Operators



Operators are special functions that take one or more operands and return a value. An operand is a value—usually a literal, a variable, or an expression—that an operator uses as input. For example, in the following code, the addition ( + ) and multiplication ( * ) operators are used with three literal operands (2, 3 , and 4 ) to return a value. This value is then used by the assignment ( = ) operator to assign the returned value, 14, to the variable sumNumber .

var sumNumber:uint = 2 + 3 * 4; // uint = 14

Operators can be unary, binary, or ternary. A unary operator takes one operand. For example, the increment ( ++ ) operator is a unary operator, because it takes only one operand. A binary operator takes two operands. For example, the division ( / ) operator takes two operands. A ternary operator takes three operands. For example, the conditional ( ?: ) operator takes three operands.

Some operators are overloaded , which means that they behave differently depending on the type or quantity of operands passed to them. The addition ( + ) operator is an example of an overloaded operator that behaves differently depending on the data type of the operands. If both operands are numbers, the addition operator returns the sum of the values. If both operands are strings, the addition operator returns the concatenation of the two operands. The following example code shows how the operator behaves differently depending on the operands:

trace(5 + 5); // 10 
trace("5" + "5"); // 55

Operators can also behave differently based on the number of operands supplied. The subtraction ( - ) operator is both a unary and binary operator. When supplied with only one operand, the subtraction operator negates the operand and returns the result. When supplied with two operands, the subtraction operator returns the difference between the operands. The following example shows the subtraction operator used first as a unary operator, and then as a binary operator.

trace(-3); // -3 
trace(7 - 2); // 5

Operator precedence and associativity

Operator precedence and associativity determine the order in which operators are processed. Although it may seem natural to those familiar with arithmetic that the compiler processes the multiplication ( * ) operator before the addition ( + ) operator, the compiler needs explicit instructions about which operators to process first. Such instructions are collectively referred to as operator precedence . ActionScript defines a default operator precedence that you can alter using the parentheses ( () ) operator. For example, the following code alters the default precedence in the previous example to force the compiler to process the addition operator before the multiplication operator:

var sumNumber:uint = (2 + 3) * 4; // uint == 20

You may encounter situations in which two or more operators of the same precedence appear in the same expression. In these cases, the compiler uses the rules of associativity to determine which operator to process first. All of the binary operators, except the assignment operators, are left-associative , which means that operators on the left are processed before operators on the right. The assignment operators and the conditional ( ?: ) operator are right-associative , which means that the operators on the right are processed before operators on the left.

For example, consider the less-than ( < ) and greater-than ( > ) operators, which have the same precedence. If both operators are used in the same expression, the operator on the left is processed first because both operators are left-associative. This means that the following two statements produce the same output:

trace(3 > 2 < 1); // false 
trace((3 > 2) < 1); // false

The greater-than operator is processed first, which results in a value of true , because the operand 3 is greater than the operand 2. The value true is then passed to the less-than operator along with the operand 1. The following code represents this intermediate state:

trace((true) < 1); 

The less-than operator converts the value true to the numeric value 1 and compares that numeric value to the second operand 1 to return the value false (the value 1 is not less than 1).

trace(1 < 1); // false

You can alter the default left associativity with the parentheses operator. You can instruct the compiler to process the less-than operator first by enclosing that operator and its operands in parentheses. The following example uses the parentheses operator to produce a different output using the same numbers as the previous example:

trace(3 > (2 < 1)); // true

The less-than operator is processed first, which results in a value of false because the operand 2 is not less than the operand 1. The value false is then passed to the greater-than operator along with the operand 3. The following code represents this intermediate state:

trace(3 > (false)); 

The greater-than operator converts the value false to the numeric value 0 and compares that numeric value to the other operand 3 to return true (the value 3 is greater than 0).

trace(3 > 0); // true

The following table lists the operators for ActionScript 3.0 in order of decreasing precedence. Each row of the table contains operators of the same precedence. Each row of operators has higher precedence than the row appearing below it in the table.

Group

Operators

Primary

[] {x:y} () f(x) new x.y x[y] <></> @ :: ..

Postfix

x++ x--

Unary

++x --x + - ~ ! delete typeof void

Multiplicative

* / %

Additive

+ -

Bitwise shift

<< >> >>>

Relational

< > <= >= as in instanceof is

Equality

== != === !==

Bitwise AND

&

Bitwise XOR

^

Bitwise OR

|

Logical AND

&&

Logical OR

||

Conditional

?:

Assignment

= *= /= %= += -= <<= >>= >>>= &= ^= |=

Comma

,

Primary operators

The primary operators include those used for creating Array and Object literals, grouping expressions, calling functions, instantiating class instances, and accessing properties.

