javascript programmer''''s reference phần 7 ppt

Here is a list of object classes with a note about what sort of object they are and when how theyare managed:Array Native A collection of objects in a sequence Boolean Native A logical v

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be created and destroyed at a level that is more granular than a function body.

There is also scope for a religious debate on indentation Three space characters works great (forme) I don't like tabs because if you move the source code to another editor, the indentation can goawry Space characters for indentation ensure that the source code looks the same in any

monospaced editing window and probably looks OK in a word processor too

See also: Flow control, if( ) , while( )

Object (Definition)

There is a distinct difference between an object and an Object

We refer to the built-in Object class with a capitalised name When referring generically to objects

of other classes, the word object is all lower case Therefore we can have an Object object and aString object Native objects are built-in, host objects are also built in but created outside of theJavaScript core functionality User-defined objects are not covered here

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Here is a list of object classes with a note about what sort of object they are and when how theyare managed:

Array Native A collection of objects in a sequence

Boolean Native A logical value container

Date Native A date value container

Function Native A function code container

Global Built-in A container for global properties, methods, and functions

Image Hosted Web browser image wrapper

Math Built-in A container for math functions

Number Native A numeric value

Object Native A generic object

String Native A sequence of characters

Because you might refer to documents in many ways, possibly by means of object properties or as aproperty belonging to another window, it is not safe to assume that the document property

belonging to the Global object the script is attached to is always the document object you aretrying to access Because of this, the object references in the syntax examples assume the object isbeing referred to via a variable called myDocument or myObject etc For example, the value

myDocument is shown being assigned as a variable from the many alternative sources from whichyou can obtain a document object reference

Object (Type)

A native built-in type

Availability: ECMAScript edition – 2

An Object is an unordered collection of properties Each property consists of a name, a value and aset of attributes

See also: Alias, Data Type, Definition, Internal Method, Internal Property, Object object,

Property, Property attribute, Type

Cross-references:

ECMA 262 edition 2 – section 8.6

Wrox Instant JavaScript – page 28

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O – Object constant (Definition)

Object constant (Definition)

See also: Array(), Boolean(), Closure(), Constant expression, Date(),

Function(), Global object, Math object, Number(), Objectliteral, Object()

Cross-references:

Object inspector (Useful tip)

A debugging tool for inspecting object properties and classes

Here is a small debugging utility that breaks an object down and displays some of its properties

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O – Object literal (Definition)

Object literal (Definition)

An object initialiser that creates the object as well

Property/method value type: Object object

JavaScript version 1.2 introduces Object literals

The object is created and returned by the expression This would normally be assigned to a

variable, which effectively names the object It isn't the object class but it can be copied Its class isstill "Object"

Object literals can be nested so that the properties of the topmost object can in fact be object

// Create a simple object literal

var simple = { prop:100 };

// Create a nested object literal

var nested = { reference: { prop:100 } };

// Create a nested object literal with expression derived value

var evaluated = { reference: { prop:(Math.random()*100) } };

See also: Object constant, Object.constructor

Cross-references:

ECMA 262 edition 3 – section – 11.1.5

O'Reilly JavaScript Definitive Guide – page – 45

Object model (Definition)

There are several different object models that are realized in JavaScript implementations

In the Netscape and MSIE web browsers, the object models are provided as representations of thedocument, the browser, event capturing mechanisms, and the style sheet In addition, some

implementations model the environs, the operating system, and the file system Each of these objectmodels interacts with the others and is a way of representing the tangible real-world objects

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Although these are generally arranged in a tree-like structure, there are many short cut referencesthat mean you can refer to the same object in a variety of ways For example the Netscape

JavaPackage object can be referred to with the following properties in a Netscape browser:

Object object (Object/core)

An object of the class "Object"

JavaScript – 1.1JScript – 3.0Internet Explorer – 4.0Netscape – 3.0Netscape Enterprise Server 2.0Opera 3.0

JavaScript syntax:

Object properties: parent , proto , constructor, name, prototype

Object methods: assign(), eval(), hasOwnProperty(), isPrototypeOf(),propertyIsEnumerable(), toLocaleString(),

toSource(), toString(), unwatch(), valueOf(),watch()

An instance of the class "Object" is created by using the new operator on the Object()

constructor The new object adopts the behavior of the built-in prototype object through theprototype-inheritance mechanisms

