Dart Conditional expressions

The conditional operators covered here are distinct from conditional statements such as if-else. They refer to the ternary operator and null-coalescing operator.

Two Types of Conditional Operators

Dart provides two concise conditional operators to replace verbose if-else blocks, letting you handle conditional assignment or return values in a single line.

1. Ternary conditional operator: condition ? expr1 : expr2
2. Null-coalescing operator: expr1 ?? expr2

Ternary operator syntax: condition ? expr1 : expr2

Syntax breakdown:

booleanCondition ? expressionIfTrue : expressionIfFalse

If condition evaluates to true, the operator returns expr1; if false, it returns expr2.

The condition must produce a boolean value, which determines one of the two results for assignment or return.

Example:

void main() {
    bool isRain = true;
	// Assign value via ternary expression
	var result = isRain ? 'Raining' : 'Not raining';
	print(result); // Raining

	isRain = false;
	result = isRain ? 'Raining' : 'Not raining';
	print(result); // Not raining
}Code language: Dart (dart)

Program output:

D:\dartdemo\firstdart>dart run
Building package executable...
Built firstdart:firstdart.
Raining
Not rainingCode language: CSS (css)

If you’re already familiar with if-else, the equivalent logic can be written like this:

void main() {
    bool isRain = true;

	String result;
	if (isRain) {
	  result = 'Raining';
	} else {
	  result = 'Not raining';
	}
	print(result);//Raining
}Code language: Dart (dart)

Null-Coalescing Operator

Checks if a variable is null and quickly assigns a fallback default value.

Syntax: expr1 ?? expr2

nullableExpression ?? defaultValue

• If expr1 is not null, return expr1 directly;
• If expr1 equals null, return expr2

Official sample:

void main() {
	// Accepts a nullable String parameter String?
	String playerName(String? name) => name ?? 'Guest';

	print(playerName("Tom"));  // Tom
	print(playerName(null));   // Guest
}Code language: JavaScript (javascript)

We define a function playerName that takes a nullable string. The ?? operator sets a fallback value: if the passed name is null, it defaults to “Guest”.

Additional Note

playerName here uses an arrow function, a shorthand syntax for functions which we will cover later. It is equivalent to the full function below:

String playerName(String? name) {
  return name ?? 'Guest';
}Code language: Dart (dart)

Equivalent Implementation Comparisons

Implemented with ternary operator

String playerName(String? name) => name != null ? name : 'Guest';Code language: JavaScript (javascript)

Full if-else version

String playerName(String? name) {
  if (name != null) {
    return name;
  } else {
    return 'Guest';
  }
}Code language: JavaScript (javascript)