This chapter describes JavaScript expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, and special operators.
Expressions
An expression is any valid unit of code that resolves to a value.
Conceptually, there are two types of expressions: those that assign a value to a variable and those that simply have a value.
The expression x = 7
is an example of the first type. This expression uses the = operator to assign the value seven to the variable x
. The expression itself evaluates to seven.
The code 3 + 4
is an example of the second expression type. This expression uses the + operator to add three and four together without assigning the result, seven, to a variable.
JavaScript has the following expression categories:
 Arithmetic: evaluates to a number, for example 3.14159. (Generally uses arithmetic operators.)
 String: evaluates to a character string, for example, "Fred" or "234". (Generally uses string operators.)
 Logical: evaluates to true or false. (Often involves logical operators.)
 Object: evaluates to an object. (See special operators for various ones that evaluate to objects.)
Operators
JavaScript has the following types of operators. This section describes the operators and contains information about operator precedence.
 Assignment operators
 Comparison operators
 Arithmetic operators
 Bitwise operators
 Logical operators
 String operators
 Special operators
JavaScript has both binary and unary operators, and one special ternary operator, the conditional operator. A binary operator requires two operands, one before the operator and one after the operator:
operand1 operator operand2
For example, 3+4
or x*y
.
A unary operator requires a single operand, either before or after the operator:
operator operand
or
operand operator
For example, x++
or ++x
.
Assignment operators
An assignment operator assigns a value to its left operand based on the value of its right operand. 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 shorthand for the operations listed in the following table:
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 
Comparison operators
This seems to me kind of poorly explained, mostly the difference betwen "==" and "==="... A comparison operator compares its operands and returns a logical value based on whether the comparison is true. The operands can be numerical, string, logical, or object values. Strings are compared based on standard lexicographical ordering, using Unicode values. In most cases, if the two operands are not of the same type, JavaScript attempts to convert them to an appropriate type for the comparison. This behavior generally results in comparing the operands numerically. The sole exceptions to type conversion within comparisons involve the ===
and !==
operators, which perform strict equality and inequality comparisons. These operators do not attempt to convert the operands to compatible types before checking equality. The following table describes the comparison operators in terms of this sample code:
var var1 = 3, var2 = 4;
Operator  Description  Examples returning true 

Equal (== ) 
Returns true if the operands are equal.  3 == var1
3 == '3' 
Not equal (!= ) 
Returns true if the operands are not equal.  var1 != 4 
Strict equal (=== ) 
Returns true if the operands are equal and of the same type. See also Object.is and sameness in JS. 
3 === var1 
Strict not equal (!== ) 
Returns true if the operands are not equal and/or not of the same type.  var1 !== "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 
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 
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 (/). These operators work as they do in most other programming languages when used with floating point numbers (in particular, note that division by zero produces Infinity
). For example:
console.log(1 / 2); /* prints 0.5 */ console.log(1 / 2 == 1.0 / 2.0); /* also this is true */
In addition, JavaScript provides the arithmetic operators listed in the following table.
Operator  Description  Example 

% (Modulus) 
Binary operator. Returns the integer remainder of dividing the two operands.  12 % 5 returns 2. 
++ (Increment) 
Unary operator. Adds one to its operand. If used as a prefix operator (++x ), returns the value of its operand after adding one; if used as a postfix operator (x++ ), returns the value of its operand before adding one. 
If x is 3, then ++x sets x to 4 and returns 4, whereas x++ returns 3 and, only then, sets x to 4. 
 (Decrement) 
Unary operator. Subtracts one from its operand. The return value is analogous to that for the increment operator.  If x is 3, then x sets x to 2 and returns 2, whereas x returns 3 and, only then, sets x to 2. 
 (Unary negation) 
Unary operator. Returns the negation of its operand.  If x is 3, then x returns 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.
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 the left, shifting in zeros from the right. 
Signpropagating right shift  a >> b 
Shifts a in binary representation b bits to the right, discarding bits shifted off. 
Zerofill 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. 
Bitwise logical operators
Conceptually, the bitwise logical operators work as follows:
 The operands are converted to thirtytwobit 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:
Expression  Result  Binary Description 

15 & 9 
9 
1111 & 1001 = 1001 
15  9 
15 
1111  1001 = 1111 
15 ^ 9 
6 
1111 ^ 1001 = 0110 
~15 
16 
~ 00000000... 00001111 = 1111 1111 ... 11110000 
~9 
10 
~ 00000000 ... 0000 1001 = 1111 1111 ... 1111 0110 
Note that all 32 bits are inverted using the Bitwise NOT operator, and that values with the most significant (leftmost) bit set to 1 represent negative numbers (two'scomplement representation).
Bitwise shift operators
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 thirtytwobit integers and return a result of the same type as the left operand.
The shift operators are listed in the following table.
Operator  Description  Example 

