JavaScript has assignment, comparison, arithmetic, bitwise, logical, string, and special operators. This chapter describes the operators and contains information about operator precedence.
The following table summarizes the JavaScript operators.
(Logical AND) Returns the first operand if it can be converted to false; otherwise, returns the second operand. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.
||
(Logical OR) Returns the first operand if it can be converted to true; otherwise, returns the second operand. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.
!
(Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true.
(Bitwise AND) Returns a one in each bit position if bits of both operands are ones.
^
(Bitwise XOR) Returns a one in a bit position if bits of one but not both operands are one.
|
(Bitwise OR) Returns a one in a bit if bits of either operand is one.
~
(Bitwise NOT) Flips the bits of its operand.
<<
(Left shift) Shifts its first operand in binary representation the number of bits to the left specified in the second operand, shifting in zeros from the right.
>>
(Sign-propagating right shift) Shifts the first operand in binary representation the number of bits to the right specified in the second operand, discarding bits shifted off.
>>>
(Zero-fill right shift) Shifts the first operand in binary representation the number of bits to the right specified in the second operand, discarding bits shifted off, and shifting in zeros from the left.
Specifies an expression to be evaluated without returning a value.
Assignment Operators
An assignment operator assigns a value to its left operand based on the value of its right operand.
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
The basic assignment operator is equal (=), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x. The other assignment operators are usually shorthand for standard operations, as shown in the following table.
Table 5.2 Assignment operators
Shorthand operator
Meaning
x += y
x = x + y
x -= y
x = x - y
x *= y
x = x * y
x /= y
x = x / y
x %= y
x = x % y
x <<= y
x = x << y
x >>= y
x = x >> y
x >>>= y
x = x >>> y
x &= y
x = x & y
x ^= y
x = x ^ y
x |= y
x = x | y
In unusual situations, the assignment operator is not identical to the Meaning expression in Table 5.2. When the left operand of an assignment operator itself contains an assignment operator, the left operand is evaluated only once. For example:
a[i++] += 5 //i is evaluated only once
a[i++] = a[i++] + 5 //i is evaluated twice
Comparison Operators
A comparison operator compares its operands and returns a logical value based on whether the comparison is true.
Implemented in
JavaScript 1.0
JavaScript 1.3: Added the === and !== operators.
JavaScript 1.4: Deprecated == for comparison of two JSObject objects. Use the JSObject.equals method.
ECMA version
ECMA-262 includes all comparison operators except === and !==.
ECMA-262 Edition 3 adds === and !==.
The operands can be numerical or string values. Strings are compared based on standard lexicographical ordering, using Unicode values.
A Boolean value is returned as the result of the comparison.
Two strings are equal when they have the same sequence of characters, same length, and same characters in corresponding positions.
Two numbers are equal when they are numerically equal (have the same number value). NaN is not equal to anything, including NaN. Positive and negative zeros are equal.
Two objects are equal if they refer to the same Object.
Two Boolean operands are equal if they are both true or false.
Null and Undefined types are == (but not ===).
The following table describes the comparison operators.
Returns true if the operands are equal. If the two operands are not of the same type, JavaScript attempts to convert the operands to an appropriate type for the comparison.
3 == var1
"3" == var1
3 == '3'
Not equal (!=)
Returns true if the operands are not equal. If the two operands are not of the same type, JavaScript attempts to convert the operands to an appropriate type for the comparison.
var1 != 4
var1 != "3"
Strict equal (===)
Returns true if the operands are equal and of the same type.
3 === var1
Strict not equal (!==)
Returns true if the operands are not equal and/or not of the same type.
var1 !== "3"
3 !== '3'
Greater than (>)
Returns true if the left operand is greater than the right operand.
var2 > var1
Greater than or equal (>=)
Returns true if the left operand is greater than or equal to the right operand.
var2 >= var1
var1 >= 3
Less than (<)
Returns true if the left operand is less than the right operand.
var1 < var2
Less than or equal (<=)
Returns true if the left operand is less than or equal to the right operand.
var1 <= var2
var2 <= 5
1These examples assume that var1 has been assigned the value 3 and var2 has been assigned the value 4.
Using the Equality Operators
The standard equality operators (== and !=) compare two operands without regard to their type. The strict equality operators (=== and !==) perform equality comparisons on operands of the same type. Use strict equality operators if the operands must be of a specific type as well as value or if the exact type of the operands is important. Otherwise, use the standard equality operators, which allow you to compare the identity of two operands even if they are not of the same type.
When type conversion is needed, JavaScript converts String, Number, Boolean, or Object operands as follows.
When comparing a number and a string, the string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.
If one of the operands is Boolean, the Boolean operand is converted to 1 if it is true and +0 if it is false.
If an object is compared with a number or string, JavaScript attempts to return the default value for the object. Operators attempt to convert the object to a primitive value, a String or Number value, using the valueOf and toString methods of the objects. If this attempt to convert the object fails, a runtime error is generated.
