sheetjs/bits/56_commaify.js

function commaify(s/*:string*/)/*:string*/ {
	if(s.length <= 3) return s;
	var j = (s.length % 3), o = s.substr(0,j);
	for(; j!=s.length; j+=3) o+=(o.length > 0 ? "," : "") + s.substr(j,3);
	return o;
}