Numbers and dates

Numbers

Numbers are, well, numbers, and they are implemented in JavaScript as such. All numbers are implemented in JavaScript as IEEE-754 doubles, which is a standard for defining floating-point numbers, allowing for up to 16 significant figures. This, incidentally, means that JavaScript has no integers. It also means that numbers will often behave strangely due to rounding off.

Math Object

The predefined Math object has properties and methods for mathematical constants and functions. For example, the Math object's PI property has the value of pi (3.141...), 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.

The following table summarizes the Math object's methods.

Unlike many other objects, you never create a Math object of your own. You always use the predefined Math object.

Number Object

The Number object has properties for numerical constants, such as maximum value, not-a-number, and infinity. You cannot change the values of these properties and you use them as follows:

var biggestNum = Number.MAX_VALUE;
var smallestNum = Number.MIN_VALUE;
var infiniteNum = Number.POSITIVE_INFINITY;
var negInfiniteNum = Number.NEGATIVE_INFINITY;
var notANum = Number.NaN;

You always refer to a property of the predefined Number object as shown above, and not as a property of a Number object you create yourself.

The following table summarizes the Number object's properties.

The Number prototype provides methods for retrieving information from Number objects in various formats. The following table summarizes the methods of Number.prototype.

Date Object

JavaScript does not have a date data type. However, you can use the Date object and its methods 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 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.

The Date object range is -100,000,000 days to 100,000,000 days relative to 01 January, 1970 UTC.

To create a Date object:

var 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.

Calling Date without the new keyword simply converts the provided date to a string representation.

The parameters in the preceding syntax can be any of the following:

• Nothing: creates today's date and time. For example, today = new Date();.
• A string representing a date in the following form: "Month day, year hours:minutes:seconds." For example, var Xmas95 = new Date("December 25, 1995 13:30:00"). If you omit hours, minutes, or seconds, the value will be set to zero.
• A set of integer values for year, month, and day. For example, var Xmas95 = new Date(1995, 11, 25).
• A set of integer values for year, month, day, hour, minute, and seconds. For example, var Xmas95 = new Date(1995, 11, 25, 9, 30, 0);.

JavaScript 1.2 and earlier
The Date object behaves as follows:

• Dates prior to 1970 are not allowed.
• JavaScript depends on platform-specific date facilities and behavior; the behavior of the Date object varies from platform to platform.

Methods of the Date Object

The Date object methods for handling dates and times fall into these broad categories:

• "set" methods, for setting date and time values in Date objects.
• "get" methods, for getting date and time values from Date objects.
• "to" methods, for returning string values from Date objects.
• parse and UTC methods, for parsing Date strings.

With the "get" and "set" methods you can 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:

• Seconds and minutes: 0 to 59
• Hours: 0 to 23
• Day: 0 (Sunday) to 6 (Saturday)
• Date: 1 to 31 (day of the month)
• Months: 0 (January) to 11 (December)
• Year: years since 1900

For example, suppose you define the following date:

var Xmas95 = new Date("December 25, 1995");

Then Xmas95.getMonth() returns 11, and Xmas95.getFullYear() returns 1995.

The getTime and setTime methods are useful for comparing dates. The getTime method returns the number of milliseconds since January 1, 1970, 00:00:00 for a Date object.

For example, the following code displays the number of days left in the current year:

var today = new Date();
var endYear = new Date(1995, 11, 31, 23, 59, 59, 999); // Set day and month
endYear.setFullYear(today.getFullYear()); // Set year to this year
var msPerDay = 24 * 60 * 60 * 1000; // Number of milliseconds per day
var daysLeft = (endYear.getTime() - today.getTime()) / msPerDay;
var daysLeft = Math.round(daysLeft); //returns days left in the year

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:

var IPOdate = new Date();
IPOdate.setTime(Date.parse("Aug 9, 1995"));

Using the Date Object: an Example

In the following example, the function JSClock() returns the time in the format of a digital clock.

function JSClock() {
  var time = new Date();
  var hour = time.getHours();
  var minute = time.getMinutes();
  var second = time.getSeconds();
  var temp = "" + ((hour > 12) ? hour - 12 : hour);
  if (hour == 0)
    temp = "12";
  temp += ((minute < 10) ? ":0" : ":") + minute;
  temp += ((second < 10) ? ":0" : ":") + second;
  temp += (hour >= 12) ? " P.M." : " A.M.";
  return temp;
}

The JSClock function first creates a new Date object called time; since no arguments are given, time is created with the current date and time. Then calls to the getHours, getMinutes, and getSeconds methods assign the value of the current hour, minute, and second to hour, minute, and second.

The next four statements build a string value based on the time. The first statement creates a variable temp, assigning it a value using a conditional expression; if hour is greater than 12, (hour - 12), otherwise simply hour, unless hour is 0, in which case it becomes 12.

The next statement appends a minute value to temp. If the value of minute is less than 10, the conditional expression adds a string with a preceding zero; otherwise it adds a string with a demarcating colon. Then a statement appends a seconds value to temp in the same way.

Finally, a conditional expression appends "P.M." to temp if hour is 12 or greater; otherwise, it appends "A.M." to temp.