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396 . Chapter 20 The setDefault() function works with any type of select object, whether it is a multiple one or not. It simply loops through the options of the select object and sets the selected property of each to its defaultSelected property, reverting all options to their default selection state. The value handed to the function is the select object, referenced as this.form.myMenu. Note: Microsoft Internet Explorer does not fully support the select object. The entire discussion is based on Navigator because it simply does not work for Microsoft s browser. As you can see, Microsoft has focused on the user interface of its browser but did not pay much attention to script-level access via JavaScript. The average web surfer does not feel the disadvantages because web content providers use only features that are supported by both browsers. text There is no HTML attribute that defines the option s label. Take a look at Example 20-11. The strings First option, Second option, and so forth are not specified in any HTML attribute as you could expect. They are simply appended to the definitions. The text property can be set at any time, immediately affecting the display. Example 20-12 demonstrates the use of this property. When the user selects an option from the list, that option is automatically placed at the top of the list, shifting all other options downwards. See the listings for the value property if this is not completely clear.

First option Second option Third option Fourth option Fifth option

Example 20-11. A simple function resets a select object. Chapter
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394 . Chapter 20 CNN MSNBC USA TODAY Example 20-10. Another version of the URL picker this one uses a button. The only difference between Example 20-10 and Example 20-9 is that in Example 20-10 the event handler is attached to a button object, while in Example 20-9 it is attached to a select object. In order to avoid changing the function, we have modified the argument handed to the function to keep passing the select object. The expression this.form references the form and elements[0] references the select object which is the first element in the form. Properties (options Array) An element in the options array reflects a select object s option defined by the tag in HTML. Properties of the options array are properties of specific options in a select object. defaultSelected The defaultSelected property evaluates to a Boolean value. If the specified option is defined with a SELECTED attribute (), the value of defaultSelected is true. Otherwise, it is false. defaultSelected initially reflects whether the SELECTED attribute is used within an tag. Setting the defaultSelected property via a script overrides the initial HTML setting. In a select object without a MULTIPLE specification you can only have one option selected by default. Therefore, setting the defaultSelected property of a given option to true clears any previous default selections, including those set with SELECTED. Nevertheless, if you set defaultSelected in a select object defined with the MULTIPLE attribute, previous default selections are not cleared. index The index property of a single option in a select object is the number identifying the position of the object in the selection list, starting from zero. Under normal circumstances, there is no justification for the existence of the index property, because in order to reference an option, you need to know its index: document.selectObject.options[indexValue] and when you reference the index property, you supposedly know the index already: document.selectObject.options[indexValue].index
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Forms . 393 MSNBCUSATODAY Chapter Example 20-9. A URL picker. When the user selects an option in the select object, the loadPage() function is invoked by the onChange event handler, passing the select object as an argument. The URL associated with each option is stored as the option s VALUE attribute, or value property in terms of JavaScript. The selected option is list.options[list.selectedIndex] because list.selectedIndex is an integer representing the index of the selected option. The value property is used to access the URL of the selected object, which is then assigned to location.href, in order to load that page to the browser window. You may prefer to use a button in place of the onChange event handler:

Figure 20-11. Using the selectedIndex property.
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392 . Chapter 20 (). Suppose the first element of the first form in a document is a select object with three options. These options can be referenced in JavaScript as: document.forms[0].elements[0].options[0] document.forms[0].elements[0].options[1] document.forms[0].elements[0].options[2] As always, the length of the array, which is equal to the number of options in the given select object, is stored in the array s length property. In the preceding examples that would be: document.forms[0].elements[0].options.length Elements of the options array are read-only. Although it does not generate a JavaScript error, assigning a value to any of the elements has no effect. The bare-bones selectObject.options reference evaluates to the full HTML syntax used to create the specified selectObject. Note that you can also access the properties of the select object as if they were direct properties of the options array. selectedIndex The selectedIndex property is an integer specifying the index of the selected option in a select object. Options in a select object are indexed in the order of definition, i.e., in the same order they are entered in the options array. You can also set the value of selectedIndex in a script to immediately update the state (the selected option) of a select object. The selectedIndex property is not useful with multiple select objects because it can only refer to the first selected option in the list. You can work around this problem by using the selected property of the options array and a simple loop. See the listings for this property for further details. Example 20-9 and Figure 20-11 demonstrate the selectedIndex property. Although we have not discussed the value property yet, its role is obvious. If needed, refer to its description later in this chapter. CNN
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Forms . 391 Properties (select Object) The select object features various properties, but the most useful are actually properties of the options array, as you will find out later. In this section we discuss only the properties that belong directly to the select object. length You can access the number of options in a select object through the length property. Since this value is also referenced as the length property of the options array, you have the freedom to choose which length to use. Since length is known to be a stan-20 dard property of all arrays in JavaScript, we personally prefer referencing length via the options array. Netscape Navigator accepts any of the following references to the number of options in the select object: Chapter selectName.lengthselectName.options.lengthselectName.options.options.lengthselectName.options.options.options.length Microsoft Internet Explorer accepts only the first two, but, since the third one does not make any sense, you should avoid using it anyway. The value of the length property should not be assigned by a script. It is dynamic, so it changes whenever an option is added to the corresponding select object. name The name property reflects the NAME attribute of the tag. You can modify this property freely. Adjusting it via a script overrides the initial HTML setting. The following script segment shows how to display the name property ( products ) via an alert dialog box: SonyJVCToshiba options You can reference the options in a select object by the options object. Generally speaking, this array contains an entry for each option ()ina select object
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390 . Chapter 20 Event Handlers onBlur A blur event occurs when a select object loses focus. The onBlur event handler s script executes upon a blur event. There are a few ways to generate a blur event: . Select an option from the list and then click outside the select object, either on the page s body or in another form element. . Select an option from the menu and then send the browser s window to the background. The new window you focus on may be a different browser window or a window associated with any foreign application. This event handler is not very useful, because it relies on the user to remove focus from the object, which is not that intuitive. onFocus A focus event associated with a select object occurs when the user gives it focus; that is, when the user attempts to select an option from the menu but before the menu pops up or modifies the currently selected option. The onFocus event handler responds to a focus event. When the user clicks somewhere inside the select object, you can, for example, pop up an alert box with user instructions for that particular menu. The following source demonstrates this suggestion:

