I want to intercept Boolean Coercion for Objects in JavaScript

MDN takes a strong stance against using new with Boolean:

…all objects, including a Boolean object whose wrapped value is false, are truthy and evaluate to true in places such as conditional statements. — MDN: Boolean: Boolean primitives and Boolean objects

(…though it makes use of it on other pages without a warning)

!!(Boolean(false)) // false

// Don’t use this!
!!(new Boolean(false)) // true

A quick shout out to previously unknown (to me) new.target for detecting use of new in your own classes.

But how do I customize this behavior? I want to make my own Boolean-esque object and I need a low-level mechanism to intercept boolean primitive coercion to do it!

Consider this class that extends Boolean:

class MyBoolean extends Boolean {
	[Symbol.toPrimitive]() {
		return this.valueOf();
	}
}

!!(new MyBoolean(true)) // true
!!(new MyBoolean(false)) // true (I want `false`)

Or this example creating a new Boolean-esque class:

class MyBoolean {
	#v;
	constructor(val) {
		this.#v = Boolean(val);
	}
	// not triggered
	valueOf() {
		return this.#v;
	}
	// not triggered
	[Symbol.toPrimitive]() {
		return this.#v;
	}
}

!!(new MyBoolean(true)) // true
!!(new MyBoolean(false)) // true (I want `false`)

MDN again, with a quick quip:

Note: Unlike other type conversions like string coercion or number coercion, boolean coercion does not attempt to convert objects to primitives by calling user methods.—MDN: Boolean: Boolean coercion

I wish I could end this blog post with some magical hack that I found to workaround this behavior, but I have yet to make such a discovery. I’m only left with this blog-post-as-comment-card hoping that someone will see my plea to unlock this new power. ECMAScriptarians, help!

Related:

• a gist from Dr. Axel Rauschmayer
• Fake Operator Overloading (a blog post from Dr. Axel Rauschmayer)