You cannot use the standard equality operator (==) to compare instances of JSObject. Use the JSObject.equals method for such comparisons.
Backward Compatibility The behavior of the standard equality operators (== and !=) depends on the JavaScript version.
JavaScript 1.3 and earlier versions.You can use either the standard equality operator (==) or JSObject.equals to compare instances of JSObject.
JavaScript 1.2.The standard equality operators (== and !=) do not perform a type conversion before the comparison is made. The strict equality operators (=== and !==) are unavailable.
JavaScript 1.1 and earlier versions.The standard equality operators (== and !=) perform a type conversion before the comparison is made. The strict equality operators (=== and !==) are unavailable.
Arithmetic Operators
Arithmetic operators take numerical values (either literals or variables) as their operands and return a single numerical value. The standard arithmetic operators are addition (+), subtraction (-), multiplication (*), and division (/).
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
These operators work as they do in most other programming languages, except the / operator returns a floating-point division in JavaScript, not a truncated division as it does in languages such as C or Java. For example:
1/2 //returns 0.5 in JavaScript
1/2 //returns 0 in Java
% (Modulus)
The modulus operator is used as follows:
var1 % var2
The modulus operator returns the first operand modulo the second operand, that is, var1 modulo var2, in the preceding statement, where var1 and var2 are variables. The modulo function is the integer remainder of dividing var1 by var2. For example, 12 % 5 returns 2.
++ (Increment)
The increment operator is used as follows:
var++ or ++var
This operator increments (adds one to) its operand and returns a value. If used postfix, with operator after operand (for example, x++), then it returns the value before incrementing. If used prefix with operator before operand (for example, ++x), then it returns the value after incrementing.
For example, if x is three, then the statement y = x++ sets y to 3 and increments x to 4. If x is 3, then the statement y = ++x increments x to 4 and sets y to 4.
-- (Decrement)
The decrement operator is used as follows:
var-- or --var
This operator decrements (subtracts one from) its operand and returns a value. If used postfix (for example, x--), then it returns the value before decrementing. If used prefix (for example, --x), then it returns the value after decrementing.
For example, if x is three, then the statement y = x-- sets y to 3 and decrements x to 2. If x is 3, then the statement y = --x decrements x to 2 and sets y to 2.
- (Unary Negation)
The unary negation operator precedes its operand and negates it. For example, y = -x negates the value of x and assigns that to y; that is, if x were 3, y would get the value -3 and x would retain the value 3.
Bitwise Operators
Bitwise operators treat their operands as a set of 32 bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.
The following table summarizes JavaScript's bitwise operators:
Table 5.4 Bitwise operators
Operator
Usage
Description
Bitwise AND
a & b
Returns a one in each bit position for which the corresponding bits of both operands are ones.
Bitwise OR
a | b
Returns a one in each bit position for which the corresponding bits of either or both operands are ones.
Bitwise XOR
a ^ b
Returns a one in each bit position for which the corresponding bits of either but not both operands are ones.
Bitwise NOT
~ a
Inverts the bits of its operand.
Left shift
a << b
Shifts a in binary representation b bits to left, shifting in zeros from the right.
Sign-propagating right shift
a >> b
Shifts a in binary representation b bits to right, discarding bits shifted off.
Zero-fill right shift
a >>> b
Shifts a in binary representation b bits to the right, discarding bits shifted off, and shifting in zeros from the left.
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
"> Bitwise Logical Operators
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
Conceptually, the bitwise logical operators work as follows:
The operands are converted to thirty-two-bit integers and expressed by a series of bits (zeros and ones).
Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on.
The operator is applied to each pair of bits, and the result is constructed bitwise.
For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:
15 & 9 yields 9 (1111 & 1001 = 1001)
15 | 9 yields 15 (1111 | 1001 = 1111)
15 ^ 9 yields 6 (1111 ^ 1001 = 0110)
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
"> Bitwise Shift Operators
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.
Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operator.
<< (Left Shift)
This operator shifts the first operand the specified number of bits to the left. Excess bits shifted off to the left are discarded. Zero bits are shifted in from the right.
For example, 9<<2 yields thirty-six, because 1001 shifted two bits to the left becomes 100100, which is thirty-six.
>> (Sign-Propagating Right Shift)
This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Copies of the leftmost bit are shifted in from the left.
For example, 9>>2 yields two, because 1001 shifted two bits to the right becomes 10, which is two. Likewise, -9>>2 yields -3, because the sign is preserved.
>>> (Zero-Fill Right Shift)
This operator shifts the first operand the specified number of bits to the right. Excess bits shifted off to the right are discarded. Zero bits are shifted in from the left.
For example, 19>>>2 yields four, because 10011 shifted two bits to the right becomes 100, which is four. For non-negative numbers, zero-fill right shift and sign-propagating right shift yield the same result.
