Data Types and Type Inference

Every value in Dart has a type. The core types:

Type inference

Dart works out the type from the value:

void main() {
  var count = 10;
  var price = 9.99;
  var title = 'Dart';
  var ready = true;

  print(count.runtimeType);
  print(price.runtimeType);
  print(title.runtimeType);
  print(ready.runtimeType);
}

int
double
String
bool

Type annotations

You can write the type instead of var, or after final:

int score = 100;
double ratio = 3;      // an int literal is accepted: stored as 3.0
final String name = 'Ada';

Annotations are common for function parameters and class fields, and whenever they make code clearer.

Types are fixed

A variable keeps its type. var level = 1; makes level an int, so this fails:

void main() {
  var level = 1;
  level = 'two';
}

hello.dart:3:11: Error: A value of type 'String' can't be assigned to a variable of type 'int'.

Everything is an object

In Dart, every value — even a number or null — is an object with methods: (-5).abs(), 3.7.round(), 'dart'.toUpperCase().

Dart also has dynamic, which turns off type checking for a variable. Avoid it — it throws away the safety the compiler gives you.