https://nwgrgr.com/scripts/autoTab.js

1var isNN = (navigator.appName.indexOf('Netscape')!=-1);
2function autoTab(input,len, e) 
3{
4    var keyCode = (isNN) ? e.which : e.keyCode; 
5    var filter = (isNN) ? [0,8,9] : [0,8,9,16,17,18,37,38,39,40,46];
6    if(input.value.length >= len && !containsElement(filter,keyCode)) 
7    {
8        input.value = input.value.slice(0, len);
9        input.form[(getIndex(input)+1) % input.form.length].focus();
10    }
11    
12    return true;
13}
14
15function containsElement(arr, ele) 
16{
17    var found = false, index = 0;
18    while(!found && index < arr.length)
19    {
20        if(arr[index] == ele)
21        {
22            found = true;
23        }
24        else
25        {
26            index++;
27        }
28    }
29            
30    return found;
31}
32
33function getIndex(input) 
34{
35    var index = -1, i = 0, found = false;
36    
37    while (i < input.form.length && index == -1)
38    {
39        if (input.form[i] == input)
40        {
41            index = i;
42        }
43        else 
44        {
45            i++;
46        }
47    }
48            
49    return index;
50}