oop - Does OO Javascript have a `to_bool` or `__len__` equivalent?

Question

Does OO JS have a mechanism for casting instance objects to boolean? I would like to be able to use custom instance objects directly in conditionals, and make assertions along the lines of !!(new Foo(0)) === false, !!(new Foo(1)) === true.

• Python has __nonzero__ and __len__ (see here)

• Ruby has to_bool.

How does JS do this for String literals "" and zero 0?

Answer 1

No, JS does not provide a trap method for casting to boolean. Truthiness of a value is statically determined by the language rules and cannot be changed.

You should give your instances a method to be explicitly invoked like isValid(), isTruthy(), isEmpty() or whatever concept your object represents.