How to replace ‘undefined’ with an text message in JavaScript

I made a statement in the options. However, some options that I don’t show in the text statement will appear undefined

Instead of displaying the word undefined, I want to replace it with a text message.

Here’s my code below:

function idenmalaria() {
  var a, b, c, d, text;

  a = document.getElementById("rbc_size").value;
  b = document.getElementById("rbc_dis").value;
  c = document.getElementById("dot").value;
  d = document.getElementById("distribusi").value;
   
  if (a == 1 && b == 1 && c == 1 && d == 1) {
    text = "Plasmodium falciparum";
  } else if (a == 1 && b == 2 && c == 3 && d == 2) {
    text = "Plasmodium malariae";
  } else if (a == 2 && b == 1 && c == 2 && d == 2) {
    text = "Plasmodium vivax";
  } else if (a == 2 && b == 2 && c == 2 && d == 2) {
    text = "Plasmodium ovale";
  }

  document.getElementById("result").value = text;

}

HTML code :

<select id="rbc_size">
    <option value="1">RBC normal</option>
    <option value="2">RBC big</option>
</select>
<select id="rbc_dis">
    <option value="1">RBC distorted</option>
    <option value="2">RBC undistorted</option>
</select>
<select id="dot">
    <option value="1">Maurer dots</option>
    <option value="2">Sch├╝ffner dots</option>
    <option value="3">None</option>
</select>
<select id="distribusi">
    <option value="1">Homogeneous</option>
    <option value="2">Heterogeneous</option>
</select>

<button onclick="idenmalaria()" type="submit">Parasit</button>
<input type="text" id="result" />

Any help would be appreciated. Thank you.