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JavaScript Code to Restrict Keyboard Entries

By Stephen Bucaro

You can restrict form textbox input to only the characters you desire. This example shows how to restrict textbox input characters to only the characters 0 and 1. The character code for 0 is 48, the character code for 1 is 49. The code shown below restricts input to those two characters.

var unicode = e.charCode || e.keyCode;
if(unicode != 48 && unicode != 49) return false;

The character code input from the user's keyboard is stored in the variable unicode and the if statement tests if that code is not equal to 0 and is not equal to 1, if true, it return a false. A return of false means exit this section of code without doing anything else. This effectively restricts the input.

We do want to permit certain other key functions besides 0 and 1. For example we want to allow the character code for backspace (8), tab (9), and delete (46) so that the user can edit their input. To do that we nest the previous ifstatement in another if statement that does not allow those keyCodes to be tested by the first if statement, as shown below.

if(unicode != 8 && unicode != 9 && unicode != 46)
{
  if(unicode != 48 && unicode != 49) return false;
}

All this key input restricting prevents the form from responding to the normal Tab function that allows the user to tab between form elements, which is something that you probably want to preserve, so before letting program flow get to the statements above, I use an if statement to catch the character code for tab (13), and use it to send the focus to the next element in the form. The code for this is shown below.

if(unicode == 13)
{
   document.getElementById("result").focus();
}

Shown below is the entire code for a function to restrict the input from a form text box to only 0 and 1.

<script>
function restrictKey(e)
{
   var unicode = e.charCode || e.keyCode;
   // cause enter key to tab
   if(unicode == 13)
   {
      document.getElementById("result").focus();
   }

   // allow tab and backspace key
   if(unicode != 8 && unicode != 9 && unicode != 46)
   {
     if(unicode != 48 && unicode != 49) return false;
   }
}

function showEntry(form)
{
   alert(form.bindigits.value);
}
</script>

<form action="javascript:return false;">
Binary Number: <input type="text" id="bindigits" size="12" maxLength="12" style="text-align:right" onkeypress="return restrictKey(event)" /><br />
<input type="button" id="result" value="Show" onclick="showEntry(this.form)" />
</form>

You should know enough now to be able to adjust the code for your own purposes. To aid in that, below is a table of some keys and their related character codes.

Key	keyCode
Backspace	8
Tab	9
Enter	13
Shift	16
Ctrl	17
Alt	18
Esc	27
Space	32
Page Up	33
Page Down	34
End	35
Home	36
Left arrow	37
Up arrow	38
Right arrow	39
Down arrow	40
PrntScrn	44
Insert	45
Delete	46
0-9	48-57
A-Z	65-90