How to implement a writable stream

Question

I want to pipe data from an amazon kinesis stream to a an s3 log or a bunyan log.

The sample works with a file write stream or stdout. How would I implmeny my own writable stream?

//this works
var file = fs.createWriteStream('my.log')
kinesisSource.pipe(file)

this doesn't work saying it has no method 'on'

var stream = {}; //process.stdout works however
stream.writable = true;
stream.write =function(data){
    console.log(data);
};
kinesisSource.pipe(stream);

what methods do I have to implement for my own custom writable stream, the docs seem to indicate I need to implement 'write' and not 'on'

Answer 1

Actually to create a writeable stream is quite simple. Here's is the example:

var fs = require('fs');
var Stream = require('stream');

var ws = new Stream;
ws.writable = true;
ws.bytes = 0;

ws.write = function(buf) {
   ws.bytes += buf.length;
}

ws.end = function(buf) {
   if(arguments.length) ws.write(buf);
   ws.writable = false;

   console.log('bytes length: ' + ws.bytes);
}

fs.createReadStream('file path').pipe(ws);

Also if you want to create your own class, @Paul give a good answer.

Answer 2

Here is an example directly from nodejs docs
https://nodejs.org/api/stream.html#an-example-writable-stream

const { Writable } = require('stream');
class MyWritable extends Writable {
  _write(chunk, encoding, callback) {
    if (chunk.toString().indexOf('a') >= 0) {
      callback(new Error('chunk is invalid'));
    } else {
      callback();
    }
  }
}