text: function tabuada() {
      var num = document.getElementById('txtn')
      var tab = document.getElementById('seltab')
      if (num.value === "") {
          window.alert('Por favor digite um número')
      }
      else {
          var n = Number(num.value)
          var c = 1
          tab.innerHTML = ""
          while (c <= 10) {
              var item = document.createElement('option')
              item.text = `${n} x ${c} = ${n*c}`
              tab.appendChild(item)
              c++
          }
      }
  }