All properties and methods of the prototype are available as if they were part of the instance

An Object is a member of the type Object It is an unordered collection of properties, each ofwhich may contain a primitive value, another object, or a function

The constructor is invoked by the new operator or by calling it as a Constructor function For example:

new String("Some Text");

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O – Object object (Object/core)

This will create a new object of the String type You can invoke the constructor without the newoperator but the consequences will depend on the constructor as to what it will yield as a result Inthe case of the String data type, the constructor could be invoked like this:

String("Some Text");

However, you would get a primitive string as a result from this and not a String object JavaScript

is somewhat forgiving and you may not notice this happening until later on when it becomesimportant that you have a String object and not a simple string

Because this object is the topmost parent object in the prototype inheritance hierarchy, all otherobject classes inherit its methods and properties However, in some cases they will get overridden

constructors Each constructor has a Prototype property that is used to facilitate inheritance based

on the prototype It also provides for shared properties, which is similar to but not the same as theClass properties that you find in true object-oriented languages

Externally, the objects in JavaScript exhibit most of the attributes of a class based object orientedsystem and some commentators argue that this qualifies JavaScript as being a genuine objectoriented system However I think the following points declassify it as a truly object orientedsystem, meaning that it is an "object like" system:

❑ Global variables and the scope chain mechanism

❑ Prototype based inheritance

❑ Creation of multiple objects and calling them within a single script

❑ Object data is not truly private

It’s a close enough call that JavaScript 2.0 may well move it into the class-based object-orientedcategory at which time the prototype inheritance would be replaced with super-class/sub-classmechanisms and the arguments become null and void

Warnings:

❑ Be very careful not to confuse this generic top-level core object with the object that MSIE instantiates

to represent an <OBJECT> tag MSIE creates OBJECT objects for that purpose but also supportsObject objects For this reason, it may be the case that interpreters cannot become case-insensitivewhen matching class names If they did, then it would be impossible to distinguish between Objectand OBJECT class names

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See also: Aggregate type, Array object, Boolean object, Date object, delete, Function

object, Math object, Native object, Number object, Object, OBJECT object,Object(), Object(), Object.Class, Object.prototype, String object,userDefined object

Deprecatedeval() 1.1 + 3.0 + 3.0 + 4.0 + 3.0 + 2.0 + - Warning,

DeprecatedhasOwn

unwatch() 1.2 + - 4.0 + - - 3.0 + - WarningvalueOf() 1.1 + 3.0 + 3.0 + 4.0 + 3.0 + 2.0 + 2 + -

Cross-references:

ECMA 262 edition 2 – section 4.2.1

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O – Object() (Constructor)

O'Reilly JavaScript Definitive Guide – page 44

Object() (Constructor)

An Object object constructor

JavaScript – 1.1JScript – 3.0Internet Explorer – 4.0Netscape – 3.0Netscape Enterprise Server 2.0Opera 3.0

When Object is called as part of a new expression, it is a constructor that may create an object instance.There are limitations what is sensible for the new operator to be able to do with the Object

constructor Since the Object is considered to be the highest ancestor of all objects in the prototypeinheritance chain, you cannot logically have more than one Object object Passing other nativeobjects to the Object Constructor implies a type casting from their native object type to theObject type That's not logical either The main use of the Object Constructor then is to

manufacturer object instantiations from non-object data types

The table summarizes the resulting values from using the Object() constructor with the new operator

No argument Creates a new empty object

null Creates a new empty object

undefined Creates a new empty object

Boolean Create a new boolean object whose default value is the input valueNumber Create a new number object whose default value is the input valueString Create a new string object whose default value is the input valueNative object Return the native object itself

Host object Host implementation dependant behavior Objects are cloned if

necessary but some may not be

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Unless you assign the result of the new operation, an object will simply consume memory Youneed to store a reference to it at the time it is instantiated You can do this by assigning it to avariable or a property of another object, passing it in to a function and making sure it gets retained

in there, or storing it as an element in an array

Warnings:

❑ You can refer to objects without the parentheses but then you are not referring to a constructorfunction but to the object itself The behavior varies between browsers and depends on the kind ofobject being instantiated and probably depends on whether it is a wrapper for a primitive data type

or a more complex aggregated type

❑ These all appear to create the same kind of object in both MSIE and Netscape

myObject = Object();

myNewObject = new Object();

myOtherObject = new Object;

