Operators

Dart supports the operators shown in the following table. The table shows Dart’s operator associativity and operator precedence from highest to lowest, which are an approximation of Dart’s operator relationships. You can implement many of these operators as class members.

When you use operators, you create expressions. Here are some examples of operator expressions:

a++
a + b
a = b
a == b
c ? a : b
a is T

Operator precedence example

In the operator table, each operator has higher precedence than the operators in the rows that follow it. For example, the multiplicative operator % has higher precedence than (and thus executes before) the equality operator ==, which has higher precedence than the logical AND operator &&. That precedence means that the following two lines of code execute the same way:

// Parentheses improve readability.
if ((n % i == 0) && (d % i == 0)) ...

// Harder to read, but equivalent.
if (n % i == 0 && d % i == 0) ...

Arithmetic operators

Dart supports the usual arithmetic operators, as shown in the following table.

Example:

assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder

assert('5/2 = ${5 ~/ 2} r ${5 % 2}' == '5/2 = 2 r 1');

Dart also supports both prefix and postfix increment and decrement operators.

Example:

int a;
int b;

a = 0;
b = ++a; // Increment a before b gets its value.
assert(a == b); // 1 == 1

a = 0;
b = a++; // Increment a after b gets its value.
assert(a != b); // 1 != 0

a = 0;
b = --a; // Decrement a before b gets its value.
assert(a == b); // -1 == -1

a = 0;
b = a--; // Decrement a after b gets its value.
assert(a != b); // -1 != 0

Equality and relational operators

The following table lists the meanings of equality and relational operators.

To test whether two objects x and y represent the same thing, use the == operator. (In the rare case where you need to know whether two objects are the exact same object, use the identical() function instead.) Here’s how the == operator works:

1. If x or y is null, return true if both are null, and false if only one is null.

2. Return the result of invoking the == method on x with the argument y. (That’s right, operators such as == are methods that are invoked on their first operand. For details, see Operators.)

Here’s an example of using each of the equality and relational operators:

assert(2 == 2);
assert(2 != 3);
assert(3 > 2);
assert(2 < 3);
assert(3 >= 3);
assert(2 <= 3);

Type test operators

The as, is, and is! operators are handy for checking types at runtime.

The result of obj is T is true if obj implements the interface specified by T. For example, obj is Object? is always true.

Use the as operator to cast an object to a particular type if and only if you are sure that the object is of that type. Example:

(employee as Person).firstName = 'Bob';

If you aren’t sure that the object is of type T, then use is T to check the type before using the object.

if (employee is Person) {
  // Type check
  employee.firstName = 'Bob';
}

Assignment operators

As you’ve already seen, you can assign values using the = operator. To assign only if the assigned-to variable is null, use the ??= operator.

// Assign value to a
a = value;
// Assign value to b if b is null; otherwise, b stays the same
b ??= value;

Compound assignment operators such as += combine an operation with an assignment.

Here’s how compound assignment operators work:

The following example uses assignment and compound assignment operators:

var a = 2; // Assign using =
a *= 3; // Assign and multiply: a = a * 3
assert(a == 6);

Logical operators

You can invert or combine boolean expressions using the logical operators.

Here’s an example of using the logical operators:

if (!done && (col == 0 || col == 3)) {
  // ...Do something...
}

Bitwise and shift operators

You can manipulate the individual bits of numbers in Dart. Usually, you’d use these bitwise and shift operators with integers.

Here’s an example of using bitwise and shift operators:

final value = 0x22;
final bitmask = 0x0f;

assert((value & bitmask) == 0x02); // AND
assert((value & ~bitmask) == 0x20); // AND NOT
assert((value | bitmask) == 0x2f); // OR
assert((value ^ bitmask) == 0x2d); // XOR

assert((value << 4) == 0x220); // Shift left
assert((value >> 4) == 0x02); // Shift right

// Shift right example that results in different behavior on web
// because the operand value changes when masked to 32 bits:
assert((-value >> 4) == -0x03);

assert((value >>> 4) == 0x02); // Unsigned shift right
assert((-value >>> 4) > 0); // Unsigned shift right

Conditional expressions

Dart has two operators that let you concisely evaluate expressions that might otherwise require if-else statements:

condition ? expr1 : expr2

If condition is true, evaluates expr1 (and returns its value); otherwise, evaluates and returns the value of expr2.

expr1 ?? expr2

If expr1 is non-null, returns its value; otherwise, evaluates and returns the value of expr2.

When you need to assign a value based on a boolean expression, consider using ? and :.

var visibility = isPublic ? 'public' : 'private';

If the boolean expression tests for null, consider using ??.

String playerName(String? name) => name ?? 'Guest';

The previous example could have been written at least two other ways, but not as succinctly:

// Slightly longer version uses ?: operator.
String playerName(String? name) => name != null ? name : 'Guest';

// Very long version uses if-else statement.
String playerName(String? name) {
  if (name != null) {
    return name;
  } else {
    return 'Guest';
  }
}

Cascade notation

Cascades (.., ?..) allow you to make a sequence of operations on the same object. In addition to accessing instance members, you can also call instance methods on that same object. This often saves you the step of creating a temporary variable and allows you to write more fluid code.

Consider the following code:

var paint = Paint()
  ..color = Colors.black
  ..strokeCap = StrokeCap.round
  ..strokeWidth = 5.0;

The constructor, Paint(), returns a Paint object. The code that follows the cascade notation operates on this object, ignoring any values that might be returned.

The previous example is equivalent to this code:

var paint = Paint();
paint.color = Colors.black;
paint.strokeCap = StrokeCap.round;
paint.strokeWidth = 5.0;

If the object that the cascade operates on can be null, then use a null-shorting cascade (?..) for the first operation. Starting with ?.. guarantees that none of the cascade operations are attempted on that null object.

querySelector('#confirm') // Get an object.
  ?..text = 'Confirm' // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'))
  ..scrollIntoView();

The previous code is equivalent to the following:

var button = querySelector('#confirm');
button?.text = 'Confirm';
button?.classes.add('important');
button?.onClick.listen((e) => window.alert('Confirmed!'));
button?.scrollIntoView();

You can also nest cascades. For example:

final addressBook = (AddressBookBuilder()
      ..name = 'jenny'
      ..email = '[email protected]'
      ..phone = (PhoneNumberBuilder()
            ..number = '415-555-0100'
            ..label = 'home')
          .build())
    .build();

Be careful to construct your cascade on a function that returns an actual object. For example, the following code fails:

var sb = StringBuffer();
sb.write('foo')
  ..write('bar'); // Error: method 'write' isn't defined for 'void'.

The sb.write() call returns void, and you can’t construct a cascade on void.

Other operators

You’ve seen most of the remaining operators in other examples:

For more information about the ., ?., and .. operators, see Classes.