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?

"*since a valueOf() prototype for numbers exists*" - not sure how `Number.prototype.valueOf` is related? — Bergi, Mar 29 '20 at 18:56
Don't objects have Object.prototype.valueOf() that is used for type conversion of objects to numbers? — Israel, Mar 29 '20 at 18:59

score 2 · Accepted Answer · answered Mar 29 '20 at 19:00

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);

score -2 · Answer 2 · answered Mar 29 '20 at 18:44

-2

Your should to change de toString() function by parseInt() function to get NaN response.

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

let n = +obj; 

alert(n);

answered Mar 29 '20 at 18:44

judlup

119
2
15

Okay. But I want to understand why it is that way – Israel Mar 29 '20 at 18:48
Israel, the reason is because you are trying to sum a number with a string. – judlup Mar 29 '20 at 18:57
I am trying to convert the object to a number with unary plus – Israel Mar 29 '20 at 19:00

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

2 Answers2