Check if character is numeric

I've written a client-side validation function (for a textbox) that checks if the key pressed by the user is numeric. I cannot get it to work properly, however. The following code does not prevent the user from entering a non-numeric character, but it does prevent the user from entering two non-numeric characters. For example if the user enters a "1" and the a "d", the textbox will display "1d". The next time the user presses a key, the "d" is removed and whatever character the user pressed replaces the "d". I don't want the textbox to display the character unless it is numeric.


'The following is entered in the codebehind

Code:

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Dim txtQuantity As TextBox = e.Item.Cells(4).FindControl("txtQuantity")
txtQuantity.Attributes.Add("onkeypress", "IsNumeric(" & txtQuantity.ClientID & ",'Integer')")

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'this is the javascript function called when the textbox's onkeypress event is fired

Code:

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function IsNumeric(TextBox,strType)
//  check for valid numeric strings     
{
if (strType="Decimal")
  {
  var strValidChars = "0123456789.";
  }
else if (strType="Integer")
  {
  var strValidChars = "0123456789";
  }
var strString;
var strChar;
strString=TextBox.value;

for (i = 0; i < strString.length; i++)
  {
  strChar = strString.charAt(i);
  if (strValidChars.indexOf(strChar) == -1)
      {
      TextBox.value = TextBox.value.substring(0, i);
    TextBox.value.length = i
      event.returnValue = false;
      }
  }
}

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I'm not very proficient in Javascript so I'd appreciate any suggestions.

Resolved