Dart Lesson 3 of 102 2 min read

Your First Dart Program: Hello World Explained

Write Hello World in Dart and understand every part of it: the main function, print, statements, semicolons and how to run the file.

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Every Dart program, from a five-line script to a full mobile app, starts running at the same place: a function called main.

Hello, world #

Create a file named hello.dart and type this in:

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

Run it from the terminal with dart run hello.dart, or press Run above.

Hello, world!

Reading the program piece by piece #

Part	Meaning
`void`	The function gives nothing back when it finishes
`main`	The special name Dart looks for to start the program
`()`	The function takes no input here
`{ ... }`	The body: the statements that run, top to bottom
`print(...)`	A built-in function that writes a line to the console
`'Hello, world!'`	A string, which is a piece of text
`;`	Ends a statement. Dart requires it

Statements run in order #

void main() {
  print('Step 1: boil water');
  print('Step 2: add tea');
  print('Step 3: wait three minutes');
}

Step 1: boil water
Step 2: add tea
Step 3: wait three minutes

Printing values, not just text #

print accepts any value. Put a $ in front of a variable name inside a string to drop its value in. Use ${...} for a calculation.

void main() {
  var name = 'Maya';
  var age = 21;

  print(name);
  print(age);
  print('$name is $age years old.');
  print('Next year she will be ${age + 1}.');
}

Maya
21
Maya is 21 years old.
Next year she will be 22.

A shorter main #

When a function body is a single expression, you can write it with an arrow.

void main() => print('Hello, world!');

Reading command-line arguments #

main can receive the words typed after the file name.

void main(List<String> args) {
  print('You passed ${args.length} argument(s): $args');
}

Running dart run hello.dart red blue prints You passed 2 argument(s): [red, blue].

Common mistakes #

Forgetting the semicolon. print('hi') without ; is an error.
Wrong capital letters. Dart is case sensitive. Print and Main do not exist.
Mismatched quotes. A string that starts with ' must end with '.
Code outside a function. Statements such as print(...) must be inside main or another function.

Try it yourself #

Write a program that prints your name, your city and your favourite food on three separate lines. Then change it so all three appear in a single sentence using $ interpolation.