https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString#using_options

var number = 1000;

// request a currency format
console.log(number.toLocaleString('en-US', { style: 'currency', currency: 'USD' }));

Without methods and just formatting with commas

        function addComma(value) {
            return value.split('').map((v, i) => {
                if ( i === value.length - 2) {
                    return ',' + v
                }
                return v
            }).join('')
        }
        console.log(addComma('1000'))
        console.log(addComma('10000'))
        console.log(addComma('100000'))

https://www.delftstack.com/howto/javascript/javascript-currency-format/

you can transform your string in number and then use toLocaleString()

here is an example from the webpage

const number = 2000;
number.toLocaleString('en-IN', {style: 'currency',currency: 'INR', minimumFractionDigits: 2})
console.log(number);

Another solution, just splitting the string into whole and cents:

function addComma(n)
{
    if (n.length == 0) {
       return '0,00';
    }

    if (n.length == 1) {
       return '0,0' + n;
    }

    if (n.length == 2) {
       return '0,' + n;
    }

    let whole = n.substring(0, n.length-2);
    let cents = n.substring(n.length-2);
    return whole + ',' + cents;
}