I created a simple function that does the trick with a regex expression.

The function receives either a string or a number and returns the formatted number with correct commas:

formatNumber(number) {
    number = number.toString()
    return number.replace(/(\d)(?=(\d\d\d)+(?!\d))/g, "$1,")
}

I wrote this function in TypeScript and added tests.

I've exported it to bit.dev with a live demo and docs.

Also, it can be easily consumed with NPM/Yarn/Bit.
https://bit.dev/joshk/jotils/format-number

If you have a better way to write this function, please write a comment.