transform.js

function transform(emit) {
  // any state kept between input chunks will be setup here
  return function (item) {
    // Forward the EOS marker
    if (item === undefined) return emit();
    // Skip cheese items
    if (item === "cheese") return;
    // Double anything else
    emit(item);
    emit(item);
    
    // This example is silly, but it shows that you can call emit 0 or more times for every item input.
  };
}

someObject.on("data", transform(function (out) {
  // every data event will run through the filter and emit here.
}));