text: function tabuada(){
      let num = document.getElementById('txtn')
      let tab = document.getElementById('seltab')
      if(num.value.length == 0) {
          window.alert('[ERROR] Digite um número!')
      } else {
          let n1 = Number(num.value)
          tab.innerHTML = ''
          for(let c = 1; c <= 10; c++){
              let item = document.createElement('option')
              item.text = `${n1} × ${c} = ${n1*c}`
              item.value = `tab${c}`
              tab.appendChild(item)
          }
      }
  }