How to make thousand comma separator in JS calculator?

Question

I'm trying to make thousand separator in JS calculator.

When user enters: 1234 -> 1,234.

I'm thinking about doing like that:

var commas = result.textContent.toLocaleString("en-US");

 var commas = result.textContent.toString().replace(/\B(?=(\d{2})+(?!\d))/g, ",");

console.log(commas);

result.innerHTML=commas;

But it gives me weird output. What am I doing wrong?

Thank you in advance!