phoneStr= '12345678912346';

phoneStr.replace(/(d{3})(d{3})(d{4})(d{3})/, '($1) - $2 - $3 Ext - $4');

let phoneStr= '12345678912346';
let formatted = '(' + phoneStr.slice(0,3) + ') - ' + phoneStr.slice(3,6) + ' - ' + phoneStr.slice(6,10) + ' Ext - ' + phoneStr.slice(10,15);
console.log(formatted);