Microsoft Netscape Sun

onChange A change event associated with a select object occurs when the user changes the selected option. Unlike change events associated with other objects, the select object does not have to lose focus in order to generate the event. The onChange event handler executes when a change event occurs. The onChange event handler is commonly used for exchanging information with the user. We will demonstrate this event handler in later examples, when we discuss the select object s methods and properties. Methods blur, focus The only explicit methods of the select object are blur() and focus(). See the listings for the text object at the beginning of this chapter for complete coverage of these methods.
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Forms . 389 The text in the box is not the VALUE attribute specified inside the tag, but is rather the string placed outside the tag. The string has no effect other than being the menu label. The main advantage of this menu is that, due to its pop-up configuration, it does not take up much space on the page. Its disadvantage is that the user can select only one option. To work around this deficiency, you can specify the MULTIPLE attribute, but, since all options are laid out instead of popping up, you lose the space advantage. Here is a simple example: MicrosoftNetscapeSun Chapter You can select multiple options by holding the Shift button down and selecting or deselecting options from the menu. The Control key can also be pressed in order to select each option individually. These keys are commonly used in all major operating systems. The complete syntax for the select object is as follows: textToDisplay[ textToDisplay] You should recognize all attributes except for the event handlers, which are explained later. JavaScript Access There are several ways to reference a select object: [window.]document.formName.selectName[window.]document.formName.elements[index] [window.]document.forms[index].selectName[window.]document.forms[index].elements[index] You can access a specific option in a select object by appending an options property to its reference. This property is actually an array of options starting at index 0.You can reference, for example, the first option of a select object in the following fashion: [window.]document.formName.selectName.options[0] The options array is discussed later as one of the select object s properties.
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388 . Chapter 20 select Object HTML Syntax Scrolling menus are a flexible means of input you can use on your forms. The tag is used to create various types of scrolling menus. This is the common way to enable the user to select an option from a list. The tag is specified by a pair. You should always specify the menu s name in the following fashion: It is preferred to use a name that meets the JavaScript identifier naming conventions. As you might expect, the interior of the section includes the list s options. An option is specified in the following form: optionText For example, the following element creates a simple menu of computer firms: Microsoft Netscape Sun The following image shows two identical menus; the first is in its initial state and the second one is opened by the user: Figure 20-10. A form with two menus, one closed and one open. You can use the SELECT attribute (no value is necessary) to automatically select a default option when the page loads. For example, the following script creates a menu, where the default option (which is also the value that appears highlighted in the box) is Sun : Microsoft Netscape Sun
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Forms . 387 that all radio objects are elements of a unique array document.animation.lamps. The second statement handles the boundary case when the checking loop needs to advance from the last button of the row to the first one (wraparound). The expression current == document.animation.lamps.length 1 evaluates to true when the value of current is equal to the index of the last element in the document.animation.lamps array (the last radio button). In this case, current is set to 0, forcing the first button on the row to be checked next. In all other cases, the value of current is incremented, advancing the checked radio button. Note that when a radio button belonging to a group is clicked, any previously selected button is deselected. The last 20 statement of the function recursively calls itself after a pause of speed milliseconds (speed is a global variable). Properties checked The checked property evaluates to a Boolean value. If its calling radio object is checked (highlighted), the value of checked is true; otherwise, it is false. Due to the structure of a radio object group, the checked property of a single radio object in a group is always true. You can set the value of checked in order to modify the radio object s display. Since all buttons of a group can be deselected, setting a checked property to false simply deselects the calling radio object, without causing any side effects to the other buttons. Checking an empty radio object, however, does deselect any previously highlighted member of the group. See the section on the checkbox object for full coverage of this property. defaultChecked The defaultChecked property reflects the HTML CHECKED attribute. Refer to the section on the checkbox object for further details. name The name property initially reflects the NAME attribute in the HTML definition. See the section on the checkbox object s name property for a complete explanation. value The value property initially corresponds to the VALUE attribute of a radio button HTML tag, but it can be adjusted to any valid string. Once again, refer to the section on the checkbox object for the syntax and a full description. Chapter
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