let arr=[1,2,3,4,5,6,7,8,9] arr.splice(0,0,'(') arr.splice(4,0,')') arr.splice(5,0,' ') arr.splice(9,0,'-') console.log(arr.join('')) // function createPhoneNumber(numbers){ // var format = "(xxx) xxx-xxxx"; // for(var i = 0; i < numbers.length; i++) // { // format = format.replace('x', numbers[i]); // } // return format; // }