Preventing a keystroke value from being added to string.

[Claus Mygind]

with the "onKeyPress " event handler I see that I can capture and examine the value of a keystroke before it is displayed on the screen. I have made good use of this for single character fields where I want to examine if the value is correct and also convert the value to upper case like so:

 
<input 
   type="text" 
   name="f1" 
   id  ="f1"  
   maxLength="1"
   onKeyPress="return isHorS(event);" 
 />
 
function isHorS(evt)
{
   evt = (evt) ? evt : event; 
   var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode : ((evt.which) ? evt.which : 0));   
 
      if (charCode != 8 && charCode != 9 && charCode != 32 && charCode != 72 && charCode != 104 && charCode != 83 && charCode != 115 )
      {  
            alert("You must either leave blank or enter H or S!"); 
             document.getElementById("f1").value = "";
      }else{
          if (charCode == 104  || charCode == 115 )
          {
            var cUp = String.fromCharCode(charCode)
            document.getElementById("f1").value = cUp.toUpperCase();
          }
      }
 
}
 

As I said this works great when I am only dealing with one character. But for longer strings than one character, I would like to prevent unwanted characters from becoming part of the string. Can I use this technique by intercepting what the user is typing give them an alert and then cancel that keystroke entry?

[Logician]

It's much simpler to use a regular expression to remove unwanted characters from the string after each keystroke. Use the onkeyup event.

[Claus Mygind]

great do you have an example I can use. although I did get it to work like this:

 onKeyPress=" return isValidCode(event, this);"
 
function isValidCode(evt, obj)
{
   evt = (evt) ? evt : event; 
   var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode :  
       ((evt.which) ? evt.which : 0));   
    if ( charCode == 8 || 
         charCode == 9 || 
        (charCode > 47 && charCode < 58) || 
        (charCode > 64 && charCode < 91) ||    
        (charCode > 96 && charCode < 123) )
    {
        if (charCode > 96 && charCode < 123)
        {
            Here I would like to make it an uppercase value
        }
    }else{
        alert("That is not a valid character ");
        return false;
    }
}
 
 

Perhaps your example of a regular expression can also convert the keystroke upper case if allowed?

[Claus Mygind]

Ok I have learned that only IE let's you intercept a keystroke and alter it before it reaches the form (generally considered a security risk as programmer can alter what the user is entering)

var charCode = event.keyCode;
 
event.keyCode = charCode - 32

And as I am working exclusively with fireFox that will not work for me.

will look at the keyup option