Symbol.toPrimitive manipulation

test.js

var obj = {
    [Symbol.toPrimitive](hint) {
        if (hint == "number") {
            return Object.keys(this).length;
        }
        if (hint == "string") {
            return JSON.stringify(this);
        }
    },
    key: 'value',
    2: 89,
    cats: 'good',
    dogs: 'good',
    goats: 'bad'
};
console.log(+obj); // 5
console.log(String(obj)); // {"2":89,"key":"value","cats":"good","dogs":"good","goats":"bad"}

I wonder whether I could overload the Objectconstructor to apply that value to every incoming object

What does obj + "" do?

I wrote a constructor for it, but I can't seem to overload the wrapper Object constructor:

function ObjQuill(obj){
    var manipulatedObject = {
        [Symbol.toPrimitive](hint) {
            if (hint == "number") {
                return Object.keys(this).length;
            }
            if (hint == "string") {
                return JSON.stringify(this);
            }
        }
    };
    return Object.assign(manipulatedObject, obj);
}
var testObj = ObjQuill({cat: 'dog', dog: 'cat'});
console.log(+testObj ); // 2
console.log(String(testObj )); //"{"cat":"dog","dog":"cat"}"