What is the purpose of new Boolean() in Javascript?

The global function Boolean() can be used for type casting when called without new, eg

var foo = Boolean(bar); // equivalent to `var foo = !!bar`

When called with new, a wrapper object will be created additionally, which means that you can assign arbitrary properties to the object:

var foo = new Boolean(bar); // equivalent to `var foo = Object(Boolean(bar));`
foo.baz = 'quux';
alert(foo.baz);

This is not possible with primitive values as primitives can't hold properties:

var foo = true;
foo.baz = 'quux';
alert(foo.baz); // `foo.baz` is `undefined`

Assigning a property to a primitive doesn't produce an error because of auto-boxing, ie

foo.baz = 'quux';

will be interpreted as

// create and immediately discard a wrapper object:
(new Boolean(foo)).baz = 'quux';

To get the primitive value back, you'll have to invoke the valueOf() method. This is needed if you want to actually use the wrapped value, because objects always evaluate to true in boolean contexts - even if the wrapped value is false.

I've never come across a useful application of being able to assign properties to booleans, but boxing might be useful in cases where a reference to a primitive value is needed.

While others mentioned the theory, let me talk about the practical part:

Because Boolean objects (as objects in general) are always truthy, it is considered bad practice to use them. In many years of JS programming, I have never used them, and I can't remember seeing Booleans in other peoples' code either. Not even once.

Using primitive values will avoid confusion and will make your code a little bit shorter.

If you ever need a bool wrapped in an object, you might as well use an Object object like so:

foo = { value: false };

Also, calling the Boolean() constructor as a function (as in foo = Boolean(bar)) has the same effect as explicit typecasting using !!, and the latter is generally preferred over the former.