These do not:

myBoolean = Boolean();

myNewBoolean = new Boolean();

myOtherBoolean = new Boolean;

❑ In the case of the Boolean, Number, and String object types, only the first form will initialize the newobject with a value Placing the result of these examples in document.write() statements

illustrates the behavior You may want to examine the objects returned in more detail by developing

an object inspector script

See also: Boolean(), Constructor function, constructor property, Garbage collection, Global

object, Memory leak, new, Number(), Object constant, Object object,Object.prototype, Reference counting, String()

Cross-references:

ECMA 262 edition 2 – section 15.1.3.1

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O – Object() (Function)

Object() (Function)

An Object object constructor

JavaScript – 1.1JScript – 1.0Internet Explorer – 3.02Netscape – 3.0

Property/method value type: An object of a type that depends on the passed in argument

Argument list: aValue A value to be stored in the new object

The Object Constructor can be called as a function When this happens, the value passed in

undergoes a type conversion

In an ECMA-compliant implementation, the Object constructor function uses the ToObjectconversion However it handles input values undefined and null as special cases and creates anew object as if the constructor had been used with the new operator

The table summarizes the results based on the input value data types

See also: Cast operator, Constructor function, constructor property, Implicit

conversion, Object object

Cross-references:

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Object. parent (Property)

A special property in which to access the scope chain during function execution

Netscape – 4.0

Property/method value type: ScopeChain object

Object. proto (Property)

A special property in which to access the prototype inheritance chain during construction

Netscape – 4.0

Property/method value type: Function object

See also: Lexical scoping, Prototype Based Inheritance, Prototype chain,

Warnings:

❑ This method is deprecated in favor of the Object.watch() and Object.unwatch() methods

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O – Object.Class (Property/internal)

Object.Class (Property/internal)

Internal property that returns an object class

This is an internal property that describes the class that an Object object instance is a member of.The reserved words suggest that in the future, this property may be externalized

Property attributes:

DontEnum, Internal

Cross-references:

ECMA 262 edition 2 – section – 15.2.2.1

Object.constructor (Property)

A reference to a constructor object

Opera browser – 3.0

The initial value of the Object.prototype.constructor is the built-in Object constructor.You can use this as one way of creating objects although it is more popular to use the new

Object() technique

This property is useful if you have an object that you want to clone but you don't know what sort

of object it is Simply access the constructor belonging to the object you have a reference to

Netscape provides constructors for many objects, virtually all of them in fact, even when it ishighly inappropriate to do so MSIE is far more selective and there are some occasions when youmight wish for a constructor that MSIE does not make available

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Object.eval() (Method)

Evaluate the JavaScript source text passed in a string argument

JScript – 3.0Internet Explorer – 4.0Netscape – 3.0Netscape Enterprise Server version – 2.0Opera browser – 3.0

Deprecated

Property/method value type: Function result

A method that can be used to test whether a property exists and belongs to the receiving object

Property/method value type: Boolean primitive

For this method to yield a Boolean true value, the property named in the argument must exist andmust belong to the receiving object If the property is inherited from a prototype or earlier ancestorthen this method returns false

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O – Object.isPrototypeOf() (Method)

This method would be useful if it could test for the existence of a property in the inheritance chain.There is an internal HasProperty() method but the ECMA standard indicate that it is not

exposed to the script interface

Cross-references:

Object.isPrototypeOf() (Method)

A test for the relationship between two objects to ascertain direct parentage

Argument list: anObject The object whose prototype is to be tested

The receiving object is tested for identity against the object referred to by the prototype property ofthe object passed as an argument If the object in the argument is a direct child object through theprototype chain, then this method returns a true value

Property/method value type: String primitive

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Objects are identified either by the NAME="… " HTML tag attribute or by the ID="… " HTMLtag attribute.

Netscape shows a marginal preference for the name property while MSIE seems slightly betterdisposed towards the ID property However in many cases, both browsers support either techniqueand in some cases will locate items named with either tag as if they existed in a single namespace

Object.propertyIsEnumerable() (Method)

A test for whether a property has the don't enumerate flag set or not

Argument list: aName The name of the object property to test

If the receiving object has a member property of the name that is passed in the argument, and if theDontEnum attribute of that property is false, then this method returns the Boolean true value

Cross-references:

Object.prototype (Property)

The prototype for the Object object, which can be used to extend the interface for all Object objects

JavaScript – 1.1JScript – 3.0Internet Explorer – 4.0Netscape – 3.0Opera browser – 3.0

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The prototype property for the Object prototype object is null.

