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?
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.
来源：https://stackoverflow.com/questions/37055255/does-oo-javascript-have-a-to-bool-or-len-equivalent