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 or dynamic type.

dynamic name = 'Bob'; 

Another option is to explicitly declare the type that would be inferred: 

String name = 'Bob';

Default value

Uninitialized variables 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);

Note: Production code ignores the assert() call. During development, on the other hand, assert(condition) throws an exception if condition is false. For details, see Assert.

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.) A final top-level or class variable is initialized the first time it’s used.

Note: Instance variables can be final but not const. Final instance variables must be initialized before the constructor body starts — at the variable declaration, by a constructor parameter, or in the constructor’s initializer list.

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.

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. 

As of Dart 2.5, you can define constants that use type checks and casts (is and as), collection if and collection for, and spread operators (... and ...?):

// Valid compile-time constants as of Dart 2.5. 
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.