Boolean
The Boolean
object is an object wrapper for a boolean value.
Description
The value passed as the first parameter is converted to a boolean value, if necessary. If the value is omitted or is 0
, -0
, null
, false
, NaN
, undefined
, or the empty string (""
), the object has an initial value of false
. All other values, including any object, an empty array ([]
), or the string "false
", create an object with an initial value of true
.
Do not confuse the primitive Boolean
values true
and false
with the true
and false
values of the Boolean
object.
Any object of which the 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
:
var x = new Boolean(false);
if (x) {
// this code is executed
}
This behavior does not apply to Boolean
primitives. For example, the condition in the following if
statement evaluates to false
:
var x = false;
if (x) {
// this code is not executed
}
Do not use a Boolean
object to convert a non-boolean value to a boolean value. To perform this task, instead, use Boolean
as a function, or a double NOT operator:
var x = Boolean(expression); // use this...
var x = !!(expression); // ...or this
var x = new Boolean(expression); // don't use this!
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
.
var myFalse = new Boolean(false); // initial value of false
var g = Boolean(myFalse); // initial value of true
var myString = new String('Hello'); // string object
var s = Boolean(myString); // initial value of true
Do not use a Boolean
object in place of a Boolean
primitive.
Note: When the non-standard property document.all
is used as an argument for this constructor, the result is a Boolean
object with the value false
. This property is legacy and non-standard and should not be used.
When using ==
to loosely compare an object to a boolean primitive, it's important to have a clear understanding of what's actually being compared. Consider the following example:
if ([]) { console.log("[] is truthy")} // logs "[] is truthy"
if ([] == false) { console.log("[] == false")} // logs "[] == false"
The reason for [] == false
even though []
is truthy is: the comparison [] == false
compares the value of []
to false
. And to get the value of []
, the JavaScript engine first calls [].toString()
. That results in ""
, and that is what's actually compared to false
. In other words, [] == false
is equivalent to "" == false
. And ""
is falsy — and so that's what explains the behavior in the example.
Constructor
Boolean()
-
Creates a new
Boolean
object.
Instance methods
Boolean.prototype.toString()
-
Returns a string of either
true
orfalse
depending upon the value of the object. Overrides theObject.prototype.toString()
method. Boolean.prototype.valueOf()
-
Returns the primitive value of the
Boolean
object. Overrides theObject.prototype.valueOf()
method.
Examples
Creating Boolean
objects with an initial value of false
var bNoParam = new Boolean();
var bZero = new Boolean(0);
var bNull = new Boolean(null);
var bEmptyString = new Boolean('');
var bfalse = new Boolean(false);
Creating Boolean
objects with an initial value of true
var btrue = new Boolean(true);
var btrueString = new Boolean('true');
var bfalseString = new Boolean('false');
var bSuLin = new Boolean('Su Lin');
var bArrayProto = new Boolean([]);
var bObjProto = new Boolean({});
Specifications
Specification |
---|
ECMAScript Language Specification # sec-boolean-objects |
Browser compatibility
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