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Operators

Here, the code asks, “Is variable

an instance of the class

String

?” Yes it is, so the result is

“true”

Although not as versatile as

typeof

instanceof

does offer enough help for the cases when

typeof

returns

“object”

Operators

ECMA-262 describes a set of

operators

that can be used to manipulate variables. The operators range

from mathematical operators (such as addition and subtraction) and bitwise operators to relational

operators and equality operators. Any time a native action is performed on a value, it is considered

an operator.

Unary operators

Unary operators take only one parameter: the object or value to operate on. They are the simplest opera-

tors in ECMAScript.

delete

The

delete

operator erases a reference to an object property or method that was previously defined.

Example:

var o = new Object;

o.name = “Nicholas”;

alert(o.name); //outputs “Nicholas”

delete o.name;

alert(o.name); //outputs “undefined”

In this example, the

name

property is deleted, which means that it is forcibly de-referenced and set

undefined

(which you will remember is the same value a variable has when it is created and not

initialized).

The

delete

operator

cannot

be used to delete properties and methods that are not defined by the devel-

oper. For instance, the following line causes an error:

delete o.toString;

Even though

toString

is a valid name of a method, this code line causes an error because the

toString()

method is native to ECMAScript and not developer-defined.

void

The

void

operator returns

undefined

for any value. This is typically used to avoid outputting a value

that shouldn’t be output, such as when calling a JavaScript function from an HTML

<a>

element. To do

this properly, the function cannot return a valid value; otherwise the browser erases the page and dis-

plays only the result of the function. For example:

<a href=”javascript:window.open(‘about:blank’)”>Click Me</a>

ECMAScript Basics

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