Function to Check if field is blank using javascript

function IsBlank(ObjId, fieldName) {
    if (obj) {
        var val = document.all(ObjId).value;
        if (val == "") {
            alert("Please enter value for \"" + fieldName + "\"");
            document.all(ObjId).focus();
            return true;
        }
        return false;
    }
}