A peculiar JavaScript language feature from its early days: if the object x has a valueOf() method that returns a number, you can use numerical operators on the object x directly, such as x + 1 or x < 4. This method is also called when coercing a value to a string by concatenating it with another string, as in x + ''. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/valueOf