const barStream = fs.createWriteStream('bar');
const foo = spawn('foo', [], { stdio: ['ignore', barStream, 'inherit'] });

throws an error:

Incorrect value for stdio stream: WriteStream

Doing this in reverse like foo.stdout.pipe(barStream) likely does the trick in some cases.

But why exactly WriteStream cannot be supplied as stdout stream? Is there a way to make barStream suitable for stdout?

const barStream = fs.createWriteStream('bar');
const foo = spawn('foo', [], { stdio: ['ignore', barStream, 'inherit'] });

throws an error:

Incorrect value for stdio stream: WriteStream

Doing this in reverse like foo.stdout.pipe(barStream) likely does the trick in some cases.

But why exactly WriteStream cannot be supplied as stdout stream? Is there a way to make barStream suitable for stdout?

• javascript
• node.js

1 Answer 1

From the documentation for the stdio option, emphasis mine:

<Stream> object - Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).

So:

const barStream = fs.createWriteStream('bar');

barStream.on('open', () => {
    const foo = spawn('foo', [], { stdio: ['ignore', barStream, 'inherit'] });
    ⋮
});

I’ll admit that error message could definitely be more helpful.