Dart Beginner Lesson 5 of 107 5 min read

Dart Syntax: The Basic Rules of Dart in One Page

Learn Dart syntax fast: program structure, semicolons, variables, types, strings, if else, loops, functions, classes, null safety and naming rules, with easy examples.

This page is a tour of Dart syntax for beginners. Each section shows one rule in a tiny program. You do not need to remember everything now: every topic links to a full lesson later in the course. If you have programmed before, this page alone is enough to start reading Dart code.

Every program starts in main

void main() {
  print('Hello, Dart!');
}

Hello, Dart!

• void main() is where the program begins. void means it returns nothing.
• Code that belongs together sits inside curly braces { }.
• Each statement ends with a semicolon ;. Forgetting it is the most common beginner error.
• print writes a line to the console.

Comments

void main() {
  // A single-line comment
  /* A comment that can
     cover several lines */
  print('Comments are ignored when the program runs');
}

Comments are ignored when the program runs

/// starts a documentation comment. See Comments.

Variables and types

void main() {
  var name = 'Asha';      // Dart works out the type: String
  int age = 21;
  double height = 1.62;
  bool isStudent = true;
  final country = 'Nepal'; // set once
  const pi = 3.14159;      // known at compile time

  print('Name: $name');
  print('Age next year: ${age + 1}');
  print('Height: $height m, student: $isStudent');
  print('$country, pi = $pi');
}

Name: Asha
Age next year: 22
Height: 1.62 m, student: true
Nepal, pi = 3.14159

Dart is statically typed: once age is an int, you cannot put text in it. The compiler tells you before the program runs. Read more in Variables, Data types and final vs const.

Strings and interpolation

Use $name to insert a variable and ${...} for any expression. Single and double quotes both work.

void main() {
  var item = 'tea';
  var price = 1.5;
  print('One $item costs \$$price');
  print('Two cost \$${price * 2}');
  print('Upper case: ${item.toUpperCase()}');
}

One tea costs $1.5
Two cost $3.0
Upper case: TEA

Write \$ when you want a real dollar sign. All the string tools are in Dart string methods.

Operators

void main() {
  print(7 + 2);   // 9
  print(7 / 2);   // 3.5  (always a double)
  print(7 ~/ 2);  // 3    (whole-number division)
  print(7 % 2);   // 1    (remainder)
  print(7 > 2 && 2 > 1);
  print(7 == 7.0);
}

9
3.5
3
1
true
true

See Operators for the full list.

if, else if and else

void main() {
  var score = 72;

  if (score >= 80) {
    print('Grade A');
  } else if (score >= 60) {
    print('Grade B');
  } else {
    print('Grade C');
  }

  var result = score >= 40 ? 'pass' : 'fail';
  print(result);
}

Grade B
pass

Conditions must be a real bool. Dart does not treat 0 or an empty string as false. More in if else and the ternary operator.

Loops

void main() {
  for (var i = 1; i <= 3; i++) {
    print('Round $i');
  }

  for (final fruit in ['apple', 'mango']) {
    print(fruit);
  }

  var n = 3;
  while (n > 0) {
    print(n);
    n--;
  }
}

Round 1
Round 2
Round 3
apple
mango
3
2
1

See Loops and switch.

Functions

int add(int a, int b) {
  return a + b;
}

int square(int x) => x * x; // arrow syntax for one expression

String greet(String name, {String greeting = 'Hello'}) {
  return '$greeting, $name!';
}

void main() {
  print(add(2, 3));
  print(square(4));
  print(greet('Ram'));
  print(greet('Sita', greeting: 'Namaste'));
}

5
16
Hello, Ram!
Namaste, Sita!

The type before the name is what the function returns. Named parameters go inside { }. Full details in Functions and Function parameters.

Lists, sets and maps

void main() {
  var list = [1, 2, 3];               // ordered, allows duplicates
  var tags = {'a', 'b'};              // unique values
  var map = {'apple': 3, 'mango': 5}; // key: value

  list.add(4);
  print(list);
  print(tags);
  print(map['mango']);
}

[1, 2, 3, 4]
{a, b}
5

See Collections and Dart list methods.

Classes and objects

class Dog {
  final String name;
  Dog(this.name);

  void bark() => print('$name says woof');
}

void main() {
  var dog = Dog('Bruno'); // no "new" keyword needed
  dog.bark();
}

Bruno says woof

Start with Classes and objects.

Null safety

A normal variable can never be null. Add ? to the type to allow it.

void main() {
  String? nickname;               // may be null
  print(nickname ?? 'no nickname');

  nickname = 'Bibek';
  print(nickname.length);         // safe: Dart knows it is not null now
}

no nickname
5

This is explained step by step in Null safety in Dart.

Naming rules and style

Other rules worth knowing:

• Dart is case-sensitive: name and Name are different.
• Names can contain letters, digits, _ and $, but cannot start with a digit.
• Keywords such as class, if, var and return cannot be used as names.
• Run dart format . to lay out your code the standard way. See Formatting and linting.

Try it yourself

Write a program with a function describe(String name, int age) that returns '<name> is an adult' when age is 18 or more and '<name> is a minor' otherwise. Call it for three people stored in a list of records, such as [('Asha', 21), ('Kiran', 15), ('Leo', 18)], using a for loop.