How to combine streams and not lose data

[ORESoftware]

I have this question on SO:

https://stackoverflow.com/questions/56945909/combine-fs-createreadstream-with-process-stdin-stream

Basically I am continually writing to a file to keep track of old data.
At some point, I want to read the data from the file and then start reading the new data from stdin. I don't want to duplicate data nor do I want to lose data.

Is there some reliable way to do this?

process.stdin.resume().pipe(fs.createWriteStream('file'));

{
  // much later on

  const sws = new SomeWriteStream({end:false});

  fs.createReadStream('file').pipe(sws).once('end', () => {
    process.stdin.pipe(sws);
  });

}

[RaisinTen]

Is this still relevant?

• once should be on
• pipe returns the destination and in this case you need the readable
• end as an option for fs.createReadStream is different from using it as an option for readable.pipe, in this case you need to pass it as an option for readable.pipe

Additionally, I added a slight delay of 5 seconds to your code to signify when sws starts reading from stdin.

const fs = require('fs');

process.stdin.resume().pipe(fs.createWriteStream('file'));

{
  // much later on

  setTimeout(() => {

    const sws = fs.createWriteStream('sws');
    const file = fs.createReadStream('file');

    file.pipe(sws, {end: false});

    file.on('end', () => {
      console.log('reading sws from stdin');
      process.stdin.pipe(sws);
    });
  }, 5000);

}

Here is the log of a demo run:

❯ node index.js
1
2
3
reading sws from stdin
4
5
6
❯ cat file
1
2
3
4
5
6
❯ cat sws
1
2
3
4
5
6

[ORESoftware]

thx

[RaisinTen]

Feel free to close this if you are satisfied with the answer :)