## Chapter 5 Operators

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.

## 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 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 oncea[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.

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.

The following table describes the comparison operators.

### 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.

#### Backward Compatibility

The behavior of the standard equality operators (== and !=) depends on the JavaScript version.

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 (/).

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 JavaScript1/2 //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:

### Bitwise Logical Operators Implemented in JavaScript 1.0 ECMA version ECMA-262

Conceptually, the bitwise logical operators work as follows:

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:

### 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.

The logical operators are described in the following table.

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:

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:

#### Examples

The following code shows examples of the && (logical AND) operator.

`a1=true && true       // t && t returns truea2=true && false      // t && f returns falsea3=false && true      // f && t returns falsea4=false && (3 == 4)  // f && f returns falsea5="Cat" && "Dog"     // t && t returns Doga6=false && "Cat"     // f && t returns falsea7="Cat" && false     // t && f returns false`
The following code shows examples of the || (logical OR) operator.

`o1=true || true       // t || t returns trueo2=false || true      // f || t returns trueo3=true || false      // t || f returns trueo4=false || (3 == 4)  // f || f returns falseo5="Cat" || "Dog"     // t || t returns Cato6=false || "Cat"     // f || t returns Cato7="Cat" || false     // t || f returns Cat`
The following code shows examples of the ! (logical NOT) operator.

`n1=!true              // !t returns falsen2=!false             // !f returns truen3=!"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"`.

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.

#### 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.

#### 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.

#### Syntax

`delete objectNamedelete objectName.propertydelete 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=42var y= 43myobj=new Number()myobj.h=4      // create property hdelete 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, a is still a and a is undefined.

When the `delete` operator removes an array element, that element is no longer in the array. In the following example, trees is removed with `delete`.

`trees=new Array("redwood","bay","cedar","oak","maple")delete treesif (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 is assigned the value undefined, but the array element still exists:

`trees=new Array("redwood","bay","cedar","oak","maple")trees=undefinedif (3 in trees) {   // this gets executed}`

### new

The new operator creates an instance of a user-defined object type or of one of the built-in object types that has a constructor function.

#### Syntax

`objectName = new objectType (param1 [,param2] ...[,paramN])`

#### Parameters

 `objectName` Name of the new object instance. `objectType` Object type. It must be a function that defines an object type. `param1...paramN` Property values for the object. These properties are parameters defined for the `objectType` function.

#### Description

Creating a user-defined object type requires two steps:

To define an object type, create a function for the object type that specifies its name, properties, and methods. An object can have a property that is itself another object. See the examples below.

You can always add a property to a previously defined object. For example, the statement `car1.color = "black"` adds a property `color` to `car1`, and assigns it a value of `"black"`. However, this does not affect any other objects. To add the new property to all objects of the same type, you must add the property to the definition of the `car` object type.

You can add a property to a previously defined object type by using the `Function.prototype` property. This defines a property that is shared by all objects created with that function, rather than by just one instance of the object type. The following code adds a `color` property to all objects of type `car`, and then assigns a value to the `color` property of the object `car1`. For more information, see `prototype`

`Car.prototype.color=nullcar1.color="black"birthday.description="The day you were born"`

#### Examples

Example 1: Object type and object instance. Suppose you want to create an object type for cars. You want this type of object to be called `car`, and you want it to have properties for make, model, and year. To do this, you would write the following function:

`function car(make, model, year) {   this.make = make   this.model = model   this.year = year}`
Now you can create an object called `mycar` as follows:

`mycar = new car("Eagle", "Talon TSi", 1993)`
This statement creates `mycar` and assigns it the specified values for its properties. Then the value of `mycar.make` is the string `"Eagle"`, `mycar.year` is the integer `1993`, and so on.

You can create any number of `car` objects by calls to `new`. For example,

`kenscar = new car("Nissan", "300ZX", 1992)`
Example 2: Object property that is itself another object. Suppose you define an object called `person` as follows:

`function person(name, age, sex) {   this.name = name   this.age = age   this.sex = sex}`
And then instantiate two new `person` objects as follows:

`rand = new person("Rand McNally", 33, "M")ken = new person("Ken Jones", 39, "M")`
Then you can rewrite the definition of `car` to include an owner property that takes a `person` object, as follows:

`function car(make, model, year, owner) {   this.make = make;    this.model = model;   this.year = year;   this.owner = owner;}`
To instantiate the new objects, you then use the following:

`car1 = new car("Eagle", "Talon TSi", 1993, rand);car2 = new car("Nissan", "300ZX", 1992, ken)`
Instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects `rand` and `ken` as the parameters for the owners. To find out the name of the owner of `car2`, you can access the following property:

`car2.owner.name`

### this

The this keyword refers to the current object. In general, in a method `this` refers to the calling object.

#### Syntax

``this`[.propertyName]`

#### Examples

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)">`

### typeof

The `typeof` operator is used in either of the following ways:

`1. typeof operand2. 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=1var today=new Date()`
The `typeof` operator returns the following results for these variables:

`typeof myFun is objecttypeof shape is stringtypeof size is numbertypeof today is objecttypeof dontExist is undefined`
For the keywords `true` and `null`, the `typeof` operator returns the following results:

`typeof true is booleantypeof null is object`
For a number or string, the `typeof` operator returns the following results:

`typeof 62 is numbertypeof 'Hello world' is string`
For property values, the `typeof` operator returns the type of value the property contains:

`typeof document.lastModified is stringtypeof window.length is numbertypeof Math.LN2 is number`
For methods and functions, the `typeof` operator returns results as follows:

`typeof blur is functiontypeof eval is functiontypeof parseInt is functiontypeof shape.split is function`
For predefined objects, the `typeof` operator returns results as follows:

`typeof Date is functiontypeof Function is functiontypeof Math is functiontypeof Option is functiontypeof String is function`

### void

The void operator is used in either of the following ways:

`1. void (expression)2. 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 0, but that 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>`