How can a Node.js readable stream be duplicated and consumed independently?

Question

How can a Node.js readable stream be duplicated and consumed independently?

Is there a way to duplicate or tee a Node.js readable stream so that the streams can be consumed independently from the same source?

My use case looks something like this:

f(inputStream, callback(err, data) {
  // do stuff with data to change how g behaves
  g(inputStream);
});

The requirement is that f and g start reading from the beginning of the stream. I would normally use readable.pipe here but g needs to happen in the callback.

javascript
node.js

Answer 1

You can pass the same readable stream to both f and g. They will read from the stream independent of one another.

For example:-

var fs = require('fs');
var str = fs.createReadStream('<file location>');


function f(s) {
s.on('data', function (d) {
        console.log(d.toString());
});
s.on('end', function() {
        console.log('ended');
});
}
function g(s) {
s.on('data', function (d) {
        console.log(d.toString());
});
s.on('end', function() {
        console.log('ended');
});
}

g(str);
f(str);