text: function tabuada(){
      let num = document.getElementById('txtn')
      let tab = document.getElementById('txttab')
      
      if( num.value.length == 0){
          alert('Erro! Digite um número')
      }else{
          let n = Number(num.value)                
          let c = 1
          tab.innerHTML = ''
          while (c <= 10){
              let item = document.createElement('option')
              item.text = `${n} X ${c} = ${c*n}`
              c++
              tab.appendChild(item)
          }
      }
  }