The example demonstrates how to provide extensions to all instances of this class by adding afunction to the prototype object

// Create a new object

myObject = new Object();

See also: Arguments object, Function.arguments[], JellyScript, Object

object, Object(), Object.constructor, Object.toString(),Object.valueOf(), prototype property

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ReadOnly, DontDelete, DontEnum

Cross-references:

The locale context supplies some special conversion rules for strings Depending on the locale, thismight include special characters or a means of using double byte characters It may also affect thedirection of the text, for certain Asian locales for example

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O – Object.toString() (Method)

This is an alternative way to deliver a string version of an object's internal values In this case, it isformatted as an Object literal and can then be used in an eval() function to assign another object.The exact format of what you see depends on the object being examined

The result of calling this method is string version of the object formatted as an Object literal

Warnings:

❑ Note that this is not available in the MSIE browser but can be useful when constructing an Objectinspector for use in Netscape

Object.toString() (Method)

Return a string primitive version of an object

JavaScript – 1.1JScript – 3.0Internet Explorer – 4.0Netscape – 3.0Opera browser – 3.0

When the toString() method of an Object.prototype is invoked, the class name of the object

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Object.unwatch() (Method)

A method to disable a watch that was set up on a property change

Availability: JavaScript – 1.2

Netscape – 4.0Netscape Enterprise Server – 3.0

This is inherited by most object classes in Netscape

The primitive numeric value of the object

JavaScript – 1.1JScript – 3.0Internet Explorer – 4.0Netscape – 3.0Netscape Enterprise Server – 2.0Opera – 3.0

Property/method value type: Depends on the object value

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O – Object.watch() (Method)

As a general rule, the valueOf() method for an object simply returns the this property of theobject itself However, the object may be a wrapper for a host object some kind It may thereforehave been created by invoking the Object constructor In that case, the host object should be

returned in an ECMA-compliant implementation

Implementations may choose to return the this property of an object or some other value if they choose.The result of this method will be implementation-and object-dependant The native core objects arewell defined and will return predictable value types Generally these will be defined by ECMA orW3C standards It is up to the hosting environment to provide the valueOf() interface to its ownsuite of objects

See also: Cast operator, Object.prototype, valueOf()

Cross-references:

Object.watch() (Method)

A means of establishing a call back when a property value changes

Availability: JavaScript – 1.2

Netscape – 4.0Netscape Enterprise Server version – 3.0

This is inherited by most object classes in Netscape

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Object property delimiter (.) (Delimiter)

A token to delimit object properties from their object

JavaScript – 1.0JScript – 1.0Internet Explorer – 3.0Netscape – 2.0Netscape Enterprise Server version – 2.0Opera browser – 3.0

- myObject.aProperty.aProperty

Argument list: aProperty The identifier name of property to be accessed

The dot delimits properties and objects It can find properties of properties of objects too

The associativity is left to right

Refer to the Operator Precedence topic for details of execution order

You can also access the property values as if the object were an array This:

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O – OBJECT object (Object/HTML)

OBJECT object (Object/HTML)

This is an object that encapsulates an ActiveX plugin Do not confuse it with the Object object that

is the super-class of all objects in JavaScript

Availability: DOM level – 1

Inherits from: Element object

HTML syntax: <OBJECT> </OBJECT>

anIndex A reference to an element in a collection

aName An associative array reference

Argument list:

anElementID The ID value of an Element object

Object properties: accessKey, align, altHtml, archive, border, classid,code, codeBase, codeType, data, dataFld, dataSrc,

declare, form, height, hspace, name, object, readyState,standby, tabIndex, type, useMap, vspace, width

