n addition to LiveWire objects defined for server-side JavaScript and Navigator objects introduced in
n addition to LiveWire objects defined for server-side JavaScript and Navigator objects introduced in Chapter 2, "Using Navigator objects," three additional objects are built in to JavaScript and can be used in either client or server scripts: String, Math, and Date. There are also a handful of built-in functions that can be used in both client and server scripts.
mystring = "Hello, World!"creates a String object called mystring. String literals are also String objects; for example, the literal "Howdy" is a String object. A String object has one predefined property, length, that indicates the number of characters in the string. So, using the previous example, the expression
x = mystring.lengthassigns a value of thirteen to x, because "Hello, World!" has thirteen characters. A String object has two kinds of methods: those that return a variation on the string itself, such as substring and toUpperCase, and those that return an HTML-formatted version of the string, such as bold and link. For example, using the previous example, both
mystring.toUpperCase() and "hello, world!".toUpperCase() return the string "HELLO, WORLD!"
The substring method takes two arguments and returns a subset of the string between the two arguments. Using the previous example, mystring.substring(4, 9) returns the string "o, Wo." For more information, see the reference topic for substring.
The String object also has a number of methods for automatic HTML formatting, such as bold to create boldface text and link to create a hyperlink. For example, you could create a hyperlink to a hypothetical URL with the link method as follows:
mystring.link("http://www.helloworld.com")
Math.PISimilarly, standard mathematical functions are methods of Math. These include trigonometric, logarithmic, exponential, and other functions. For example, if you want to use the trigonometric function sine, you would write
Math.sin(1.56)Note that all trigonometric methods of Math take arguments in radians. The following table summarizes Math's methods.
It is often convenient to use the with statement when a section of code uses several math constants and methods, so you don't have to type "Math" repeatedly. For example,
with (Math) {
a = PI * r*r
y = r*sin(theta)
x = r*cos(theta)
}
Note
Currently, you cannot work with dates prior to January 1, 1970.To create a Date object:
dateObjectName = new Date([parameters])where dateObjectName is the name of the Date object being created; it can be a new object or a property of an existing object. The parameters in the preceding syntax can be any of the following:
today = new Date().
Xmas95 = new Date("December 25, 1995 13:30:00"). If you omit hours, minutes, or seconds, the value will be set to zero.
Xmas95 = new Date(95,11,25). A set of values for year, month, day, hour, minute, and seconds. For example, Xmas95 = new Date(95,11,25,9,30,0).
Xmas95 = new Date("December 25, 1995")
Then Xmas95.getMonth() returns eleven, and Xmas95.getYear() returns ninety-five.
The getTime and setTime methods are useful for comparing dates. The getTime method returns the number of milliseconds since the epoch for a Date object.
For example, the following code displays the number of days left in the current year:
today = new Date()This example creates a Date object named today that contains today's date. It then creates a Date object named endYear and sets the year to the current year. Then, using the number of milliseconds per day, it computes the number of days between today and endYear, using getTime and rounding to a whole number of days. The parse method is useful for assigning values from date strings to existing Date objects. For example, the following code uses parse and setTime to assign a date value to the IPOdate object:
endYear = new Date("December 31, 1990") // Set day and month
endYear.setYear(today.getYear()) // Set year to this year
msPerDay = 24 * 60 * 60 * 1000 // Number of milliseconds per day
daysLeft = (endYear.getTime() - today.getTime()) / msPerDay
daysLeft = Math.round(daysLeft)
document.write("Number of days left in the year: " + daysLeft)
IPOdate = new Date()
IPOdate.setTime(Date.parse("Aug 9, 1995"))
<SCRIPT>The display in Navigator looks like this: The eval function is not limited to evaluating numerical expressions, however. Its argument can include object references or even JavaScript statements. For example, you could define a function called setValue that would take two arguments, an object and a value, as follows:
function compute(obj) {
obj.result.value = eval(obj.expr.value)
}
</SCRIPT>
<FORM NAME="evalform">
Enter an expression: <INPUT TYPE="text" NAME="expr" SIZE=20 >
<BR>
Result: <INPUT TYPE="text" NAME="result" SIZE=20 >
<BR>
<INPUT TYPE="button" VALUE="Click Me" onClick="compute(this.form)">
</FORM>
function setValue (myobj, myvalue) {
eval ("document.forms[0]." + myobj + ".value") = myvalue
}
Then, for example, you could call this function to set the value of a form element "text1" as follows:
setValue(text1, 42)
parseFloat(str)parseFloat parses its argument, the string str, and attempts to return a floating-point number. If it encounters a character other than a sign (+ or -), a numeral (0-9), a decimal point, or an exponent, then it returns the value up to that point and ignores that character and all succeeding characters. If the first character cannot be converted to a number, it returns "NaN" (not a number). The syntax of parseInt is
parseInt(str [, radix])parseInt parses its first argument, the string str, and attempts to return an integer of the specified radix (base), indicated by the second, optional argument, radix. For example, a radix of ten indicates to convert to a decimal number, eight octal, sixteen hexadecimal, and so on. For radixes above ten, the letters of the alphabet indicate numerals greater than nine. For example, for hexadecimal numbers (base 16), A through F are used. If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point. If the first character cannot be converted to a number in the specified radix, it returns "NaN." The parseInt function truncates numbers to integer values.