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Are booleans objects in JavaScript? Is it true that "everything is an object" in JavaScript?

beth
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2 Answers2

8

Primitives are not objects, everything else (any standard object) is an object. However, most primitives (all apart from undefined and null) have an object counterpart.

So 

var a = false;

is not an object, but

var b = new Boolean(false);

is.

Since two objects are only equal if they refer to one and the same object, using the object version of primitives should better be avoided:

a === false; // is true
b === false // is false   <- this is a problem

Or especially with boolean objects, using them with any boolean operators will create unexpected results. An object reference always evaluates to true, so the outcome of using b would be:

// remember
// a is the primitive value false
// b is a boolean object with value false

// NOT
!a // true
// but
!b // false

// AND
a && true // false
// but
b && true // true

There is no real advantage of using these object versions anyway, since JavaScript is autoboxing primitives when you try to call methods on them. That's why calls like:

var s = "HI THERE!".toLowerCase();
s = s.substring(0,2);

are possible.

Felix Kling
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    However, you virtually never want object-wrapped primitives. – gsnedders Aug 15 '12 at 23:16
  • there is a purpose (although not very useful: http://stackoverflow.com/a/856330/908879) _"...is to convert non boolean objects into a boolean"_ – ajax333221 Aug 15 '12 at 23:36
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    @ajax333221: True, you could indeed use it for type conversion. In this case, you could also use double negation though, e.g. `!!"someString"`. Also it is important to note that there is a difference between calling `Boolean` (and any other of these functions) as function or as constructor. As function they all(?) return primitive values. – Felix Kling Aug 15 '12 at 23:38
  • not conformed with already an accepted answer and +7 votes, he keeps doing awesome edits. If everyone were like Felix Fling, there would probably be world peace and we will likely find cure to most of incurable diseases of today – ajax333221 Aug 15 '12 at 23:52
  • @ajax333221: Thanks for the compliment :) – Felix Kling Aug 16 '12 at 00:05
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Booleans, numbers and strings are object-like types - they have methods, but they are immutable.

Vlad
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