Core JavaScript Reference 1.5: | 15 | WebReference

Core JavaScript Reference 1.5: | 15

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Core JavaScript Reference 1.5





Number

Lets you work with numeric values. The Number object is an object wrapper for primitive numeric values.


Core object

Implemented in  

JavaScript 1.1, NES 2.0

JavaScript 1.2: modified behavior of Number constructor.

JavaScript 1.3: added toSource method.

JavaScript 1.5, NES 6.0: added toExponential, toFixed, and toPrecision methods.  

ECMA version  

ECMA-262  


Created by
The Number constructor:

new Number(value)


Parameters


value

 

The numeric value of the object being created.  


Description
The primary uses for the Number object are:

The properties of Number are properties of the class itself, not of individual Number objects.

JavaScript 1.2: Number(x) now produces NaN rather than an error if x is a string that does not contain a well-formed numeric literal. For example,

x=Number("three");

document.write(x + "<BR>");

prints NaN

You can convert any object to a number using the top-level Number function.


Property Summary


Property

Description

constructor

 

Specifies the function that creates an object's prototype.  

MAX_VALUE

 

The largest representable number.  

MIN_VALUE

 

The smallest representable number.  

NaN

 

Special "not a number" value.  

NEGATIVE_INFINITY

 

Special value representing negative infinity; returned on overflow.  

POSITIVE_INFINITY

 

Special value representing infinity; returned on overflow.  

prototype

 

Allows the addition of properties to a Number object.  


Method Summary


Method

Description

toExponential

 

Returns a string representing the number in exponential notation.  

toFixed

 

Returns a string representing the number in fixed-point notation.  

toPrecision

 

Returns a string representing the number to a specified precision in fixed-point notation.  

toSource

 

Returns an object literal representing the specified Number object; you can use this value to create a new object. Overrides the Object.toSource method.  

toString

 

Returns a string representing the specified object. Overrides the Object.toString method.  

valueOf

 

Returns the primitive value of the specified object. Overrides the Object.valueOf method.  

In addition, this object inherits the watch and unwatch methods from Object.


Examples
Example 1. The following example uses the Number object's properties to assign values to several numeric variables:

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

Example 2. The following example creates a Number object, myNum, then adds a description property to all Number objects. Then a value is assigned to the myNum object's description property.

myNum = new Number(65);
Number.prototype.description=null;
myNum.description="wind speed";


constructor

Specifies the function that creates an object's prototype. Note that the value of this property is a reference to the function itself, not a string containing the function's name.



Property of  

Number  

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


Description
See Object.constructor.


MAX_VALUE

The maximum numeric value representable in JavaScript.



Property of  

Number  

Static, Read-only

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


Description
The MAX_VALUE property has a value of approximately 1.79E+308. Values larger than MAX_VALUE are represented as "Infinity".

Because MAX_VALUE is a static property of Number, you always use it as Number.MAX_VALUE, rather than as a property of a Number object you created.


Examples
The following code multiplies two numeric values. If the result is less than or equal to MAX_VALUE, the func1 function is called; otherwise, the func2 function is called.

if (num1 * num2 <= Number.MAX_VALUE)
   func1()
else
   func2()


MIN_VALUE

The smallest positive numeric value representable in JavaScript.



Property of  

Number  

Static, Read-only

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


Description
The MIN_VALUE property is the number closest to 0, not the most negative number, that JavaScript can represent.

MIN_VALUE has a value of approximately 5e-324. Values smaller than MIN_VALUE ("underflow values") are converted to 0.

Because MIN_VALUE is a static property of Number, you always use it as Number.MIN_VALUE, rather than as a property of a Number object you created.


Examples
The following code divides two numeric values. If the result is greater than or equal to MIN_VALUE, the func1 function is called; otherwise, the func2 function is called.

if (num1 / num2 >= Number.MIN_VALUE)
   func1()
else
   func2()


NaN

A special value representing Not-A-Number. This value is represented as the unquoted literal NaN.



Property of  

Number  

Read-only  

 

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


Description
JavaScript prints the value Number.NaN as NaN.

NaN is always unequal to any other number, including NaN itself; you cannot check for the not-a-number value by comparing to Number.NaN. Use the isNaN function instead.

You might use the NaN property to indicate an error condition for a function that should return a valid number.


