Dart Lesson 19 of 102 2 min read

assert in Dart: Catching Mistakes During Development

Learn how assert statements work in Dart, when they run, and how to use them to catch programming mistakes early.

An assert states something that must be true at that point in the program. If it is false, Dart stops with an error that points to the exact line. Asserts are a way of writing down your assumptions so the computer can check them.

Syntax #

void main() {
  var age = 25;
  assert(age >= 0, 'Age cannot be negative');
  print('Age is $age');
}

The second argument is an optional message shown when the check fails.

Asserts only run in development #

This is the important part. Asserts are ignored in production, so they cost nothing in a released app.

A failing assert #

double discountedPrice(double price, double percent) {
  assert(percent >= 0 && percent <= 100, 'percent must be 0-100, got $percent');
  return price * (1 - percent / 100);
}

void main() {
  print(discountedPrice(200, 15));
  print(discountedPrice(200, 150)); // a bug in the calling code
}

Run with asserts enabled, the second call stops the program:

170.0
Failed assertion: 'percent >= 0 && percent <= 100': percent must be 0-100, got 150.0

assert in constructors #

Asserts are often placed in a constructor’s initializer list to validate arguments.

class Rectangle {
  final double width;
  final double height;

  Rectangle(this.width, this.height)
      : assert(width > 0, 'width must be positive'),
        assert(height > 0, 'height must be positive');
}

void main() {
  var r = Rectangle(3, 4);
  print(r.width * r.height);
}

12.0

assert or exception? #

Use an assert for mistakes made by a programmer: a function called with arguments that should never happen. Use an exception for problems that can occur in a correct program: a missing file, no internet, a user typing letters into a number field. Because asserts vanish in release builds, never rely on them to validate user input.

void withdraw(double balance, double amount) {
  // A user can really ask for too much, so this must always be checked.
  if (amount > balance) {
    throw ArgumentError('Insufficient balance');
  }
  print('Withdrew $amount');
}

void main() {
  withdraw(100, 30);
}

Withdrew 30.0

Never put code with side effects inside an assert. assert(list.remove(item)) would remove the item in debug mode and do nothing in release.

Try it yourself #

Write double average(List<int> numbers) with an assert that the list is not empty. Call it with an empty list using dart run --enable-asserts and read the failure message.