Zez

<form onchange="funkcija(a,b,c)">
  <label for="a">A:</label>
    <input type="text" id="a" />
   <span>+</span>
    <label for="b">B:</label>
    <input type="text" id="b" />
  <span>=</span>
  <input type="text" id="c" readonly><br>
  <button onclick="prep(a,b,c)" type="button">
    Permutacija
  </button><br>
  <label id="perm">a</label>
</form>




let a = document.getElementById("a");
let b = document.getElementById("b");
let c = document.getElementById("c");
let perm = document.getElementById("perm");
function funkcija(a,b,c) {
  c.value=parseInt(a.value)+parseInt(b.value);

}
let final=1;
function prep(a,b,c) {
let numA=parseInt(a.value);
let numB=parseInt(b.value);
let numC=parseInt(c.value);
let len = a.value.length+b.value.length+c.value.length;
for(let i=len;i>=1;i--) {
  final*=i;
}
  console.log(final);
  permutate(numA,numB,numC);
 }
let brojPoziva=0;
function permutate(a,b,c) {
  brojPoziva++;
  if(final>0) {
    console.log(a*100+b*10+c+":"+final);
    final--;
    permutate(a,c,b,final);
    permutate(c,b,a,final);
    //return a*100+b*10+c;
  }
  else
    return;
}