mask a number

opc1.js

function format(mask, number) {
  const string = number.replace('-','')
  let result = ''

  for(let iMask = 0, iString = -1; iMask < mask.length && iString < string.length; iMask += 1) {
    const currentChar = mask.charAt(iMask)
    result += currentChar === 'X' ? string.charAt(iString += 1) : currentChar
  }

  return result
}

const number = '1234567890'
const mask = 'XX-XXXXXXX-X'
console.log(format(mask, number))

opc2.js

const number = '1234567890';
var formatted = number.replace(/^(\d{2})(\d{7})(\d{1}).*/, '$1-$2-$3');

console.log(formatted)