不断返回[object HTMLSpanElement]？ （JavaScript）

Question

Ok so im trying to add two (or more) variables and make it appear in a table cell. I get the variables from lets say the 1st and 2nd cells and i want to display them on the 7th cell, so this is what i have for now. This is the basic code for the cell that gets the info.

function prompta1()
    {
      var a1=prompt("Unesi broj","")
      var spana1 = document.getElementById ("spana1");
      spana1.innerHTML = a1;
function prompta2()
    {
      var a2=prompt("Unesi broj","")
      var spana2 = document.getElementById ("spana2");
      spana2.innerHTML = a2;

And this is the code for the cell that needs to show them.

  function functiona7()
    {
      var a7= spana1+spana2;
      var spana7 = document.getElementById ("spana7");
      spana7.innerHTML = a7;
    } 

Now , when i try this it only gives.

[object HTMLSpanElement][object HTMLSpanElement]

Please help! HTML for getting the values

enter code here   <tr>
  <td><h1>1</h1></td>

  <td><input type="button" onclick="prompta1()" value="Izmeni"> <br> <span id='spana1'></span></td>
  <td><input type="button" onclick="prompta2()" value="Izmeni"> <br> <span id='spana2'></span></td>
</tr>
<tr>

HTML for showing the value

      <td>   <input type="button" onclick="functiona7()" value="Izracunaj"> <br> <span id='spana7'></span> </td>

Answer 1

Try something like this:

function prompta1() {
    var a1 = prompt("Unesi broj", "");
    var spana1 = document.getElementById("spana1");
    spana1.innerHTML = a1;
}

function prompta2() {
    var a2 = prompt("Unesi broj", "");
    var spana2 = document.getElementById("spana2");
    spana2.innerHTML = a2;
}

function functiona7() {
    var spana1 = document.getElementById("spana1").innerHTML;
    var spana2 = document.getElementById("spana2").innerHTML;
    var spana7 = document.getElementById("spana7");
    if (!(spana1 && spana2 && spana7)) return;
    var a7 = spana1 + spana2;
    spana7.innerHTML = a7;
}

Fiddle

In your current example the values for spana1/2 are not pulling from the DOM, so first they need to be populated using getElementById, after that a simple combination of the Inner Html is all that is needed (although you may want to add a space in between too).