JavaScript by Example: JavaScript Core Objects. Pt. 1

Basic units of time.

Unit of Time

Milliseconds

1 second

1000

1 minute

second * 60

(1000 * 60)

1 hour

minute * 60

(1000 * 60 * 60)

1 day

hour * 24

(1000 * 60 * 60 * 24 )

1 week

day * 7

(1000 * 60 * 60 * 24 * 7 )

<html><head><title>Countdown 'till Christmas</title></head>

1 var today = new Date();

2 var fullyear = today.getFullYear();

3 var future = new Date("December 25, "+ fullyear);

4 var diff = future.getTime() - today.getTime();

// Number of milliseconds

5 var days = Math.floor(diff / (1000 * 60 * 60 * 24 ));

// Convert to days

6 var str="Only <u>" + days + "</u> shopping days left

\'til Christmas! ";

document.write(str+"<br>");

</script>

</body>

</html>

A new Date object called today is created.

The getFullYear() method returns the year as 2003.
Another Date object called future is created. It will contain the future date, Christmas, passed as its argument.
The difference between the future time and the present time is calculated and returned in milliseconds with the getTime() method.
The Math object is used to round down the result of converting milliseconds to days.
This string contains the number of days between the present date and Christmas. (See Figure 9.15.)


<html><head><title>The Prototype Property</title> <script language = "javascript"> // Customize the Date 1 function weekDay(){ 2 var now = this.getDay(); 3 var names = new Array(7); names[0]="Sunday"; names[1]="Monday"; names[2]="Tuesday"; names[3]="Wednesday"; names[4]="Thursday"; names[5]="Friday"; names[6]="Saturday"; 4 return(names[now]); } 5 Date.prototype.DayOfWeek=weekDay; </script> </head> <body bgcolor="pink"> <font face="arial" size="+1"> <center> <script language="JavaScript"> 6 var today=new Date(); 7 document.write("Today is " + today.DayOfWeek() + ".<br>"); </script> </body></html>

<html><head><title>The Prototype Property</title>

// Customize the Date

1 function weekDay(){

2 var now = this.getDay();

3 var names = new Array(7);

names[0]="Sunday";

names[1]="Monday";

names[2]="Tuesday";

names[3]="Wednesday";

names[4]="Thursday";

names[5]="Friday";

names[6]="Saturday";

4 return(names[now]);

}

5 Date.prototype.DayOfWeek=weekDay;

</script>

</head>

6 var today=new Date();

7 document.write("Today is " + today.DayOfWeek() + ".<br>");

</script>

</body></html>


The function called weekday() is defined.
The variable now is assigned a number representing the day of the week, where 0 is Sunday. A new Array object called names is created. It will contain seven elements. Each element will be assigned the name of the weekday, e.g., "Sunday" . The value in now, a number between 0 and 6, will be used as an index in the names array. If now is 6, then the value of names[6], "Saturday", will be returned . A prototype method called DayOfWeek is assigned the name of the function that defines the method. A new Date object is created with the Date() constructor method . The new prototyped method is called, and returns the string value of today's date, "Saturday". (See Figure 9.16.) The capabilities of the Date object have been extended to provide a method that will return the name of the weekday.

The function called weekday() is defined.

The variable now is assigned a number representing the day of the week, where 0 is Sunday.
A new Array object called names is created. It will contain seven elements. Each element will be assigned the name of the weekday, e.g., "Sunday"
.
The value in now, a number between 0 and 6, will be used as an index in the names array. If now is 6, then the value of names[6], "Saturday", will be returned
.
A prototype method called DayOfWeek is assigned the name of the function that defines the method.
A new Date object is created with the Date() constructor method
.
The new prototyped method is called, and returns the string value of today's date, "Saturday". (See Figure 9.16.) The capabilities of the Date object have been extended to provide a method that will return the name of the weekday.