Event handlers: onAfterUpdate, onBeforeUpdate, onBlur, onClick,

onDataAvailable, onDataSetChanged, onDataSetComplete,onDblClick, onDragStart, onError, onErrorUpdate,onFilterChange, onFocus, onHelp, onKeyDown, onKeyPress,onKeyUp, onMouseDown, onMouseMove, onMouseOut,

onMouseOver, onMouseUp, onReadyStateChange, onRowEnter,onRowExit, onSelectStart

This is an object representing an <OBJECT> HTML tag

The <OBJECT> tag is a block-level tag That means that it forces a line break before and after itself.This object is specific to the MSIE browser when it runs on the Windows operating system Noother browser supports ActiveX as well as MSIE and no other operating system properly or

completely supports the ActiveX infrastructure

The events handled, and the properties and the methods of this object will depend on the kind ofActiveX object that is created

The DOM level 1 specification refers to this as an ObjectElement object

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-O – -OBJECT object (-Object/HTML)

-onBlur 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - - WarningonClick 1.5+ 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + Warning

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OBJECT.align (Property)

An alignment control for an <OBJECT> tag's position with respect to its parent object

The alignment of the ActiveX OBJECT object with respect to its containing parent object is defined

in this property The expected and widely available set of alignment specifiers are available:

The alternative HTML is used in case the base <OBJECT> tag experiences a problem when loading

or the browser cannot use ActiveX objects as embeds Of course if this property is accessible fromthe OBJECT object during scripting, the browser must have parsed the <OBJECT> tag, although itmay still have had problems with the component

This HTML is enclosed between the <OBJECT> and </OBJECT> tags in the HTML document source

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O – OBJECT.archive (Property)

Certain tags are likely to be omitted from the altHTML property value <OBJECT> blocks contain

<PARAMETER> tags for passing values to the embedded ActiveX component Clearly you won'twant these appearing in the display if the component fails to load The <PARAMETER> tags areconsidered integral to the <OBJECT> and it’s smart enough to disregard them as it constructs itsalternative HTML block

Browsers that cannot understand and render <OBJECT> tags should also ignore the <PARAMETER>tags too

This is a new attribute of the DOM HTMLObjectElement but is shown here as this is the existingobject type it is to be added to

OBJECT.border (Property)

The width of the border around the object when it is rendered into the display

This property can be set from script and although its type is a String primitive, it will expect anumeric value JavaScript will coerce as necessary during the assignment

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OBJECT.classid (Property)

The URL that locates the registered ActiveX control within the local file system when MSIE is used

on the Windows platform

Internet Explorer – 4.0

This is a special URL value used to locate ActiveX objects within the file system of the PC runningthe client browser This is likely only available in Windows versions of the MSIE browser becausethat is the only platform that supports ActiveX objects It is not supported on the Macintosh version

of MSIE because ActiveX objects aren't available there This is because they are compiled x86 code and therefore cannot run in a non-Intel environment (unless the x86 CPU is being emulated).The ActiveX control needs to have been registered and installed already It is possible to construct

micro-an <OBJECT> tag that conveys sufficient information to locate and install a missing ActiveX controlbut this can be a quite involved process

ReadOnly

OBJECT.code (Property)

The name of a Java applet to be used with the <OBJECT> tag

This specifies the main code class to be loaded when the object is instantiated This value is added

to the codebase property to form a fully qualified URL

There is conflicting information in the reference sources regarding the read/write ability of thisproperty Some suggest it is ReadOnly and others suggest you can assign a value to it It may bethat you can assign a value to it without the JavaScript interpreter complaining but that any valueyou assign is ignored

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The codebase is the path to the directory where the classes specified in the code property arelocated The actual path to the required files is generated by a string concatenation of

codebase+code to generate a fully specified URL

Due to security limitations it is not permitted to access a codebase value that is outside the domainspecified by the containing document

There is conflicting information in the reference sources regarding the read/write ability of thisproperty Some suggest it is ReadOnly and others suggest you can assign a value to it It may bethat you can assign a value to it without the JavaScript interpreter complaining but that any valueyou assign is ignored

OBJECT.codeType (Property)

A description of the type of code in the object referred to by the CLASSID="… " HTML tag attribute

This is a MIME type value that describes the kind of code being embedded by the <OBJECT> tag.There is conflicting information in the reference sources regarding the read/write ability of thisproperty Some suggest it is ReadOnly and other suggest you can assign a value to it It may bethat you can assign a value to it without the JavaScript interpreter complaining but that any valueyou assign is ignored

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OBJECT.data (Property)