Logical Operators
Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value.
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
The logical operators are described in the following table.
Table 5.5 Logical operators
Operator
Usage
Description
&&
expr1 && expr2
(Logical AND) Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false.
||
expr1 || expr2
(Logical OR) Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.
!
!expr
(Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true.
Examples of expressions that can be converted to false are those that evaluate to null, 0, the empty string (""), or undefined.
Even though the && and || operators can be used with operands that are not Boolean values, they can still be considered Boolean operators since their return values can always be converted to Boolean values.
Short-Circuit Evaluation.As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:
false && anything is short-circuit evaluated to false.
true || anything is short-circuit evaluated to true.
The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.
Backward Compatibility
JavaScript 1.0 and 1.1.The && and || operators behave as follows:
Operator
Behavior
&&
If the first operand (expr1) can be converted to false, the && operator returns false rather than the value of expr1.
||
If the first operand (expr1) can be converted to true, the || operator returns true rather than the value of expr1.
Examples The following code shows examples of the && (logical AND) operator.
a1=true && true // t && t returns true
a2=true && false // t && f returns false
a3=false && true // f && t returns false
a4=false && (3 == 4) // f && f returns false
a5="Cat" && "Dog" // t && t returns Dog
a6=false && "Cat" // f && t returns false
a7="Cat" && false // t && f returns false
The following code shows examples of the || (logical OR) operator.
o1=true || true // t || t returns true
o2=false || true // f || t returns true
o3=true || false // t || f returns true
o4=false || (3 == 4) // f || f returns false
o5="Cat" || "Dog" // t || t returns Cat
o6=false || "Cat" // f || t returns Cat
o7="Cat" || false // t || f returns Cat
The following code shows examples of the ! (logical NOT) operator.
In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings. For example, "my " + "string" returns the string "my string".
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
The shorthand assignment operator += can also be used to concatenate strings. For example, if the variable mystring has the value "alpha," then the expression mystring += "bet" evaluates to "alphabet" and assigns this value to mystring.
Special Operators
?: (Conditional operator)
The conditional operator is the only JavaScript operator that takes three operands. This operator is frequently used as a shortcut for the if statement.
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
Syntax condition ? expr1 : expr2
Parameters
condition
An expression that evaluates to true or false.
expr1, expr2
Expressions with values of any type.
Description If condition is true, the operator returns the value of expr1; otherwise, it returns the value of expr2. For example, to display a different message based on the value of the isMember variable, you could use this statement:
document.write ("The fee is " + (isMember ? "$2.00" : "$10.00"))
, (Comma operator)
The comma operator evaluates both of its operands and returns the value of the second operand.
Implemented in
JavaScript 1.0
ECMA version
ECMA-262
Syntax expr1, expr2
Parameters
expr1, expr2
Any expressions.
Description You can use the comma operator when you want to include multiple expressions in a location that requires a single expression. The most common usage of this operator is to supply multiple parameters in a for loop.
For example, if a is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to increment two variables at once. The code prints the values of the diagonal elements in the array:
for (var i=0, j=9; i <= 9; i++, j--)
document.writeln("a["+i+","+j+"]= " + a[i,j])
delete
The delete operator deletes an object, an object's property, or an element at a specified index in an array.
Implemented in
JavaScript 1.2, NES 3.0
ECMA version
ECMA-262
Syntax delete objectName delete objectName.property delete objectName[index]
delete property // legal only within a with statement
Parameters
objectName
The name of an object.
property
The property to delete.
index
An integer representing the array index to delete.
Description The fourth form is legal only within a with statement, to delete a property from an object.
You can use the delete operator to delete variables declared implicitly but not those declared with the var statement.
If the delete operator succeeds, it sets the property or element to undefined. The delete operator returns true if the operation is possible; it returns false if the operation is not possible.
x=42
var y= 43
myobj=new Number()
myobj.h=4 // create property h
delete x // returns true (can delete if declared implicitly)
delete y // returns false (cannot delete if declared with var)
delete Math.PI // returns false (cannot delete predefined properties)
delete myobj.h // returns true (can delete user-defined properties)
delete myobj // returns true (can delete objects)
Deleting array elements.When you delete an array element, the array length is not affected. For example, if you delete a[3], a[4] is still a[4] and a[3] is undefined.
When the delete operator removes an array element, that element is no longer in the array. In the following example, trees[3] is removed with delete.
trees=new Array("redwood","bay","cedar","oak","maple")
delete trees[3]
if (3 in trees) {
// this does not get executed
}
If you want an array element to exist but have an undefined value, use the undefined keyword instead of the delete operator. In the following example, trees[3] is assigned the value undefined, but the array element still exists:
trees=new Array("redwood","bay","cedar","oak","maple")
trees[3]=undefined
if (3 in trees) {
// this gets executed
}
function
The function operator defines an anonymous function inside an expression.
