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JavaScript Programmer’s Reference Off-screen image caching (Useful tip) A technique for caching images locally in readiness for an animation. Refer to: Image preloading offscreenBuffering (Property) An alias for the window.offScreenBuffering property. Availability: JavaScript 1.2 JScript 3.0 Internet Explorer 4.0 Netscape 4.0 Property/method value type: Booleanor String primitive -myWindow.offScreenBuffering -myWindow.offScreenBuffering = aSetting -offScreenBuffering JavaScript syntax: -offScreenBuffering = aSetting Argument list: aSetting A new value to control this functionality Refer to: Window.offScreenBuffering OL object (Object/HTML) An object that represents the ordered list contained in an

tag. Availability: DOM level 1 JavaScript 1.5 JScript 3.0 Internet Explorer 4.0 Netscape 6.0 Inherits from: Element object IE myOL = myDocument.all.anElementID IE myOL = myDocument.all.tags(”OL”)[anIndex] IE myOL = myDocument.all[aName] -myOL = myDocument.getElementById (anElementID) -myOL = myDocument.getElementsByName(aName) [anIndex] JavaScript syntax: -myOL = myDocument.getElementsByTagName (”OL”)[anIndex] 1606

O Off by one errors (Pitfall) Warnings: . Beware when you prefix decimal values with a zero character. You may want to justify a column of figures. If you add leading zero characters to a numeric string, if that string is subsequently parsed back to a numeric value, you may inadvertently export the value as a decimal but import it as an octal value. This can lead to an extremely difficult-to-diagnose fault in your software because the parsers sometimes add some intelligence and will correctly interpret the value as decimal if the characters 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 convert using the numeric parser. The example shows a simple function for doing this. Example code: See also: Decimal value, Hexadecimal value, Integer constant, Number, Number.toString() Cross-references: 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 arrays and strings. . Enumerating through a range of values and testing for equality with the target value rather than testing that you are still less than the target value. This is typically a problem when you build for loops. See also: Array index delimiter ([ ]), Array.slice(), do … while( … ), for( … ) …, Pitfalls, while( … ) … 1605

JavaScript Programmer’s Reference The object space is defined by an extent rectangle that surrounds the space occupied by it on the screen. 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. See also: Deprecated functionality Octal value (Definition) A numeric value based on a radix of 8. Availability: ECMAScript edition 3 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 binary system. 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 over bit and a three digit octal number actually represents a 9 bit value. However, an 8 bit value can be encoded conveniently if the range is limited to 0377 as a maximum. Hexadecimal values map far more conveniently although they are harder to compute mentally. 0 0 0 0 0 0 0 0 0 0 0 0 1604

O OBJECT.useMap (Property) OBJECT.useMap (Property) The URL of a