A URL that points at a file containing data that the OBJECT element can access

This is intended for passing in a URL that the ActiveX object can use to access some data servicethat is online and available for access via the network It is not the URL of the ActiveX object itself

ReadOnly

OBJECT.declare (Property)

A means of defining the object without activating it

JavaScript – 1.5Netscape – 6.0

Declaring an OBJECT in this way may be useful when referring to the object from elsewhere in thepage or from within another object Sometimes you simply want to know something about it,perhaps one of its parameters For video players, sometimes its useful to instantiate the OBJECTinto the display without playing the video right away

OBJECT.form (Property)

The form that an object belongs to if it is used for form input

Property/method value type: Form object

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O – OBJECT.height (Property)

ReadOnly

OBJECT.height (Property)

The height of an area reserved for displaying the contents of the <OBJECT> tag

Property/method value type: Number primitive

The object space is defined by an extent rectangle that surrounds the space occupied by it on thescreen An extent rectangle is the smallest rectangle that completely encloses the item This

property specifies the height of that extent rectangle

OBJECT.hspace (Property)

A horizontal margin space either side of the <OBJECT> tag with respect to its surrounding objects

Margins placed around objects are either modified separately with all four margin sides having adifferent property or by adjusting the horizontal margins and vertical margins using just two values.The hspace property controls the margin to the left and right of the object

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OBJECT.name (Property)

The value of the NAME="… " HTML tag attribute

Objects are identified either by the NAME="… " HTML tag attribute or by the ID="… "

HTML tag attribute

Netscape shows a marginal preference for the name property while MSIE seems slightly betterdisposed towards the ID property However in many cases, both browsers support either techniqueand in some cases will locate items named with either tag as if they existed in a single namespace

See also: NAME="… ", Namespace

If a property is a public property of the ActiveX object, and that name coincides with a property ofthe JavaScript object that is instantiated by the <OBJECT> HTML tag, then access to the propertybelonging to the containing object is difficult This is because the search order will see the publicproperty of the ActiveX object first By using the object property, once can access the containingobject explicitly and retrieve a property of that object even if there is an identically named propertybelonging to the enclosed ActiveX object

This access mechanism applies to method invocations as well

ReadOnly

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This property reflects the loading status of an <OBJECT> tag and its corresponding OBJECT objectinstantiation

Sometimes, you can design scripts to execute while the document is downloading Inline scripts forexample At that time, you may even be able to trigger interval timed deferred executions as well

If it is important that the document has completed loading, you can check this property for one ofthe following values:

uninitialized The object is first instantiated but has not begun loading

loading The object has commenced loading

loaded The object has completed loading

interactive The object is loaded but not yet closed but is ready to handle

interaction

complete The object body has been closed and the loading is finished

An object may not need to reflect the complete status before you can commence operating on it Otherobjects may require that they are completely loaded For example, you cannot create an OBJECTobject that represents an <OBJECT> tag until the <BODY> has completed loading This is because theActiveX object construction requires a complete document body structure to attach itself to

Every time this readyState value changes, it triggers an onReadyStateChange event call-back

ReadOnly

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OBJECT.standby (Property)

Sets or resets the message text displayed while the object is loading

OBJECT.tabIndex (Property)

A control of where the OBJECT object appears in the tabbing order of the page

This value indicates where in the tabbing sequence this object and any of its children will beplaced The tabbing order is used when filling in forms or moving focus Pressing the [tab] keymoves from one form element to the next according to the cascaded tabbing order defined bybuilding a tree-like structure with the tab index values

OBJECT.type (Property)

An indication of the MIME type of the object if its codeType property is undefined

The MIME type of the object is accessible through the value of this property

Refer to the MIME type topic for details of the available MIME types you will likely see in this property

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O – OBJECT.useMap (Property)

OBJECT.useMap (Property)

The URL of a <MAP> defined hash element that defines a client-side image map

This property reflects the value of the USEMAP="… " HTML tag attribute, which should refer to thenamed <MAP> tag containing an image map The reference is by means of a "#NAME" value in thisproperty that corresponds to the NAME="… " HTML tag attribute of the <MAP> tag describing theimage map to use with the object

OBJECT.vspace (Property)

A vertical spacing above and below the <OBJECT> with respect to its adjacent objects

Margins placed around objects are either modified separately with all four margin sides having adifferent property or by adjusting the horizontal margins and vertical margins using just two values.The vspace property controls the margin at the top and bottom of the object

