Dart Lesson 5 of 102 3 min read

Variables in Dart: var, Explicit Types and dynamic

Learn how to declare variables in Dart with var and explicit types, how type inference works, and the rules for naming them.

A variable is a name that refers to a value. You create one, give it a value, and use the name wherever you need that value.

Declaring a variable with var #

void main() {
  var city = 'Kathmandu';
  var population = 1442000;
  var isCapital = true;

  print('$city, population $population, capital: $isCapital');
}

Kathmandu, population 1442000, capital: true

var tells Dart to work out the type from the value. city is a String, population is an int and isCapital is a bool. This is called type inference.

The type is fixed #

Once Dart has decided the type, it stays. You can change the value, but only to another value of the same type.

void main() {
  var score = 10;
  score = 25;        // fine, still an int
  // score = 'high'; // error: a String can't be assigned to an int
  print(score);
}

25

Writing the type yourself #

You can state the type instead of var. The result is identical; choose whichever reads better.

void main() {
  String name = 'Aarav';
  int age = 19;
  double height = 1.74;
  bool isStudent = true;

  print('$name, $age, $height m, student: $isStudent');
}

Aarav, 19, 1.74 m, student: true

Dart style prefers var for local variables when the type is obvious from the value, and written types for function parameters and class fields.

Declaring now, assigning later #

If you do not give a value straight away, write the type so Dart knows what to expect. Dart will refuse to let you read the variable before it has a value.

void main() {
  String grade;
  var marks = 82;

  if (marks >= 80) {
    grade = 'A';
  } else {
    grade = 'B';
  }
  print(grade);
}

A

dynamic: switching type checks off #

A dynamic variable can hold anything and change type at any time. Dart stops checking it for you, so mistakes only show up when the program runs. Use it rarely.

void main() {
  dynamic value = 42;
  print(value.runtimeType);
  value = 'now a string';
  print(value.runtimeType);
}

int
String

Naming rules #

• Use lowerCamelCase: firstName, totalPrice, isLoggedIn.
• Names may contain letters, digits, _ and $, and cannot start with a digit.
• Names are case sensitive: total and Total are different.
• You cannot use reserved words such as class, if or return.
• Choose names that say what the value means. daysUntilExam beats d.

Try it yourself #

Declare variables for a book: its title, number of pages, price and whether you have read it. Print them in one sentence. Then try assigning a number to the title and read the error message.