Dart Lesson 15 of 102 2 min read

The Ternary Operator in Dart (condition ? a : b)

Use Dart's ternary operator to choose between two values in a single expression, and know when an if statement reads better.

The ternary operator is a compact if/else that produces a value.

condition ? valueIfTrue : valueIfFalse

Basic use #

void main() {
  var age = 17;
  var status = age >= 18 ? 'adult' : 'minor';
  print(status);
}

minor

The same thing with if takes five lines and needs the variable declared first:

void main() {
  var age = 17;
  String status;
  if (age >= 18) {
    status = 'adult';
  } else {
    status = 'minor';
  }
  print(status);
}

Inside other expressions #

Because it is an expression, a ternary can sit anywhere a value can.

void main() {
  var items = 1;
  print('You have $items ${items == 1 ? 'item' : 'items'} in your cart.');

  var stock = 0;
  print(stock > 0 ? 'In stock' : 'Sold out');
}

You have 1 item in your cart.
Sold out

Chaining #

You can chain ternaries to pick between more than two values. Lay each branch on its own line so it reads like a table.

void main() {
  var marks = 72;
  var grade = marks >= 80
      ? 'A'
      : marks >= 60
          ? 'B'
          : 'C';
  print(grade);
}

B

With more than two or three branches, a switch expression is clearer.

Ternary or ?? for null checks #

When the test is simply “is this null?”, the ?? operator is shorter.

void main() {
  String? nickname;

  var a = nickname != null ? nickname : 'Guest'; // works
  var b = nickname ?? 'Guest';                   // better

  print('$a $b');
}

Guest Guest

When to use it #

Use a ternary when you are choosing a value and both options are short. Use if/else when you are doing things, such as calling several functions in each branch. If you have to read a ternary twice to understand it, rewrite it as an if.

Try it yourself #

Given var number = 7;, use one ternary to print whether it is even or odd. Then extend it to print zero when the number is 0.