OBJECT.width (Property)

The height of an area reserved for displaying the contents of the <OBJECT> tag

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The object space is defined by an extent rectangle that surrounds the space occupied by it on thescreen An extent rectangle is the smallest rectangle that completely encloses the item This

property specifies the width of that extent rectangle

Obsolescent (Definition)

A feature of the language that is no longer supported

Warnings:

❑ If you use obsolescent functionality, your script may fail when it is deployed on other platforms

Octal value (Definition)

A numeric value based on a radix of 8

An octal value is an integer composed of only the following characters:

0 1 2 3 4 5 6 7

Octal values are always prefixed by a zero character

The sequence carries over for the next increment when each column reaches the value 7 Thus:

00 01 02 03 04 05 06 07 010 011 012

Octal values have a historical significance from having been used in the earliest computer systems.However these days, they are particularly useful since they map quite conveniently to the binarysystem Each octal digit corresponds to three binary digits

The most significant of the three octal digits does not have a full range since it contains a carry overbit and a three digit octal number actually represents a 9 bit value However, an 8 bit value can beencoded conveniently if the range is limited to 0377 as a maximum Hexadecimal values map farmore conveniently although they are harder to compute mentally

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O – Off by one errors (Pitfall)

Warnings:

❑ Beware when you prefix decimal values with a zero character You may want to justify a column offigures If you add leading zero characters to a numeric string, if that string is subsequently parsedback to a numeric value, you may inadvertently export the value as a decimal but import it as anoctal value This can lead to an extremely difficult-to-diagnose fault in your software because theparsers sometimes add some intelligence and will correctly interpret the value as decimal if thecharacters 8 or 9 are present, but otherwise interpret it as octal notation

❑ This may be implementation-dependant behavior to some extent

❑ Be careful that you remove any leading zero characters from the text strings that you plan to convertusing the numeric parser The example shows a simple function for doing this

ECMA 262 edition 3 – section – B.1

O'Reilly JavaScript Definitive Guide – page – 35

Off by one errors (Pitfall)

An error caused by missing the target value by one

This kind of errors are caused by the following:

❑ Forgetting than an index is zero-based and assuming it begins at 1 This typically affects arraysand strings

❑ Enumerating through a range of values and testing for equality with the target value rather thantesting that you are still less than the target value This is typically a problem when you build forloops

See also: Array index delimiter ([ ]), Array.slice(), do while( ), for(

) , Pitfalls, while( )

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Off-screen image caching (Useful tip)

A technique for caching images locally in readiness for an animation

Property/method value type: Boolean or String primitive

An object that represents the ordered list contained in an <OL> tag

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O – OL object (Object/HTML)

HTML syntax: <OL> … </OL>

anIndex A reference to an element in a collection

aName An associative array reference

Argument list:

anElementID The ID value of an Element object

Object properties: compact, start, type

Event handlers: onClick, onDblClick, onDragStart, onFilterChange,onHelp, onKeyDown, onKeyPress, onKeyUp, onMouseDown,

onMouseMove, onMouseOut, onMouseOver, onMouseUp,onSelectStart

The <OL> tag is a block-level tag That means that it forces a line break before and after itself.The DOM level 1 standard describes this as a HTMLOListElement object

See also: Element object, UL object

onKeyDown 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + WarningonKeyPress 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + WarningonKeyUp 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + WarningonMouseDown 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + WarningonMouseMove 1.5 + 3.0 + 6.0 + 4.0 + - - 4.0 + WarningonMouseOut 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + WarningonMouseOver 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + WarningonMouseUp 1.5 + 3.0 + 6.0 + 4.0 + 3.0 + - 4.0 + Warning

-Inheritance chain:

Element object, Node object

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OL.compact (Property)

A switching attribute that condenses the space required to display the ordered list on the screen

The collection of LI objects are presented in the normal spaced out style when the compact

property belonging to their owner OL object is set to false

Setting the property to true should result in the list items being squeezed closer together howeverthe functionality is rarely supported on web browsers

Its more likely that you'll apply CSS style attributes to the list to achieve the same effect

Ordered lists can start at any value This is somewhat related to the LI.value property, whichallows the list items to begin sequencing from any value you care to define The value you specifyhere must be a positive integer

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