Returns the character at the specified index.

语法

stringName.charAt(index)

stringName is any string or a property of an existing object.
index is any integer from 0 to stringName.length - 1, or a property of an existing object.

用法

string

描述

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character is stringName.length - 1. If the index you supply is out of range, JavaScript returns an empty string.

例子

var anyString="Brave new world"

document.write("The character at index 0 is " + anyString.charAt(0))
document.write("The character at index 1 is " + anyString.charAt(1))
document.write("The character at index 2 is " + anyString.charAt(2))
document.write("The character at index 3 is " + anyString.charAt(3))
document.write("The character at index 4 is " + anyString.charAt(4))

See also

A checkbox on an htm form. A checkbox is a toggle switch that lets the user set a value on or off.

语法

To define a checkbox, use standard htm 语法 with the addition of the onClick event handler:

<INPUT
   TYPE="checkbox"
   NAME="checkboxName"
   VALUE="checkboxValue"
   [CHECKED]
   [onClick="handlerText"]>
   textToDisplay

To use a checkbox object's properties and methods:

1. checkboxName.propertyName
2. checkboxName.methodName(parameters)
3. formName.elements[index].propertyName
4. formName.elements[index].methodName(parameters)

Property of

描述

A checkbox object on a form looks as follows:

A checkbox object is a form element and must be defined within a <FORM> tag.

Use the checked property to specify whether the checkbox is currently checked. Use the defaultChecked property to specify whether the checkbox is checked when the form is loaded.

Properties

Methods

Event handlers

例子

Example 1. The following example displays a group of four checkboxes that all appear checked by default.

Example 2. The following example contains a form with three text boxes and one checkbox. The checkbox lets the user choose whether the text fields are converted to upper case. Each text field has an onChange event handler that converts the field value to upper case if the checkbox is checked. The checkbox has an onClick event handler that converts all fields to upper case when the user checks the checkbox.

See also

A Boolean value specifying the selection state of a checkbox object or radio button.

语法

1. checkboxName.checked
2. radioName[index].checked

checkboxName is either the value of the NAME attribute of a checkbox object or an element in the elements array.
radioName is the value of the NAME attribute of a radio object.
index is an integer representing a radio button in a radio object.

Property of

checkbox, radio

描述

If a checkbox or radio button is selected, the value of its checked property is true; otherwise, it is false.

You can set the checked property at any time. The display of the checkbox or radio button updates immediately when you set the checked property.

例子

The following example examines an array of radio buttons called musicType on the musicForm form to determine which button is selected. The VALUE attribute of the selected button is assigned to the checkedButton variable.

function stateChecker() {
   var checkedButton = ""
   for (var i in document.musicForm.musicType) {
      if (document.musicForm.musicType[i].checked=="1") {
         checkedButton=document.musicForm.musicType[i].value
      }
   }
}

See also

Clears the document in a window.

语法

document.clear()

用法

document

描述

The clear method empties the content of a window, regardless of how the content of the window has been painted.

例子

When the following function is called, the clear method empties the contents of the msgWindow window:

function windowCleaner() {
   msgWindow.document.clear()
   msgWindow.document.close()
}

See also

Cancels a timeout that was set with the setTimeout method.

语法

clearTimeout(timeoutID)

timeoutID is a timeout setting that was returned by a previous call to the setTimeout method.

用法

frame, window

描述

See the 描述 for the setTimeout method.

例子

See the 例子 for the setTimeout method.

See also

Simulates a mouse click on the calling form element.

语法

1. buttonName.click()
2. radioName[index].click()

3. checkboxName.click()

buttonName is either the value of the NAME attribute of a button, reset, or submit object or an element in the elements array.
radioName is the value of the NAME attribute of a radio object or an element in the elements array.
index is an integer representing a radio button in a radio object.
checkboxName is either the value of the NAME attribute of a checkbox object or an element in the elements array.

用法

button, checkbox, radio, reset, submit

描述

The effect of the click method varies according to the calling element:

例子

The following example toggles the selection status of the first radio button in the musicType radio object on the musicForm form:

document.musicForm.musicType[0].click()

The following example toggles the selection status of the newAge checkbox on the musicForm form:

document.musicForm.newAge.click()

Closes an output stream and forces data sent to layout to display.

