Table 5.5 Logical operators
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Operator
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Usage
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Description
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&&
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expr1 && expr2
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(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.
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||
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expr1 || expr2
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(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.
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!
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!expr
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(Logical NOT) Returns false if its single operand can be converted to true; otherwise, returns true.
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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:
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-
false && anything is short-circuit evaluated to false.
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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:
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Operator
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Behavior
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&&
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If the first operand (expr1) can be converted to false, the && operator returns false rather than the value of expr1.
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||
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If the first operand (expr1) can be converted to true, the || operator returns true rather than the value of expr1.
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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.
n1=!true // !t returns false
n2=!false // !f returns true
n3=!"Cat" // !t returns false
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".
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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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.
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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Syntax
condition ? expr1 : expr2
Parameters
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condition
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An expression that evaluates to true or false.
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expr1, expr2
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Expressions with values of any type.
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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.
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Implemented in
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JavaScript 1.0
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ECMA version
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ECMA-262
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Syntax
expr1, expr2
Parameters
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expr1, expr2
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Any expressions.
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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.
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Implemented in
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JavaScript 1.2, NES 3.0
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ECMA version
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ECMA-262
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Syntax
delete objectName
delete objectName.property
delete objectName[index]
delete property // legal only within a with statement
Parameters
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objectName
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The name of an object.
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property
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The property to delete.
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index
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An integer representing the array index to delete.
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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.
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Implemented in
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JavaScript 1.5
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Syntax
{var | const} variableName = function(parameters) {functionBody};
Description
The following examples shows how the function operator is used.
This example declares an unnamed function inside an expression. It sets x to a function that returns the square of its argument:
var x = function(y) {return y*y};
The next example declares array a as an array of three functions:
var a = [function(y) {return y}, function y {return y*y}, function (y) [return y*y*y}];
For this array, a[0](5) returns 5, a[1](5) returns 25, and a[2](5) returns 125.
in
The in operator returns true if the specified property is in the specified object.
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Implemented in
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JavaScript 1.4
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Syntax
propNameOrNumber in objectName
Parameters
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propNameOrNumber
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A string or numeric expression representing a property name or array index.
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objectName
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Name of an object.
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Description
The following examples show some uses of the in operator.
// Arrays
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
myString=new String("coral")
"length" in myString // returns true
// Custom objects
mycar = {make:"Honda",model:"Accord",year:1998}
"make" in mycar // returns true
"model" in mycar // returns true
You must specify an object on the right side of the in operator. For example, you can specify a string created with the String constructor, but you cannot specify a string literal.
color1=new String("green")
"length" in color1 // returns true
color2="coral"
"length" in color2 // generates an error (color is not a String object)
Using in with deleted or undefined properties. If you delete a property with the delete operator, the in operator returns false for that property.
mycar = {make:"Honda",model:"Accord",year:1998}
delete mycar.make
"make" in mycar // returns false
trees=new Array("redwood","bay","cedar","oak","maple")
delete trees[3]
3 in trees // returns false
If you set a property to undefined but do not delete it, the in operator returns true for that property.
mycar = {make:"Honda",model:"Accord",year:1998}
mycar.make=undefined
"make" in mycar // returns true
trees=new Array("redwood","bay","cedar","oak","maple")
trees[3]=undefined
3 in trees // returns true
For additional information about using the in operator with deleted array elements, see delete.
instanceof
The instanceof operator returns true if the specified object is of the specified object type.
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Implemented in
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JavaScript 1.4
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Syntax
objectName instanceof objectType
Parameters
