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';

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);

If you enable null safety, then 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 value 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.