Examples
In the following example, if month has a value greater than 12, it is assigned NaN, and a message is displayed indicating valid values.

var month = 13
if (month < 1 || month > 12) {
   month = Number.NaN
   alert("Month must be between 1 and 12.")
}


See also
NaN, isNaN, parseFloat, parseInt


NEGATIVE_INFINITY

A special numeric value representing negative infinity. This value is represented as the unquoted literal "-Infinity".



Property of  

Number  

Static, Read-only

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


Description
This value behaves slightly differently than mathematical infinity:

Because NEGATIVE_INFINITY is a static property of Number, you always use it as Number.NEGATIVE_INFINITY, rather than as a property of a Number object you created.


Examples
In the following example, the variable smallNumber is assigned a value that is smaller than the minimum value. When the if statement executes, smallNumber has the value "-Infinity", so the func1 function is called.

var smallNumber = -Number.MAX_VALUE*10
if (smallNumber == Number.NEGATIVE_INFINITY)
   func1()
else
   func2()


See also
Infinity, isFinite


POSITIVE_INFINITY

A special numeric value representing infinity. This value is represented as the unquoted literal "Infinity".



Property of  

Number  

Static, Read-only

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


Description
This value behaves slightly differently than mathematical infinity:

Because POSITIVE_INFINITY is a static property of Number, you always use it as Number.POSITIVE_INFINITY, rather than as a property of a Number object you created.


Examples
In the following example, the variable bigNumber is assigned a value that is larger than the maximum value. When the if statement executes, bigNumber has the value "Infinity", so the func1 function is called.

var bigNumber = Number.MAX_VALUE * 10
if (bigNumber == Number.POSITIVE_INFINITY)
   func1()
else
   func2()


See also
Infinity, isFinite


prototype

Represents the prototype for this class. You can use the prototype to add properties or methods to all instances of a class. For information on prototypes, see Function.prototype.



Property of  

Number  

Implemented in  

JavaScript 1.1, NES 2.0  

ECMA version  

ECMA-262  


toExponential

Returns a string representing the Number object in exponential notation.



Method of  

Number  

Implemented in  

JavaScript 1.5  

ECMA version  

ECMA-262, Edition 3  


Syntax
toExponential([fractionDigits])


Parameters



fractionDigits

 

An integer specifying the number of digits after the decimal point. Defaults to as many digits as necessary to specify the number.  


Description
The Number.prototype.toExponential method returns a string representing a Number object in exponential notation with one digit before the decimal point, rounded to fractionDigits digits after the decimal point. If the fractionDigits argument is omitted, the number of digits after the decimal point defaults to the number of digits necessary to represent the value uniquely.

If you use the toExponential method for a numeric literal and the numeric literal has no exponent and no decimal point, leave a space before the dot that precedes the method call to prevent the dot from being interpreted as a decimal point.

If a number has more digits that requested by the fractionDigits parameter, the number is rounded to the nearest number represented by fractionDigits digits. See the discussion of rounding in the description of the toFixed method on page 129, which also applies to toExponential.


Examples
var num=77.1234
alert("num.toExponential() is " + num.toExponential()) //displays 7.71234e+1
alert("num.toExponential(4) is " + num.toExponential(4)) //displays 7.7123e+1   
alert("num.toExponential(2) is " + num.toExponential(2)) //displays 7.71e+1
alert("77.1234.toExponential() is " + 77.1234.toExponential())
//displays 7.71234e+1
alert("77 .toExponential() is " + 77 .toExponential()) //displays 7.7e+1


See also
toFixed, toPrecision, toString


toFixed

Returns a string representing the Number object in fixed-point notation.



Method of  

Number  

Implemented in  

JavaScript 1.5  

ECMA version  

ECMA-262, Edition 3  


Syntax
toFixed([fractionDigits])


Parameters



fractionDigits

 

An integer specifying the number of digits after the decimal point. Defaults to zero.  


Description
The Number.prototype.toFixed method returns a string representing a Number object in fixed-point notation, rounded to the number of digits after the decimal point specified by fractionDigits.

The output of toFixed may be more precise than toString for some values, because toString outputs only enough significant digits to distinguish the number from adjacent number values.

