# Operators

> Learn about the operators Dart supports.



<?code-excerpt replace="/ *\/\/\s+ignore_for_file:[^\n]+\n//g; /(^|\n) *\/\/\s+ignore:[^\n]+\n/$1/g; /(\n[^\n]+) *\/\/\s+ignore:[^\n]+\n/$1\n/g; / *\/\/\s+ignore:[^\n]+//g; /([A-Z]\w*)\d\b/$1/g"?>

<a name="operators"></a>

Dart supports the operators shown in the following table.
The table shows Dart's operator associativity 
and [operator precedence](#operator-precedence-example) from highest to lowest,
which are an **approximation** of Dart's operator relationships.
You can implement many of these [operators as class members][].

| Description                             | Operator                                                                                           | Associativity |
|-----------------------------------------|----------------------------------------------------------------------------------------------------|---------------|
| unary postfix                           | *`expr`*`++`    *`expr`*`--`    `()`    `[]`    `?[]`    `.`    `?.`    `!`                        | None          |
| unary prefix                            | `-`*`expr`*    `!`*`expr`*    `~`*`expr`*    `++`*`expr`*    `--`*`expr`*      `await` *`expr`*    | None          |
| multiplicative                          | `*`    `/`    `%`    `~/`                                                                          | Left          |
| additive                                | `+`    `-`                                                                                         | Left          |
| shift                                   | `<<`    `>>`    `>>>`                                                                              | Left          |
| bitwise AND                             | `&`                                                                                                | Left          |
| bitwise XOR                             | `^`                                                                                                | Left          |
| bitwise OR                              | <code>&#124;</code>                                                                                | Left          |
| relational and type test                | `>=`    `>`    `<=`    `<`    `as`    `is`    `is!`                                                | None          |
| equality                                | `==`    `!=`                                                                                       | None          |
| logical AND                             | `&&`                                                                                               | Left          |
| logical OR                              | <code>&#124;&#124;</code>                                                                          | Left          |
| if-null                                 | `??`                                                                                               | Left          |
| conditional                             | *`expr1`*    `?`    *`expr2`*    `:`    *`expr3`*                                                  | Right         |
| cascade                                 | `..`    `?..`                                                                                      | Left          |
| assignment                              | `=`    `*=`    `/=`    `+=`    `-=`    `&=`    `^=`    *etc.*                                      | Right         |
| spread  ([See note](#spread-operators)) | `...`    `...?`                                                                                    | None          |

{:.table .table-striped}

:::warning
The previous table should only be used as a helpful guide.
The notion of operator precedence and associativity
is an approximation of the truth found in the language grammar.
You can find the authoritative behavior of Dart's operator relationships
in the grammar defined in the [Dart language specification][].
:::

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

<?code-excerpt "misc/test/language_tour/operators_test.dart (expressions)" replace="/,//g"?>
```dart
a++
a + b
a = b
a == b
c ? a : b
a is T
```

## Operator precedence example

In the [operator table](#operators),
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:

<?code-excerpt "misc/test/language_tour/operators_test.dart (precedence)"?>
```dart
// Parentheses improve readability.
if ((n % i == 0) && (d % i == 0)) {
  // ...
}

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

:::warning
For operators that take two operands, the leftmost operand determines which
method is used. For example, if you have a `Vector` object and
a `Point` object, then `aVector + aPoint` uses `Vector` addition (`+`).
:::


## Arithmetic operators

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

| Operator    | Meaning                                                                  |
|-------------|--------------------------------------------------------------------------|
| `+`         | Add                                                                      |
| `-`         | Subtract                                                                 |
| `-`*`expr`* | Unary 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)                        |

{:.table .table-striped}

Example:

<?code-excerpt "misc/test/language_tour/operators_test.dart (arithmetic)"?>
```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.

| Operator                    | Meaning                                            |
|-----------------------------|----------------------------------------------------|
| `++`*`var`* | *`var`*  `=`  *`var `*  `+ 1` (expression value is *`var`* ` + 1`) |
| *`var`*`++` | *`var`*  `=`  *`var `*  `+ 1` (expression value is *`var`*)        |
| `--`*`var`* | *`var`*  `=`  *`var `*  `- 1` (expression value is *`var`* ` - 1`) |
| *`var`*`--` | *`var`*  `=`  *`var `*  `- 1` (expression value is *`var`*)       |

{:.table .table-striped}

Example:

<?code-excerpt "misc/test/language_tour/operators_test.dart (increment-decrement)"?>
```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.

| Operator  | Meaning                                   |
|-----------|-------------------------------------------|
| `==`      | Equal; see discussion below               |
| `!=`      | Not equal                                 |
| `>`       | Greater than                              |
| `<`       | Less than                                 |
| `>=`      | Greater than or equal to                  |
| `<=`      | Less than or equal to                     |

{:.table .table-striped}

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:

<?code-excerpt "misc/test/language_tour/operators_test.dart (relational)"?>
```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.

| Operator  | Meaning                                              |
|-----------|------------------------------------------------------|
| `as`      | Typecast (also used to specify [library prefixes][]) |
| `is`      | True if the object has the specified type            |
| `is!`     | True if the object doesn't have the specified type   |

{:.table .table-striped}

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:

<?code-excerpt "misc/lib/language_tour/classes/employee.dart (emp-as-person)"?>
```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.
<?code-excerpt "misc/lib/language_tour/classes/employee.dart (emp-is-person)"?>
```dart
if (employee is Person) {
  // Type check
  employee.firstName = 'Bob';
}
```

:::note
The code isn't equivalent. If `employee` is null or not a `Person`, the
first example throws an exception; the second does nothing.
:::

## 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.

<?code-excerpt "misc/test/language_tour/operators_test.dart (assignment)"?>
```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.

