onchange Event doesn't work

[max9]

I tied the event onchange to my textbox control:\[code\]var tbValue = http://stackoverflow.com/questions/14458056/document.getElementById("tbValue"); tbValue.onchange = function () { var tbSelText = document.getElementById("selectText"); if (tbValue.value =http://stackoverflow.com/questions/14458056/= null) { tbSelText.value.replace("sada", ""); } else { tbSelText.value.replace("sada", "sada" + tbValue.value + "'"); } }\[/code\]When I change tbValue (it's my textbox control with onchange event) tbSelText must change at the same time. But it doesn't.What I did wrong?