Count wordin textbox

TO find the enter text in text box use the following function in javascript Use onkeydown and onkeyup events to call this function

function fnCheckCharacters()    
{         
    var txtcheck=document.getElementById('<%= txtQ11.ClientID %>'); 
    //here txtQ11 is my textbox id       
    var lblLeftCount = document.getElementById('<%=lblCharCounter.ClientID %>'); 
    //lblCharCounter is a labed on which i printed the remaing word                
    var len=300;       
    var Lchar = 0;           
    if (txtcheck.value.length > 300)       
    {           
        alert ("You can enter atmost 300 characters");           
        txtcheck.value=txtcheck.value.substring(0,300);       
    }
    else       
    {           
        lblLeftCount.innerHTML=txtcheck.value.length;       
    }          
}   

in this function, i fixed the length of textbox to 300. user can enter only 300 char.after inserting each char, reaming one is displayed on label.

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thnx