NodeJS streams not awaiting async - node.js

Question

I have run into an issue when testing NodeJS streams. I can't seem to get my project to wait for the output from the Duplex and Transform streams after running a stream.pipeline, even though it is returning a promise. Perhaps I'm missing something, but I believe that the script should wait for the function to return before continuing. The most important part of the project I'm trying to get working is:
// Message system is a duplex (read/write) stream
export class MessageSystem extends Duplex {
constructor() {
super({highWaterMark: 100, readableObjectMode: true, writableObjectMode: true});
}
public _read(size: number): void {
var chunk = this.read();
console.log(`Recieved ${chunk}`);
this.push(chunk);
}
public _write(chunk: Message, encoding: string,
callback: (error?: Error | null | undefined, chunk?: Message) => any): void {
if (chunk.data === null) {
callback(new Error("Message.Data is null"));
} else {
callback();
}
}
}
export class SystemStream extends Transform {
public type: MessageType = MessageType.Global;
public data: Array<Message> = new Array<Message>();
constructor() {
super({highWaterMark: 100, readableObjectMode: true, writableObjectMode: true});
}
public _transform(chunk: Message, encoding: string,
callback: TransformCallback): void {
if (chunk.single && (chunk.type === this.type || chunk.type === MessageType.Global)) {
console.log(`Adding ${chunk}`);
this.data.push(chunk);
chunk = new Message(chunk.data, MessageType.Removed, true);
callback(undefined, chunk); // TODO: Is this correct?
} else if (chunk.type === this.type || chunk.type === MessageType.Global) { // Ours and global
this.data.push(chunk);
callback(undefined, chunk);
} else { // Not ours
callback(undefined, chunk);
}
}
}
export class EngineStream extends SystemStream {
public type: MessageType = MessageType.Engine;
}
export class IOStream extends SystemStream {
public type: MessageType = MessageType.IO;
}
let ms = new MessageSystem();
let es = new EngineStream();
let io = new IOStream();
let pipeline = promisify(Stream.pipeline);
async function start() {
console.log("Running Message System");
console.log("Writing new messages");
ms.write(new Message("Hello"));
ms.write(new Message("world!"));
ms.write(new Message("Engine data", MessageType.Engine));
ms.write(new Message("IO data", MessageType.IO));
ms.write(new Message("Order matters in the pipe, even if Global", MessageType.Global, true));
ms.end(new Message("Final message in the stream"));
console.log("Piping data");
await pipeline(
ms,
es,
io
);
}
Promise.all([start()]).then(() => {
console.log(`Engine Messages to parse: ${es.data.toString()}`);
console.log(`IO Messages to parse: ${io.data.toString()}`);
});
Output should look something like:
Running message system
Writing new messages
Hello
world!
Engine Data
IO Data
Order Matters in the pipe, even if Global
Engine messages to parse: Engine Data
IO messages to parse: IO Data
Any help would be greatly appreciated. Thanks!
Note: I posted this with my other account, and not this one that is my actual account. Apologies for the duplicate.
Edit: I initially had the repo private, but have made it public to help clarify the answer. More usage can be found on the feature/inital_system branch. It can be run with npm start when checked out.
Edit: I've put my custom streams here for verbosity. I think I'm on a better track than before, but now getting a "null" object recieved down the pipeline.

Answer 1

As the documentation states, stream.pipeline is callback-based doesn't return a promise.
It has custom promisified version that can be accessed with util.promisify:
const pipeline = util.promisify(stream.pipeline);
...
await pipeline(...);

Answer 2

After some work of the past couple of days, I've found my answer. The issue was my implementation of the Duplex stream. I have since changed the MessageSystem to be a Transform stream to be easier to manage and work with.
Here is the product:
export class MessageSystem extends Transform {
constructor() {
super({highWaterMark: 100, readableObjectMode: true, writableObjectMode: true});
}
public _transform(chunk: Message, encoding: string,
callback: TransformCallback): void {
try {
let output: string = chunk.toString();
callback(undefined, output);
} catch (err) {
callback(err);
}
}
}
Thank you to #estus for the quick reply and check. Again, I find my answer in the API all along!
An archived repository of my findings can be found in this repository.