如何在创建可读流之前返回对它的引用？

Question

I have a function that needs to return a Readable stream synchronously. I have a function that creates a Readable stream with the data I require, but it's an asynchronous method. How can I return a reference to the synchronous stream?

class SomeStreamCreator {
    _requestStream() {
        fetchStream()
            .then(stream => /* stream is a Readable stream with data */)

        return /* somehow need to return the Readable stream here */
    }
}

Answer 1

Well it all depends on if you actually need the same exact stream object or just the same data. In the first case as mentioned in comments it's more or less not possible.

If it's only the data you're after the situation is quite simple if you use a simple utility stream called PassThrough available in the built-in stream module:

import {PassThrough} from "stream";

class SomeStreamCreator {
    _requestStream() {
        // first prepare an empty stream
        const out = new PassThrough();

        fetchStream() 
            // when the stream is ready pipe it to the empty one
            .then(stream => stream.pipe(out))
            // it's wise to add error handling
            .catch(error => stream.emit("error", e))

        // but immediately simply return the output
        return out;
    }
}

The output will receive all the data from the original stream and there's very little overhead so you don't need to worry about performance that much.