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<script language="javascript" type="text/javascript">

<!--

function imposeMaxLength(Object, MaxLen)

{

  return (Object.value.length <= MaxLen);

}

-->

</script>



Implementation:

<textarea name="myName" onkeypress="return imposeMaxLength(this, 50);" ></textarea> 

EDIT:

Code that doesn't freeze text:

<script type="text/javascript">



/***********************************************

* Textarea Maxlength script- © Dynamic Drive (www.dynamicdrive.com)

* This notice must stay intact for legal use.

* Visit http://www.dynamicdrive.com/ for full source code

***********************************************/



function imposeMaxLength(obj) {

        const mlength = obj.getAttribute ? parseInt(obj.getAttribute("maxlength")) : ""

        if (obj.getAttribute && obj.value.length > mlength) {

            obj.value = obj.value.substring(0, mlength)

        }

        obj.value = obj.value.trim()

    }



</script>



<textarea maxlength="40" onkeyup="return imposeMaxLength(this)"></textarea>