Validar Celular para contato WhatsApp

.js

const numberCel = '554791517799';

function addNine(number) {
  if (numberCel.slice(0, -numberCel.length + 2) !== '55') {
    //if (numberCel[0] + numberCel[1] !== '55') {
    // numberCel.slice(0,-numberCel.length +2)
    return number;
  }
  if (
    numberCel[4] === '2' ||
    numberCel[4] === '3' ||
    numberCel[4] === '4' ||
    numberCel[4] === '5'
  ) {
    return number;
  }
  var volta = 0;
  var textVolta = '';
  if (number.length < 13) {
    for (let member in number) {
      textVolta += number[member];
      if (volta === 3) {
        textVolta += '9';
      }
      volta++;
    }
  } else {
    return number;
  }
  return textVolta;
}

console.log(addNine(numberCel));