For making fields read only for the existing record

Hi Team,

Can anyone help me out for getting the Javascript code for making the fields read only when the existing record is opened?

Below is my code but its not working:

function DisableFields() {
debugger;
var kvsl_value;
var kvsl_key;
var value = Xrm.Page.ui.controls.get("kvsl_value");
var value = Xrm.Page.ui.controls.get("kvsl_key");
if (kvsl_value != null && kvsl_key != null) {
value.setDisabled(true);
key.setDisabled(true);
}
else {
value.setDisabled(false);
key.setDisabled(false);
}
}

Thanks in Advance,

Anita