How to pipe the same readable stream into multiple (writable) targets with Node.js?

To pipe the same readable stream into multiple (writable) targets with Node.js, we call the pipe method on each stream.

For instance, we write

const spawn = require("child_process").spawn;
const PassThrough = require("stream").PassThrough;

const a = spawn("echo", ["hi user"]);
const b = new PassThrough();
const c = new PassThrough();

a.stdout.pipe(b);
a.stdout.pipe(c);

let count = 0;
b.on("data", (chunk) => {
  count += chunk.length;
});
b.on("end", () => {
  console.log(count);
  c.pipe(process.stdout);
});

to create 2 PassThrough objects.

Then we call pipe to pipe our input from a to to the PassThrough objects with pipe to write to them.

We get the data written from the 'data' event handler’s chunk parameter.