How to pass Textbox value to Javascript function?

My View code is as below where I am calling a javascript function on key up event. I want to pass the control value to javascript function. I tried using 'this' and @this, but it didn't work.

@Html.TextBoxFor(m => m.LeaveDetailsList[0].LeaveTaken, new {id = "numLeaveTaken1", width = "100", @onkeyup = "checkInp2(this);" })

** Edited ** Javascript code:

function checkInp2(ControlId) {
    var x = document.getElementById(ControlID).value;
    if (isNaN(x)) {
        document.getElementById("btnSave").disabled = true;
        return false;
    }
    else {
        document.getElementById("btnSave").disabled = false;
    }
}