Variables & Data Types

Real-world analogy

What is it?

Variables are named containers that store data. Dart has three ways to declare them: 'var' (changeable), 'final' (set once at runtime), and 'const' (set at compile time). The main data types are int (whole numbers), double (decimals), String (text), bool (true/false), and dynamic (any type).

Real-world relevance

Every app uses variables constantly. A user's name is a String, their age is an int, their account balance is a double, and whether they are logged in is a bool. Choosing the right type prevents bugs and makes your code clearer.

Key points

• var - Flexible Containers — The 'var' keyword creates a variable whose value you can change later. Dart automatically figures out the type from the first value you assign. Once the type is set, you can only store that same type.
• final - Set Once, Done — 'final' creates a variable you can only set ONCE. After the first assignment, it is locked forever. Use final when you know a value will not change but it is calculated at runtime (like current time or user input).
• const - Compile-Time Constants — 'const' is even stricter than final. The value must be known at COMPILE TIME (before the app runs). Use const for values that never change and are always the same, like pi or gravity.
• int - Whole Numbers — 'int' stores whole numbers without decimals: 1, 42, -7, 1000000. Use int for counts, ages, quantities, indices -- anything that is a complete number without a fractional part.
• double - Decimal Numbers — 'double' stores numbers with decimal points: 3.14, 99.99, -0.5. Use double for prices, measurements, percentages -- anything that needs precision beyond whole numbers.
• String - Text Data — 'String' holds text. Wrap text in single quotes ('hello') or double quotes ("hello"). Dart developers usually prefer single quotes. Strings can contain letters, numbers, symbols, spaces -- any text you want.
• bool - True or False — 'bool' stores only two values: true or false. Booleans are used for decisions: Is the user logged in? Is dark mode on? Is the form valid? Every if/else statement uses a boolean to decide what to do.
• dynamic and Object — 'dynamic' turns off type checking -- the variable can hold ANY type and change types. Use it sparingly! 'Object' is the parent of all Dart types. Prefer specific types for safety; use dynamic only when truly needed.
• Type Annotations vs Inference — You can explicitly write the type (String name = 'Alice') or let Dart figure it out (var name = 'Alice'). Both work! Explicit types are clearer for complex code. Inference is great for simple, obvious cases.

Code example

void main() {
  // var - can be reassigned
  var city = 'Dhaka';
  city = 'Tokyo';  // OK!

  // final - set once, cannot change
  final birthYear = 2015;
  // birthYear = 2016;  // Error!

  // const - compile-time constant
  const pi = 3.14159;
  const maxScore = 100;

  // Data types
  int age = 10;
  double height = 4.5;
  String name = 'Flutter Learner';
  bool isHappy = true;

  // Type inference - Dart figures it out
  var score = 95;         // int
  var price = 19.99;      // double
  var greeting = 'Hi!';   // String
  var active = true;      // bool

  // String interpolation (preview!)
  print('Name: $name');
  print('Age: $age, Height: $height');
  print('Is happy? $isHappy');
  print('Score: $score / $maxScore');
}

Line-by-line walkthrough

1. 1. The main function where our program starts
2. 2. Opening curly brace
3. 3. A comment explaining var
4. 4. Creating a variable 'city' with value 'Dhaka' - can be changed
5. 5. Changing city to 'Tokyo' - allowed because we used var
6. 6.
7. 7. A comment explaining final
8. 8. Creating a final variable 'birthYear' set to 2015 - locked after this
9. 9. A comment showing that reassignment would cause an error
10. 10.
11. 11. A comment explaining const
12. 12. Creating pi as a compile-time constant
13. 13. Creating maxScore as a compile-time constant
14. 14.
15. 15. A comment for data types section
16. 16. An int variable storing the age 10
17. 17. A double variable storing the height 4.5
18. 18. A String variable storing a name
19. 19. A bool variable storing true
20. 20.
21. 21. A comment for type inference
22. 22. Dart infers this is an int from the value 95
23. 23. Dart infers this is a double from the value 19.99
24. 24. Dart infers this is a String from the text
25. 25. Dart infers this is a bool from true
26. 26.
27. 27. A comment previewing string interpolation
28. 28. Printing name using $ to embed the variable in the string
29. 29. Printing age and height using $ interpolation
30. 30. Printing the boolean value
31. 31. Printing score out of maxScore
32. 32. Closing the main function

Spot the bug

void main() {
  const currentTime = DateTime.now();
  final pi = 3.14159;
  var name = 'Alice';
  name = 42;
  print('$name $pi $currentTime');
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• Dart Variables (Dart Official)
• Dart Built-in Types (Dart Official)
• Dart Type System (Dart Official)