Format Text file to phone number in Javascript

Question

I want to replace var mobileNumber from textfile to reformat the phone number to the following: 123-456-7890 in textfile have one number like 9876543216 , 8793479876 (Text file random number so i want var change number and reformat to phone number)

var mobileNumber = (fs.readFileSync('Numbertext.txt'));
var NewformNumber = mobileNumber.replace("format phone number?");

it is possible ?

Answer 1

function formatNumber(phoneNumber)
{
    return phoneNumber.slice(0,3)+"-"+phoneNumber.slice(3,6)+"-"+phoneNumber.slice(6);
}

console.log(formatNumber("9876543216"));

I edited code from here:

Javascript dashes in phone number