What is the purpose of new Boolean() in Javascript?
boolean, javascript
Solution
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.
Problem
What is the use of: ``` var flag = new Boolean(false); ``` compared to: ``` var flag = false; ``` When would you actually use `new Boolean`?