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

Solution 1:

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"

Solution 2:

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.