toString() in JavaScript: why does toString() still get called since a valueOf() prototype for type conversion of object to number

Question

let obj = {
    toString() {
        return "2";
    }
};

let n = +obj; 

alert(n);

Since +obj requires a number, shouldn't it use the valueOf() prototype for type conversion which returns the object. Instead it uses the toString() method and alerts 2. Please why is this so?

Answer 1

Since +obj requires a number, shouldn't it use the valueOf() prototype for type conversion which returns the object.

It actually does call the valueOf method. But since - as you say - it returns an object, not a primitive value, it is found useless. Then, the alternative is called: toString(), which does return a primitive value that is subsequently cast to a number.

You can try

const obj1 = {
    valueOf() { console.log("valueOf 1"); return this; },
    toString() { console.log("toString 1"); return "1"; },
};
console.log(+obj1);

const obj2 = {
    valueOf() { console.log("valueOf 2"); return 2; },
    toString() { console.log("toString 2"); return "2"; },
};
console.log(+obj2);