convert integer to roman numeral javascript

int2roman.js

Object.defineProperty(Number.prototype,'int2roman',{
		get: function(){
            const map = {
                M:  1000,
                CM: 900,
                D:  500,
                CD: 400,
                C:  100,
                XC: 90,
                L:  50,
                XL: 40,
                X:  10,
                IX: 9,
                V:  5,
                IV: 4,
                I:  1,
            };  
            let result = '';
            let num=this.valueOf();
            for (key in map) {
                result += key.repeat(Math.floor(num / map[key]));
                num %= map[key];
            }            
            return result;
        }
	});