Personally I think it is a good rule to never use Boolean(x), String(x), etc.

The problem is that many developers are unaware that `const s = new String(x);` creates a string Object. It looks like a string, and acts like a string, except that in some places it is not a string: for example `typeof s` returns “object”! Same problem for Boolean, Number, etcetera. Yes casts are ok, but it is easier to just say never use the value object keywords because you can end up with very wierd bugs a long way from where the value object was created.

Another example:

    let x = new Boolean(true);
    console.log(x === true);

Gives false.

Okay, thanks. And why would it be faster? Optimizing !! in the JS engine seems trivial to me.

I suppose it would be easy and might already be in some engines, but it's still discouraged because you can't rely on that optimisation (there are many engines), it's semantically incorrect, and most importantly much less readable - especially for someone not tok familiar with the JS type system.