Every Dart program needs a starting point.

In Dart, that point is a function called main.

It looks like this:

void main() {
 
}

This is the simplest valid Dart program.

It does not do anything. But it is still a valid program.

What if you forget main? Dart will refuse to run your code:

Invoked Dart programs must have a 'main' function defined

So main is not optional.

If you want your program to actually do something, use print:

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

Run it. You will see Hello, world!.

That is the smallest useful Dart program.

What a Dart program is

A Dart program is a text file that ends in .dart.

You write code in that file. Dart reads it, then runs it.

You do not need to install anything to follow this course. Open DartPad in your browser. That is your coding environment.

Using print

You can call print as many times as you like:

void main() {
  print('First line.');
  print('Second line.');
}

Each call prints on its own line. The order matches the order in your code.

A note on void and print

You have seen void, (), and print. They might look unfamiliar.

They are all part of a bigger concept called functions. Functions get their own full lesson. We cover them in Lesson 10.

For now, treat void main() { ... } as the required wrapper that every Dart program needs. Treat print(...) as the thing that displays text.

That is enough to keep going.

Comments

A comment is a note you write for yourself, or for someone else reading your code later. Dart ignores it completely.

A single-line comment starts with //:

void main() {
  // This is a comment. Dart ignores this line.
  print('Hello, world!'); // A comment can also go at the end of a line.
}

A multi-line comment starts with /* and ends with */:

void main() {
  /*
    This comment spans
    multiple lines.
  */
  print('Hello, world!');
}

Use comments to explain why you wrote something, not what it does. Good code usually explains itself. But comments fill in the gaps.

The semicolon

Every statement in Dart ends with a semicolon:

void main() {
  print('Hello, world!'); // <- semicolon here
}

What if you forget it?

void main() {
  print('Hello, world!')
}

Dart will stop you:

Expected to find ';'

The semicolon rule comes from a language called ALGOL, designed in the 1950s. Dart inherited it, along with most modern languages. It tells the compiler where one instruction ends and the next begins.

You will forget it. But everyone does. The error message tells you exactly where to look.

Summary