语法

document.close()

用法

document

描述

The close method closes a stream opened with the document.open() method. If the stream was opened to layout, the close method forces the content of the stream to display. Font style tags, such as <BIG> and <CENTER>, automatically flush a layout stream.

The close method also stops the "meteor shower" in the Netscape icon and displays "Document: Done" in the status bar.

例子

function windowWriter1() {
   var myString = "Hello, world!"
   msgWindow.document.open()
   msgWindow.document.write(myString + "<P>")
   msgWindow.document.close()
}

See also

Closes the specified window.

语法

windowReference.close()

windowReference is a valid way of referring to a window, as described in the window object.

用法

window

描述

The close method closes the specified window. If you call close without specifying a windowReference, JavaScript closes the current window.

In event handlers, you must specify window.close() instead of simply using close(). Due to the scoping of static objects in JavaScript, a call to close() without specifying an object name is equivalent to document.close().

例子

Any of the following 例子 close the current window:

window.close()
self.close()
close()

The following example closes the messageWin window:

messageWin.close()

This example assumes that the window was opened in a manner similar to the following:

messageWin=window.open("")

See also

Displays a Confirm dialog box with the specified message and OK and Cancel buttons.

语法

confirm("message")

message is any string or a property of an existing object.

用法

window

描述

Use the confirm method to ask the user to make a decision that requires either an OK or a Cancel. The message argument specifies a message that prompts the user for the decision. The confirm method returns true if the user chooses OK and false if the user chooses Cancel.

Although confirm is a 用法 the window object, you do not need to specify a windowReference when you call it. For example, windowReference.confirm() is unnecessary.

例子

This example uses the confirm method in the confirmCleanUp() function to confirm that the user of an application really wants to quit. If the user chooses OK, the custom cleanUp() function closes the application.

function confirmCleanUp() {
   if (confirm("Are you sure you want to quit this application?")) {
      cleanUp()
   }
}

<INPUT TYPE="button" VALUE="Quit" onClick="confirmCleanUp()">

See also

String value of a cookie, which is a small piece of information stored by the Navigator in the cookies.txt file.

语法

document.cookie

Property of

document

描述

Use string methods such as substring, charAt, indexOf, and lastIndexOf to determine the value stored in the cookie. See the Netscape cookie specification for a complete specification of the cookie 语法.

You can set the cookie property at any time.

例子

The following function uses the cookie property to record a reminder for users of an application. The "expires=" component sets an expiration date for the cookie, so it persists beyond the current browser session. The format of the date must be

Wdy, DD-Mon-YY HH:MM:SS GMT

This is the same date format as returned by Date.toGMTString, except:

• Dashes are added between the day, month, and year
• Year is two-digit for cookies.

For example, a valid cookie expiration date is:
expires=Wednesday, 09-Nov-99 23:12:40 GMT

function RecordReminder(time, expression) {
   // Record a cookie of the form "@=" to map
   // from  in milliseconds since the epoch,
   //  returned by Date.getTime(), onto an encoded expression,
   //  (encoded to contain no white space, semicolon,
   // or comma characters)
   document.cookie = "@" + time + "=" + expression + ";"
   // set the cookie expiration time to one day
   // beyond the reminder time
   document.cookie += "expires=" + cookieDate(time + 24*60*60*1000)
   // cookieDate is a function that formats the date according to the cookie spec
}

When the user loads the page that contains this function, another function uses indexOf("@") and indexOf("=") to determine the date and time stored in the cookie.

See also

Returns the cosine of a number.

语法

Math.cos(number)

number is a numeric expression representing the size of an angle in radians, or a property of an existing object.

用法

Math

描述

The cos method returns a numeric value between -1 and 1, which represents the cosine of the angle.

例子

//Displays the value 6.123031769111886e-017
document.write("The cosine of PI/2 radians is " + Math.cos(Math.PI/2))

//Displays the value -1
document.write("<P>The cosine of PI radians is " + Math.cos(Math.PI))

//Displays the value 1
document.write("<P>The cosine of 0 radians is " + Math.cos(0))

See also