CODEWITHSUNDEEP
javascriptlanguage-design
Michiel van Oosterhout
Michiel van Oosterhout

Reputation: 23084

Why does !new Boolean(false) equals false in JavaScript?

From the jQuery documentation on JavaScript types comes this snippet of code describing the behavior of strings when converted to booleans (that topic is not related to this question, but it's just where I found the code):

!"" // true
!"hello" // false
!"true" // false
!new Boolean(false) // false

I get the first three examples, but I don't get the last example, because: 

new Boolean(false) == false //true
!false // true

So I would assume:

!new Boolean(false) // true

But instead:

!new Boolean(false) // false, mind = blown

What is this I don't even...

Is it because:

new Boolean(false) === false // false

If so, what purpose does this serve?

Upvotes: 32

Views: 5069

Answers (2)

Adam Rackis
Adam Rackis

Reputation: 83358

new Boolean(false) returns an object. All objects (except document.all in browsers) are truthy.

As a result, ! of any object will always be false.

To prove it to yourself, you can run this in your JavaScript console:

(typeof new Boolean(false)) // "object"

Also, you can use the strict equality operator === to confirm that new Boolean(false) isn’t really false:

new Boolean(false) === false // false

Incidentally, calling the Boolean function as a function—without the new—actually does return a primitive:

!Boolean(false) // true
(typeof Boolean(false)) // "boolean"

Upvotes: 30

pimvdb
pimvdb

Reputation: 154818

Because new Boolean returns an object as stated here.

The ! is defined as follows:

11.4.9 Logical NOT Operator ( ! )

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

  1. Let expr be the result of evaluating UnaryExpression.

  2. Let oldValue be ToBoolean(GetValue(expr)).

  3. If oldValue is true, return false.

  4. Return true.

and:

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

Table 11 — ToBoolean Conversions

Argument Type - Result

...

Object - true

So, it is an object, thus ToBoolean returns true, hence ! returns false.

Upvotes: 8

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