Not all values are the same kind of thing.

A name is text. An age is a number. A temperature might be a decimal. A yes-or-no answer is something else entirely.

Dart treats each kind of value differently. The kind of value is called its type.

Why types matter

Try this in DartPad:

void main() {
  print('5' + '5'); // 55
  print(5 + 5);     // 10
}

Same symbols. Different types. Different results.

'5' is text. 5 is a number. Adding text joins it together. Adding numbers does math.

If Dart did not track types, you would never know which result you were going to get.

String

A String is text. You write it inside quotes:

void main() {
  var city = 'Lahore';
  print(city); // Lahore
}

Single quotes and double quotes both work. Pick one and be consistent.

A String can be empty, one character, or thousands of characters long. It is still a String.

There is more you can do with strings. But methods like .length and .toUpperCase() require classes. We cover them in Lesson 13.

int

An int is a whole number. No decimal point:

void main() {
  var age = 25;
  var temperature = -3;
  print(age); // 25
}

Positive, negative, or zero. As long as there is no decimal point, it is an int.

double

A double is a number with a decimal point:

void main() {
  var price = 9.99;
  var pi = 3.14159;
  print(price); // 9.99
}

Even 1.0 is a double, not an int. The decimal point is what decides.

bool

A bool is either true or false. Nothing else:

void main() {
  var isLoggedIn = true;
  var hasError = false;
  print(isLoggedIn); // true
}

The name comes from George Boole, a mathematician who studied logic in the 1800s. You will use booleans constantly. Every decision your program makes comes down to one.

Writing the type explicitly

In Lesson 2 you used var and let Dart figure out the type. But you can also write the type yourself:

void main() {
  String name = 'Ali';
  int age = 25;
  double score = 98.5;
  bool passed = true;
 
  print(name); // Ali
}

String name = 'Ali' and var name = 'Ali' produce the same result. The difference is intent. When you write the type, you are saying exactly what kind of value belongs there.

Use explicit types when it makes your intent clear. Use var when the type is obvious from the value.

Type inference is not the same as no type

Here is something important.

When you write var x = 5, Dart infers that x is an int. But it does not forget. That variable is an int for the rest of its life.

Try to put text in it:

void main() {
  var x = 5;
  x = 'hello'; // Error
}

Dart will stop you:

A value of type 'String' can't be assigned to a variable of type 'int'.

var does not mean "any type." It means "figure out the type once, then lock it in."

Summary