# 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.

DescriptionOperatorAssociativity
unary postfixexpr++    expr--    ()    []    ?[]    .    ?.    !None
unary prefix-expr    !expr    ~expr    ++expr    --expr      await exprNone
multiplicative*    /    %  ~/Left
shift<<    >>    >>>Left
bitwise AND&Left
bitwise XOR^Left
bitwise OR|Left
relational and type test>=    >    <=    <    as    is    is!None
equality==    !=None
logical AND&&Left
logical OR||Left
if-null??Left
conditionalexpr1    ?    expr2    :    expr3Right
assignment=    *=    /=   +=   -=   &=   ^=   etc.Right

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

dart
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:

dart
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.

OperatorMeaning
-Subtract
-exprUnary minus, also known as negation (reverse the sign of the expression)
*Multiply
/Divide
~/Divide, returning an integer result
%Get the remainder of an integer division (modulo)

Example:

dart
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.

OperatorMeaning
++varvar  =  var + 1 (expression value is var + 1)
var++var  =  var + 1 (expression value is var)
--varvar  =  var - 1 (expression value is var - 1)
var--var  =  var - 1 (expression value is var)

Example:

dart
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.

OperatorMeaning
==Equal; see discussion below
!=Not equal
>Greater than
<Less than
>=Greater than or equal to
<=Less than or equal to

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:

dart
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.

OperatorMeaning
asTypecast (also used to specify library prefixes)
isTrue if the object has the specified type
is!True if the object doesn't have the specified type

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:

dart
(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.

dart
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.

dart
// 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:

Compound assignmentEquivalent expression
For an operator op:a op= ba = a op b
Example:a += ba = a + b

The following example uses assignment and compound assignment operators:

dart
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.

OperatorMeaning
!exprinverts the following expression (changes false to true, and vice versa)
||logical OR
&&logical AND

Here's an example of using the logical operators:

dart
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.

OperatorMeaning
&AND
|OR
^XOR
~exprUnary bitwise complement (0s become 1s; 1s become 0s)
<<Shift left
>>Shift right
>>>Unsigned shift right

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

dart
final value = 0x22;

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 the conditional operator ? and :.

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

If the boolean expression tests for null, consider using the if-null operator ?? (also known as the null-coalescing operator).

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

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

dart
// 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';
}
}

#

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:

dart
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:

dart
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.

dart
querySelector('#confirm') // Get an object.
?..text = 'Confirm' // Use its members.
..scrollIntoView();

The previous code is equivalent to the following:

dart
var button = querySelector('#confirm');
button?.text = 'Confirm';
button?.scrollIntoView();

You can also nest cascades. For example:

dart
..name = 'jenny'
..email = 'jenny@example.com'
..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:

dart
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.

#

Spread operators evaluate an expression that yields a collection, unpacks the resulting values, and inserts them into another collection.

The spread operator isn't actually an operator expression. The .../...? syntax is part of the collection literal itself. So, you can learn more about spread operators on the Collections page.

Because it isn't an operator, the syntax doesn't have any "operator precedence". Effectively, it has the lowest "precedence" — any kind of expression is valid as the spread target, such as:

dart
[...a + b]

## Other operators

#

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

OperatorNameMeaning
()Function applicationRepresents a function call
[]Subscript accessRepresents a call to the overridable [] operator; example: fooList[1] passes the int 1 to fooList to access the element at index 1
?[]Conditional subscript accessLike [], but the leftmost operand can be null; example: fooList?[1] passes the int 1 to fooList to access the element at index 1 unless fooList is null (in which case the expression evaluates to null)
.Member accessRefers to a property of an expression; example: foo.bar selects property bar from expression foo
?.Conditional member accessLike ., but the leftmost operand can be null; example: foo?.bar selects property bar from expression foo unless foo is null (in which case the value of foo?.bar is null)
!Non-null assertion operatorCasts an expression to its underlying non-nullable type, throwing a runtime exception if the cast fails; example: foo!.bar asserts foo is non-null and selects the property bar, unless foo is null in which case a runtime exception is thrown