
void main() {
description = 'Feijoada!';
print(description);
}
When you mark a variable as
late
but initialize it at its declaration,
then the initializer runs the first time the variable is used.
This lazy initialization is handy in a couple of cases:
- The variable might not be needed, and initializing it is costly.
-
You're initializing an instance variable,
and its initializer needs access to
this
.
In the following example,
if the
temperature
variable is never used,
then the expensive
readThermometer()
function is never called:
// This is the program's only call to readThermometer().
late String temperature = readThermometer(); // Lazily initialized.
Final and const
#
If you never intend to change a variable, use
final
or
const
, either
instead of
var
or in addition to a type. A final variable can be set
only once; a const variable is a compile-time constant. (Const variables
are implicitly final.)
Here's an example of creating and setting a final
variable:
final name = 'Bob'; // Without a type annotation
final String nickname = 'Bobby';
You can't change the value of a final
variable:
name = 'Alice'; // Error: a final variable can only be set once.
Use
const
for variables that you want to be
compile-time constants. If
the const variable is at the class level, mark it
static const
.
Where you declare the variable, set the value to a compile-time constant
such as a number or string literal, a const
variable, or the result of an arithmetic operation on constant numbers:
const bar = 1000000; // Unit of pressure (dynes/cm2)
const double atm = 1.01325 * bar; // Standard atmosphere
The
const
keyword isn't just for declaring constant variables.
You can also use it to create constant
values,
as well as to declare constructors that
create
constant values.
Any variable can have a constant value.
var foo = const [];
final bar = const [];
const baz = []; // Equivalent to `const []`
You can omit
const
from the initializing expression of a
const
declaration,
like for
baz
above. For details, see
DON'T use const redundantly.
You can change the reference of a non-final, non-const variable,
even if it used to have a
const
value:
foo = [1, 2, 3]; // Was const []
You can't change the value of a const
variable:
baz = [42]; // Error: Constant variables can't be assigned a value.
You can define constants that use
type checks and casts
(is
and
as
),
collection if
,
and
spread operators
(...
and
...?
):
const Object i = 3; // Where i is a const Object with an int value...
const list = [i as int]; // Use a typecast.
const map = {if (i is int) i: 'int'}; // Use is and collection if.
const set = {if (list is List<int>) ...list}; // ...and a spread.
For more information on using
const
to create constant values, see
Lists,
Maps, and
Classes.
Wildcard variables
#
A wildcard variable with the name
_
declares a local variable or parameter
that is non-binding; essentially, a placeholder.
The initializer, if there is one, is still executed, but the value isn't accessible.
Multiple declarations named
_
can exist in the same namespace without a collision error.
Top-level declarations or members where library privacy might be affected are not valid uses for wildcard variables. Declarations local to a block scope, such as the following examples, can declare a wildcard:
-
Local variable declaration.
dartmain() { var _ = 1; int _ = 2; }
-
For loop variable declaration.
dartfor (var _ in list) {}
-
Catch clause parameters.
darttry { throw '!'; } catch (_) { print('oops'); }
-
Generic type and function type parameters.
dartclass T<_> {} void genericFunction<_>() {} takeGenericCallback(<_>() => true);
-
Function parameters.
dartFoo(_, this._, super._, void _()) {} list.where((_) => true); void f(void g(int _, bool _)) {} typedef T = void Function(String _, String _);
Unless stated otherwise, the documentation on this site reflects Dart 3.9.2. Page last updated on 2025-5-5. View source or report an issue.