<< (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.  9<<2 yields 36, because 1001 shifted 2 bits to the left becomes 100100, which is 36. 
>> (Signpropagating 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.  9>>2 yields 2, because 1001 shifted 2 bits to the right becomes 10, which is 2. Likewise, 9>>2 yields 3, because the sign is preserved. 
>>> (Zerofill 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.  19>>>2 yields 4, because 10011 shifted 2 bits to the right becomes 100, which is 4. For nonnegative numbers, zerofill right shift and signpropagating 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 nonBoolean values, they may return a nonBoolean value. The logical operators are described in the following table.
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, NaN, the empty string (""), or undefined.
The following code shows examples of the && (logical AND) operator.
var a1 = true && true; // t && t returns true var a2 = true && false; // t && f returns false var a3 = false && true; // f && t returns false var a4 = false && (3 == 4); // f && f returns false var a5 = "Cat" && "Dog"; // t && t returns Dog var a6 = false && "Cat"; // f && t returns false var a7 = "Cat" && false; // t && f returns false
The following code shows examples of the  (logical OR) operator.
var o1 = true  true; // t  t returns true var o2 = false  true; // f  t returns true var o3 = true  false; // t  f returns true var o4 = false  (3 == 4); // f  f returns false var o5 = "Cat"  "Dog"; // t  t returns Cat var o6 = false  "Cat"; // f  t returns Cat var o7 = "Cat"  false; // t  f returns Cat
The following code shows examples of the ! (logical NOT) operator.
var n1 = !true; // !t returns false var n2 = !false; // !f returns true var n3 = !"Cat"; // !t returns false
Shortcircuit evaluation
As logical expressions are evaluated left to right, they are tested for possible "shortcircuit" evaluation using the following rules:
false
&& anything is shortcircuit evaluated to false.true
 anything is shortcircuit 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.
String operators
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"
.
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
JavaScript provides the following special operators:
Conditional operator
The conditional operator is the only JavaScript operator that takes three operands. The operator can have one of two values based on a condition. The syntax is:
condition ? val1 : val2
If condition
is true, the operator has the value of val1
. Otherwise it has the value of val2
. You can use the conditional operator anywhere you would use a standard operator.
For example,
var status = (age >= 18) ? "adult" : "minor";
This statement assigns the value "adult" to the variable status
if age
is eighteen or more. Otherwise, it assigns the value "minor" to status
.
Comma operator
The comma operator (,
) simply evaluates both of its operands and returns the value of the second operand. This operator is primarily used inside a for
loop, to allow multiple variables to be updated each time through the loop.
For example, if a
is a 2dimensional 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. The syntax is:
delete objectName; delete objectName.property; delete objectName[index]; delete property; // legal only within a with statement
where objectName
is the name of an object, property
is an existing property, and index
is an integer representing the location of an element in an array.
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 userdefined properties) delete myobj; // returns true (can delete if declared implicitly)
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
. However, trees[3]
is still addressable and returns undefined
.
var 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:
var trees = new Array("redwood", "bay", "cedar", "oak", "maple"); trees[3] = undefined; if (3 in trees) { // this gets executed }
in
The in
operator returns true if the specified property is in the specified object. The syntax is:
propNameOrNumber in objectName
where propNameOrNumber
is a string or numeric expression representing a property name or array index, and objectName
is the name of an object.
The following examples show some uses of the in
operator.
// Arrays var trees = new Array("redwood", "bay", "cedar", "oak", "maple"); 0 in trees; // returns true 3 in trees; // returns true 6 in trees; // returns false "bay" in trees; // returns false (you must specify the index number, // not the value at that index) "length" in trees; // returns true (length is an Array property) // Predefined objects "PI" in Math; // returns true var myString = new String("coral"); "length" in myString; // returns true // Custom objects var mycar = {make: "Honda", model: "Accord", year: 1998}; "make" in mycar; // returns true "model" in mycar; // returns true
instanceof
The instanceof
operator returns true if the specified object is of the specified object type. The syntax is:
objectName instanceof objectType
where objectName
is the name of the object to compare to objectType
, and objectType
is an object type, such as Date
or Array
.
Use instanceof
when you need to confirm the type of an object at runtime. For example, when catching exceptions, you can branch to different exceptionhandling code depending on the type of exception thrown.
For example, the following code uses instanceof
to determine whether theDay
is a Date
object. Because theDay
is a Date
object, the statements in the if
statement execute.
var theDay = new Date(1995, 12, 17); if (theDay instanceof Date) { // statements to execute }
new
You can use the new
operator to create an instance of a userdefined object type or of one of the predefined object types Array
, Boolean
, Date
, Function
, Image
, Number
, Object
, Option
, RegExp
, or String
. On the server, you can also use it with DbPool
, Lock
, File
, or SendMail
. Use new
as follows:
var objectName = new objectType([param1, param2, ..., paramN]);
You can also create objects using object initializers, as described in using object initializers.
See the new
operator page in the Core JavaScript Reference for more information.
this
Use the this
keyword to refer to the current object. In general, this
refers to the calling object in a method. Use this
as follows:
this["propertyName"]
this.propertyName
Example 1.
Suppose a function called validate
validates an object's value
property, given the object and the high and low values:
function validate(obj, lowval, hival){ if ((obj.value < lowval)  (obj.value > hival)) alert("Invalid Value!"); }
You could call validate
in each form element's onChange
event handler, using this
to pass it the form element, as in the following example:
<B>Enter a number between 18 and 99:</B> <INPUT TYPE="text" NAME="age" SIZE=3 onChange="validate(this, 18, 99);">
Example 2.
When combined with the form
property, this
can refer to the current object's parent form. In the following example, the form myForm
contains a Text
object and a button. When the user clicks the button, the value of the Text
object is set to the form's name. The button's onClick
event handler uses this.form
to refer to the parent form, myForm
.
<FORM NAME="myForm"> Form name:<INPUT TYPE="text" NAME="text1" VALUE="Beluga"/> <INPUT NAME="button1" TYPE="button" VALUE="Show Form Name" onClick="this.form.text1.value = this.form.name;"/> </FORM>
typeof
The typeof
operator is used in either of the following ways:

