copy.js

// This function writes the specified chunk to the specified stream and
// returns a Promise that will be fulfilled when it is OK to write again.
// Because it returns a Promise, it can be used with await.
function write(stream, chunk) {
    // Write the specified chunk to the specified stream
    let hasMoreRoom = stream.write(chunk);

    if (hasMoreRoom) {                     // If buffer is not full, return
        return Promise.resolve(null);      // an already resolved Promise object
    } else {
        return new Promise(resolve => {    // Otherwise, return a Promise that
            stream.once("drain", resolve); // resolves on the drain event.
        });
    }
}

// Copy data from the source stream to the destination stream
// respecting backpressure from the destination stream.
// This is much like calling source.pipe(destination).
async function copy(source, destination) {
    // Set an error handler on the destination stream in case standard
    // output closes unexpectedly (when piping output to `head`, e.g.)
    destination.on("error", err => process.exit());

    // Use a for/await loop to asynchronously read chunks from the input stream
    for await (let chunk of source) {
        // Write the chunk and wait until there is more room in the buffer.
        await write(destination, chunk);
    }
}

// Copy standard input to standard output
copy(process.stdin, process.stdout);