The JavaScript Language contains the following built-in objects and functions:
ΓÇóString object ΓÇóMath object ΓÇóDate object ΓÇóeval function
These objects and their properties and methods are built into the language. You can use these objects in both client applications with Netscape Navigator and server applications with LiveWire.
Using the String Object
Whenever you assign a string value to a variable or property, you create a string object. String literals are also string objects. For example, the statement
mystring = "Hello, World!"
creates a string object called mystring. The literal "blah" is also a string object.
The string object has methods that return:
ΓÇóa variation on the string itself, such as substring and toUpperCase. ΓÇómethods that return the string, with HTML formatting, such as bold and link.
For example, given the above object, mystring.toUpperCase() returns "HELLO, WORLD!", and so does "hello, world!".toUpperCase().
More introductory and overview information TBD.
Using the Math Object
The built-in Math object has properties and methods for mathematical constants and functions. For example, the Math object's PI property has the value of pi, which you would use in an application as
Math.PI
Similarly, 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.
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)
}
Using the Date Object
JavaScript does not have a date data type. However, the date object and its methods enable you to work with dates and times in your applications. The date object has a large number of methods for setting, getting, and manipulating dates. It does not have any properties.
JavaScript handles dates very similarly to Java. The two languages have many of the same date methods, and both languages store dates as the number of milliseconds since January 1, 1970 00:00:00.
NOTE: You cannot currently work with dates prior to 1/1/70.
To create a date object:
varName = new Date(parameters)
where varName is a JavaScript variable name for the date object being created; it can be a new object or a property of an existing object.
The parameters for the Date constructor can be any of the following:
ΓÇóNothing: creates today's date and time. For example, today = new Date() ΓÇóA string representing a date in the following form: "Month day, year hours:minutes:seconds". For example, Xmas95= new Date("December 25, 1995 13:30:00") If you omit hours, minutes, or seconds, the value will be set to zero. ΓÇóA set of integer values for year, month, and day. For example, Xmas95 = new Date(95,11,25) ΓÇóA set of values for year, month, day, hour, minute, and seconds For example, Xmas95 = new Date(95,11,25,9,30,0)
The Date object has a large number of methods for handling dates and times. The methods fall into these broad categories:
ΓÇó"set" methods, for setting date and time values in date objects ΓÇó"get" methods, for getting date and time values from date objects ΓÇó"to" methods, for returning string values from date objects. ΓÇóparse and UTC methods, for parsing date strings.
The "get" and "set" methods enable you to get and set seconds, minutes, hours, day of the month, day of the week, months, and years separately. There is a getDay method that returns the day of the week, but no corresponding setDay method, because the day of the week is set automatically. These methods use integers to represent these values as follows:
ΓÇóseconds and minutes: 0 to 59 ΓÇóhours: 0 to 23 ΓÇóday: 0 to 6 (day of the week) ΓÇódate: 1 to 31 (day of the month) ΓÇómonths: 0 (January) to 11 (December) ΓÇóyear: years since 1900
For example, suppose you define the following date:
Xmas95 = new Date("December 25, 1995")
Then Xmas95.getMonth() returns 11, and Xmas95.getYear() returns 95.
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 shopping days left until Christmas:
today = new Date()
nextXmas = new Date("December 25, 1990")
nextXmas.setYear(today.getYear())
msPerDay = 24 * 60 * 60 * 1000 ; // Number of milliseconds per day
document.write("Number of Shopping Days until Christmas: " + daysLeft);
This example creates a date object named today that contains today's date. It then creates a date object named nextXmas, 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 nextXmas, 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 to the IPOdate object.
IPOdate = new Date()
IPOdate.setTime(Date.parse("Aug 9, 1995"))
Using the eval function
The special built-in function eval takes an expression as its argument, evaluates the expression, and returns the value.
This function is useful for evaluating a string representing a numerical expression to a number. For example, input from a form element is always in a string, but you might want to convert it to a numerical value.
The following example takes input in a text field, applies the eval function and displays the result in another text field. If you type a numerical expression in the first field, and click on the button, the expression will be evaluted. For example, enter "(666 * 777) / 3", and click on the button to see the result.
<SCRIPT>
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 compute(obj) { obj.result.value = eval(obj.expr.value) } Enter an expression:
