JavaScript对象之Boolean

Boolean

The Boolean object is an object wrapper for a boolean value.

Created by
The Boolean constructor:

new Boolean(value)

Parameters

Description
Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object.

Any object whose value is not undefined or null, including a Boolean object whose value is false, evaluates to true when passed to a conditional statement. For example, the condition in the following if statement evaluates to true:

x = new Boolean(false);
if(x) //the condition is true

This behavior does not apply to Boolean primitives. For example, the condition in the following if statement evaluates to false:

x = false;
if(x) //the condition is false

Do not use a Boolean object to convert a non-boolean value to a boolean value. Instead, use Boolean as a function to perform this task:

x = Boolean(expression) //preferred
x = new Boolean(expression) //don't use

If you specify any object, including a Boolean object whose value is false, as the initial value of a Boolean object, the new Boolean object has a value of true.

myFalse=new Boolean(false)   // initial value of false
g=new Boolean(myFalse)       //initial value of true
myString=new String("Hello") // string object
s=new Boolean(myString)      //initial value of true

Do not use a Boolean object in place of a Boolean primitive.

Backward Compatibility

JavaScript 1.2 and earlier versions. The Boolean object behaves as follows:

• When a Boolean object is used as the condition in a conditional test, JavaScript returns the value of the Boolean object. For example, a Boolean object whose value is false is treated as the primitive value false, and a Boolean object whose value is true is treated as the primitive value true in conditional tests. If the Boolean object is a false object, the conditional statement evaluates to false.

• You can use a Boolean object in place of a Boolean primitive.

Method Summary

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

Examples
The following examples create Boolean objects with an initial value of false:

bNoParam = new Boolean()
bZero = new Boolean(0)
bNull = new Boolean(null)
bEmptyString = new Boolean("")
bfalse = new Boolean(false)

The following examples create Boolean objects with an initial value of true:

btrue = new Boolean(true)
btrueString = new Boolean("true")
bfalseString = new Boolean("false")
bSuLin = new Boolean("Su Lin")

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.

Description
See Object.constructor.

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.

toSource

Returns a string representing the source code of the object.

Syntax
toSource()

Parameters
None

Description
The toSource method returns the following values:

• For the built-in Boolean object, toSource returns the following string indicating that the source code is not available:

     function Boolean() {
        [native code]
     }

• For instances of Boolean, toSource returns a string representing the source code.

See also
Object.toSource

toString

Returns a string representing the specified Boolean object.

Syntax
toString()

Parameters
None.

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

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

For Boolean objects and values, the built-in toString method returns the string "true" or "false" depending on the value of the boolean object. In the following code, flag.toString returns "true".

var flag = new Boolean(true)
var myVar=flag.toString()

See also
Object.toString

valueOf

Returns the primitive value of a Boolean object.

Syntax
valueOf()

Parameters
None

Description
The valueOf method of Boolean returns the primitive value of a Boolean object or literal Boolean as a Boolean data type.

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

Examples
x = new Boolean();
myVar=x.valueOf()      //assigns false to myVar