Difference Boolean Constructor and Double Negation


In JavaScript, double negation !! is often used to convert values to boolean values while keeping the same truth value. For example:

!!"" // false
!!null // false
!!{} // true
!![] // true

// want to return true or false
const hasElement = (array, element) => !!array.find(el => el === element)
// P.S. Array.includes or Array.some would be more appropriate here

But it looks a bit ugly and you can instead use the Boolean constructor which does the same as double negation:

Boolean("") // false
Boolean(null) // false
Boolean({}) // true
Boolean([]) // true

const hasElement =
    (array, element) => Boolean(array.find(el => el === element))

It also allows you to write this short awesome-looking falsy value filter like lodash.compact:

["", null, {}, [], true, false].filter(Boolean)
// [{}, [], true]

You can just pass in the Boolean constructor itself instead of having to create an anonymous function taking the element and returning the double negation of the element. Boolean is thus better for functional programming as it is a function itself giving rise to the possibility of η-reduction. Meaning, in our example, instead of writing filter(el => Boolean(el)) we can just write filter(Boolean) instead.

Here’s the big BUT that you need to know when using Boolean: Always call the constructor like a function, never with the new keyword.

If you invoke the Boolean constructor not as a function call but by using new, it returns an object. In JavaScript, every object is a truthy value, which leads to these quirks in the language:

new Boolean(true)
> Boolean {true}

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

// you can get the primitive boolean value with valueOf()
true === new Boolean(true).valueOf() // true, but 🤮

But calling Boolean as a function returns a primitive boolean value instead which is what we want.

true === Boolean(true) // true 😃
false === Boolean(false) // true 😃

So a question one might ask is: Why would one ever want to use the constructor with new and create boolean objects instead of primitive values? Please let me know if you have an answer. Especially in modern JavaScript where the strict-equality operator === is used and the one with type coercion == is frowned upon.

Hi, I'm Christoph Michel 👋


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