All the primary operators, as listed in the following table, have equal precedence. Operators that are part of the E4X specification are indicated by the (E4X) notation.

Operator

Operation performed

[]

Initializes an array

{x:y}

Initializes an object

()

Groups expressions

f(x)

Calls a function

new

Calls a constructor

x.y x[y]

Accesses a property

<></>

Initializes an XMLList object (E4X)

@

Accesses an attribute (E4X)

::

Qualifies a name (E4X)

..

Accesses a descendant XML element (E4X)

Postfix operators

The postfix operators take one operator and either increment or decrement the value. Although these operators are unary operators, they are classified separately from the rest of the unary operators because of their higher precedence and special behavior. When a postfix operator is used as part of a larger expression, the expression’s value is returned before the postfix operator is processed. For example, the following code shows how the value of the expression xNum++ is returned before the value is incremented:

var xNum:Number = 0; 
trace(xNum++); // 0 
trace(xNum); // 1

All the postfix operators, as listed in the following table, have equal precedence:

Operator

Operation performed

++

Increments (postfix)

--

Decrements (postfix)

Unary operators

The unary operators take one operand. The increment ( ++ ) and decrement ( -- ) operators in this group are prefix operators , which means that they appear before the operand in an expression. The prefix operators differ from their postfix counterparts in that the increment or decrement operation is completed before the value of the overall expression is returned. For example, the following code shows how the value of the expression ++xNum is returned after the value is incremented:

var xNum:Number = 0; 
trace(++xNum); // 1 
trace(xNum); // 1

All the unary operators, as listed in the following table, have equal precedence:

Operator

Operation performed

++

Increments (prefix)

--

Decrements (prefix)

+

Unary +

-

Unary - (negation)

!

Logical NOT

~

Bitwise NOT

delete

Deletes a property

typeof

Returns type information

void

Returns undefined value

Multiplicative operators

The multiplicative operators take two operands and perform multiplication, division, or modulo calculations.

All the multiplicative operators, as listed in the following table, have equal precedence:

Operator

Operation performed

*

Multiplication

/

Division

%

Modulo

Additive operators

The additive operators take two operands and perform addition or subtraction calculations. All the additive operators, as listed in the following table, have equal precedence:

Operator

Operation performed

+

Addition

-

Subtraction

Bitwise shift operators

The bitwise shift operators take two operands and shift the bits of the first operand to the extent specified by the second operand. All the bitwise shift operators, as listed in the following table, have equal precedence:

Operator

Operation performed

<<

Bitwise left shift

>>

Bitwise right shift

>>>

Bitwise unsigned right shift

Relational operators

The relational operators take two operands, compare their values, and return a Boolean value. All the relational operators, as listed in the following table, have equal precedence:

Operator

Operation performed

<

Less than

>

Greater than

<=

Less than or equal to

>=

Greater than or equal to

as

Checks data type

in

Checks for object properties

instanceof

Checks prototype chain

is

Checks data type

Equality operators

The equality operators take two operands, compare their values, and return a Boolean value. All the equality operators, as listed in the following table, have equal precedence:

Operator

Operation performed

==

Equality

!=

Inequality

===

Strict equality

!==

Strict inequality

Bitwise logical operators

The bitwise logical operators take two operands and perform bit-level logical operations. The bitwise logical operators differ in precedence and are listed in the following table in order of decreasing precedence:

Operator

Operation performed

&

Bitwise AND

^

Bitwise XOR

|

Bitwise OR

Logical operators

The logical operators take two operands and return a Boolean result. The logical operators differ in precedence and are listed in the following table in order of decreasing precedence:

Operator

Operation performed

&&

Logical AND

||

Logical OR

Conditional operator

The conditional operator is a ternary operator, which means that it takes three operands. The conditional operator is a shorthand method of applying the if..else conditional statement.

Operator

Operation performed

?:

Conditional

Assignment operators

The assignment operators take two operands and assign a value to one operand, based on the value of the other operand. All the assignment operators, as listed in the following table, have equal precedence:

Operator

Operation performed

=

Assignment

*=

Multiplication assignment

/=

Division assignment

%=

Modulo assignment

+=

Addition assignment

-=

Subtraction assignment

<<=

Bitwise left shift assignment

>>=

Bitwise right shift assignment

>>>=

Bitwise unsigned right shift assignment

&=

Bitwise AND assignment

^=

Bitwise XOR assignment

|=

Bitwise OR assignment