If a number has more digits that requested by the fractionDigits parameter, the number is rounded to the nearest number represented by fractionDigits digits. If the number is exactly halfway between two representable numbers, it is rounded away from zero (up if it is positive, down if it is negative). Thus:

0.124.toFixed(2) returns "0.12".
0.125.tofixed(2) returns "0.13", because 0.125 is exactly halfway between 0.12 and 0.13.
0 .126.tofixed(2) returns "0.13".
Given this convention, one might expect 0.045.toFixed(2) to return "0.05", but it returns "0.04". This is because of the way computers represent IEEE 754 floating-point numbers. The IEEE 754 standard uses binary fractions (fractions of 0's and 1's after the dot). Just as some numbers, such as 1/3, are not representable precisely as decimal fractions, other numbers, such as 0.045, are not precisely representable as binary fractions. The IEEE 754 standard dictates that 0.045 be approximated to 0.04499999999999999833466546306226518936455249786376953125, which is precisely representable as a binary fraction. This approximation is closer to 0.04 than to 0.05, so 0.045.toFixed(2) returns "0.04".


Examples
var num=10.1234
alert("num.toFixed() is " + num.toFixed()) //displays 10
alert("num.toFixed(4) is " + num.toFixed(4)) //displays 10.1234    alert("num.toFixed(2) is " + num.toFixed(2)) //displays 10.12


See also
toExponential, toPrecision, toString


toPrecision

Returns a string representing the Number object to the specified precision.



Method of  

Number  

Implemented in  

JavaScript 1.5  

ECMA version  

ECMA-262, Edition 3  


Syntax
toPrecision([precision])


Parameters



precision

 

An integer specifying the number of digits after the decimal point.  


Description
The Number.prototype.toPrecision method returns a string representing a Number object in fixed-point or exponential notation rounded to precision significant digits.

If you use the toPrecision method for a numeric literal and the numeric literal has no exponent and no decimal point, leave a space before the dot that precedes the method call to prevent the dot from being interpreted as a decimal point.

If the precision argument is omitted, behaves as Number.prototype.toString.

If a number has more digits that requested by the precision parameter, the number is rounded to the nearest number represented by precision digits. See the discussion of rounding in the description of the toFixed method on page 129, which also applies to toPrecision.


Examples
var num=5.123456
alert("num.toPrecision() is " + num.toPrecision()) //displays 5.123456
alert("num.toPrecision(4) is " + num.toPrecision(4)) //displays 5.123
alert("num.toPrecision(2) is " + num.toPrecision(2)) //displays 5.1
alert("num.toPrecision(2) is " + num.toPrecision(1)) //displays 5
alert("num.toPrecision(2) is " + num.toPrecision(1)) //displays 5
alert("1250 .toPrecision() is " + 1250 .toPrecision(2))
//displays 1.3e+3
alert("1250 .toPrecision(5) is " + 1250 .toPrecision(5))
//displays 1250.0


See also
toExponential, toFixed, toString


toSource

Returns a string representing the source code of the object.



Method of  

Number  

Implemented in  

JavaScript 1.3  


Syntax
toSource()


Parameters
None


Description
The toSource method returns the following values:

This method is usually called internally by JavaScript and not explicitly in code.


See also
Object.toSource


toString

Returns a string representing the specified Number object.



Method of  

Number  

Implemented in  

JavaScript 1.1  

ECMA version  

ECMA-262  


Syntax
toString()
toString([radix])


Parameters



radix

 

An integer between 2 and 36 specifying the base to use for representing numeric values.  


Description
The Number object overrides the toString method of the Object object; it does not inherit Object.toString. For Number objects, the toString method returns a string representation of the object.

JavaScript calls the toString method automatically when a number is to be represented as a text value or when a number is referred to in a string concatenation.

If you use the toString method for a numeric literal and the numeric literal has no exponent and no decimal point, leave a space before the dot that precedes the method call to prevent the dot from being interpreted as a decimal point.

For Number objects and values, the built-in toString method returns the string representing the value of the number.

var howMany=10;
alert("howMany.toString() is " + howMany.toString())
alert("45 .toString() is " + 45 .toString())


See also
toExponential, toFixed, toPrecision


valueOf

Returns the primitive value of a Number object.



Method of  

Number  

Implemented in  

JavaScript 1.1  

ECMA version  

ECMA-262  


Syntax
valueOf()


Parameters
None


Description
The valueOf method of Number returns the primitive value of a Number object as a number data type.

This method is usually called internally by JavaScript and not explicitly in code.


Examples
x = new Number();
alert(x.valueOf())      //displays 0


See also
Object.valueOf


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Last Updated September 28, 2000