typeof operand

typeof (operand)
The typeof
operator returns a string indicating the type of the unevaluated operand. operand
is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.
Suppose you define the following variables:
var myFun = new Function("5 + 2"); var shape = "round"; var size = 1; var today = new Date();
The typeof
operator returns the following results for these variables:
typeof myFun; // returns "function" typeof shape; // returns "string" typeof size; // returns "number" typeof today; // returns "object" typeof dontExist; // returns "undefined"
For the keywords true
and null
, the typeof
operator returns the following results:
typeof true; // returns "boolean" typeof null; // returns "object"
For a number or string, the typeof
operator returns the following results:
typeof 62; // returns "number" typeof 'Hello world'; // returns "string"
For property values, the typeof
operator returns the type of value the property contains:
typeof document.lastModified; // returns "string" typeof window.length; // returns "number" typeof Math.LN2; // returns "number"
For methods and functions, the typeof
operator returns results as follows:
typeof blur; // returns "function" typeof eval; // returns "function" typeof parseInt; // returns "function" typeof shape.split; // returns "function"
For predefined objects, the typeof
operator returns results as follows:
typeof Date; // returns "function" typeof Function; // returns "function" typeof Math; // returns "object" typeof Option; // returns "function" typeof String; // returns "function"
void
The void
operator is used in either of the following ways:

void (expression)

void expression
The void
operator specifies an expression to be evaluated without returning a value. expression
is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.
You can use the void
operator to specify an expression as a hypertext link. The expression is evaluated but is not loaded in place of the current document.
The following code creates a hypertext link that does nothing when the user clicks it. When the user clicks the link, void(0)
evaluates to undefined, which has no effect in JavaScript.
<A HREF="javascript:void(0)">Click here to do nothing</A>
The following code creates a hypertext link that submits a form when the user clicks it.
<A HREF="javascript:void(document.form.submit())"> Click here to submit</A>
Operator precedence
The precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.
The following table describes the precedence of operators, from highest to lowest.
In accordance with relevant discussion, this table was reversed to list operators in decreasing order of priority.
Operator type  Individual operators 

member  . [] 
call / create instance  () new 
negation/increment  ! ~  + ++  typeof void delete 
multiply/divide  * / % 
addition/subtraction  +  
bitwise shift  << >> >>> 
relational  < <= > >= in instanceof 
equality  == != === !== 
bitwiseand  & 
bitwisexor  ^ 
bitwiseor   
logicaland  && 
logicalor   
conditional  ?: 
assignment  = += = *= /= %= <<= >>= >>>= &= ^= = 
comma  , 
A more detailed version of this table, complete with links to additional details about each operator, may be found in JavaScript Reference.