|      |       |       |        |                      |
|------|-------|-------|--------|----------------------|
| `=`  | `*=`  | `%=`  | `>>>=` | `^=`                 |
| `+=` | `/=`  | `<<=` | `&=`   | <code>&#124;=</code> |
| `-=` | `~/=` | `>>=` |        |                      |

{:.table}

Here's how compound assignment operators work:

|                           | Compound assignment | Equivalent expression |
|---------------------------|---------------------|-----------------------|
| **For an operator *op*:** | `a `  *`op`*`= b`   | `a = a ` *`op `* `b`  |
| **Example:**              | `a += b`            | `a = a + b`           |

{:.table}

The following example uses assignment and compound assignment
operators:

<?code-excerpt "misc/test/language_tour/operators_test.dart (op-assign)"?>
```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.

| Operator                   | Meaning                                                                  |
|----------------------------|--------------------------------------------------------------------------|
| `!`*`expr`*                | inverts the following expression (changes false to true, and vice versa) |
| <code>&#124;&#124;</code>  | logical OR                                                               |
| `&&`                       | logical AND                                                              |

{:.table .table-striped}

Here's an example of using the logical operators:

<?code-excerpt "misc/lib/language_tour/operators.dart (op-logical)"?>
```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.

| Operator            | Meaning                                               |
|---------------------|-------------------------------------------------------|
| `&`                 | AND                                                   |
| <code>&#124;</code> | OR                                                    |
| `^`                 | XOR                                                   |
| `~`*`expr`*         | Unary bitwise complement (0s become 1s; 1s become 0s) |
| `<<`                | Shift left                                            |
| `>>`                | Shift right                                           |
| `>>>`               | Unsigned shift right                                  |

{:.table .table-striped}

:::note
The behavior of bitwise operations with large or negative operands
might differ between platforms.
To learn more, check out
[Bitwise operations platform differences][].
:::

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

<?code-excerpt "misc/test/language_tour/operators_test.dart (op-bitwise)"?>
```dart
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
```

:::version-note
The `>>>` operator (known as _triple-shift_ or _unsigned shift_)
requires a [language version][] of at least 2.14.
:::

[Bitwise operations platform differences]: /resources/language/number-representation#bitwise-operations

## 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 `:`.

<?code-excerpt "misc/lib/language_tour/operators.dart (if-then-else-operator)"?>
```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).

<?code-excerpt "misc/test/language_tour/operators_test.dart (if-null)"?>
```dart
String playerName(String? name) => name ?? 'Guest';
```

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

<?code-excerpt "misc/test/language_tour/operators_test.dart (if-null-alt)"?>
```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';
  }
}
```

## 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:

<?code-excerpt "misc/lib/language_tour/cascades.dart (cascade)"?>
```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:

<?code-excerpt "misc/lib/language_tour/cascades.dart (cascade-expanded)"?>
```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.

<?code-excerpt "misc/test/language_tour/browser_test.dart (cascade-operator)"?>
```dart
document.querySelector('#confirm') // Get an object.
  ?..textContent =
      'Confirm' // Use its members.
  ..classList.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'))
  ..scrollIntoView();
```

The previous code is equivalent to the following:

<?code-excerpt "misc/test/language_tour/browser_test.dart (cascade-operator-example-expanded)"?>
```dart
final button = document.querySelector('#confirm');
button?.textContent = 'Confirm';
button?.classList.add('important');
button?.onClick.listen((e) => window.alert('Confirmed!'));
button?.scrollIntoView();
```

You can also nest cascades. For example:

<?code-excerpt "misc/lib/language_tour/operators.dart (nested-cascades)"?>
```dart
final addressBook =
    (AddressBookBuilder()
          ..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:

<?code-excerpt "misc/lib/language_tour/operators.dart (cannot-cascade-on-void)" plaster="none"?>
```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`.

:::note
Strictly speaking, the "double dot" notation for cascades isn't an operator.
It's just part of the Dart syntax.
:::

## Spread operators

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](/language/collections#spread-operators) page.

Because it isn't an operator, the syntax doesn't
have any "[operator precedence](#operators)".
Effectively, it has the lowest "precedence" &mdash;
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:

| Operator | Name                         | Meaning                                                                                                                                                                                                                                                 |
|----------|------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `()`     | Function application         | Represents a function call                                                                                                                                                                                                                              |
| `[]`     | Subscript access             | Represents a call to the overridable `[]` operator; example: `fooList[1]` passes the int `1` to `fooList` to access the element at index `1`                                                                                                            |
| `?[]`    | Conditional subscript access | Like `[]`, 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 access                | Refers to a property of an expression; example: `foo.bar` selects property `bar` from expression `foo`                                                                                                                                                  |
| `?.`     | Conditional member access    | Like `.`, 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)                                                                   |
| `!`      | Not-null assertion operator  | Casts 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) |

{:.table .table-striped}

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


[operators as class members]: /language/methods#operators
[Dart language specification]: /resources/language/spec
[identical()]: https://api.dart.dev/dart-core/identical.html
[Operators]: /language/methods#operators
[library prefixes]: /language/libraries#specifying-a-library-prefix
[if-else]: /language/branches#if
[language version]: /resources/language/evolution#language-versioning
[Classes]: /language/classes

