How to use node's transform stream in rxjs? - node.js

Question

I've been playing with rxjs for some time now and I like how I can use it's operators for logic rather than imperative programming.
However, I also like node's stream which are also highly composable so my obvious reaction was to use them both but I haven't seen it being mentioned a lot (actually, I haven't at all) besides the binding for it in rxjs's book.
So, my question really is, how do I make use of all the transform streams that are in npm on RxJS? Or, is it even possible?
Example:-
var fs = require('fs');
var csv = require('csv-parse')({delimiter:';'});
var src = fs.createReadStream('./myFile.csv');
src.pipe(csv).pipe(process.stdout);
Essentially, I would want to do this:-
var fs = require('fs');
var csv = require('csv-parse')({delimiter:';'});
var rx= require('rx-node');
var src = fs.createReadStream('./myFile.csv');
var obj = rx.fromReadableStream(src);
obj.pipe(csb).map(x=>console.log(x));
I've been told to use highland in the past but I'm strictly looking for rxjs solution here.

Answer 1

You don't have to use rx-node but you can! Remember: All streams are event emitters!.
Prepare:
input.txt
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
Run:
npm install through2 split2 rx rx-node
And in the index.js:
var Rx = require('rx');
Rx.Node = require('rx-node');
var fs = require('fs');
var th2 = require('through2');
var split2 = require('split2');
var file = fs.createReadStream('./input.txt').on('error', console.log.bind(console, 'fs err'));
var transform = th2(function(ch, en, cb) {
cb(null, ch.toString());
}).on('error', function(err) {
console.log(err, err.toString());
});
// All streams are event emitters ! (one way without using rx-node)
// var subs = Rx.Observable.fromEvent(transform, 'data').share();
// subs
// .map(value => 'Begin line: ' + value)
// .subscribe(value => console.log(value));
// rx-node has convenience functions (another way)
var subs = Rx.Node.fromTransformStream(transform).share()
.map(value => 'Begin line: ' + value)
.subscribe(value => console.log(value));
file.pipe(split2()).pipe(transform);
Output:
Begin line: Hello World!
Begin line: Hello World!
Begin line: Hello World!
Begin line: Hello World!
Begin line: Hello World!

Answer 2

EdinM gave a great general example of using RxJS with a node transform stream, but your original question remains unanswered. Since I had nearly the same question a few days ago, I wanted to make an effort at answering it for anyone who isn't familiar with using RxJS with Node. Instead of using the csv-parse module, I'm going with csv-streamify. Let's set up the basic structure:
test_data.csv:
thing,name,owner,loc
chair,sitty,billy,san fran
table,setty,bryan,new oak
Run:
$ npm install rx rx-node csv-streamify
index.js:
"use strict";
const Rx = require('rx');
Rx.Node = require('rx-node');
const fs = require('fs');
const csv = require('csv-streamify');
//Setting up the transform-stream CSV parser
let config = {
delimiter: ',', // comma, semicolon, whatever
newline: '\n', // newline character (use \r\n for CRLF files)
quote: '"', // what's considered a quote
empty: '', // empty fields are replaced by this
//objectMode: true, //parses csv table into an array of objects
//columns: true //uses column headers for the object fields
};
let parseCsv = csv(config);
//Setting up the RxJS Observer
function onNext (x) {
//do your side-effects here, after the data has
//gone through the observables operator chain
console.log('Next: ' + x);
};
function onError (err) {
console.log('Error: ' + err);
};
function onComplete () {
console.log('Completed');
};
let readStream = fs.createReadStream('test_files/test_data.csv');
readStream.pipe(parseCsv);
let subscription = Rx.Node.fromTransformStream(parseCsv)
//do something with the data with an operator such as:
//.map()
.subscribe(onNext, onError, onComplete);
Now let's run the code:
$ node index.js
And we'll get this output:
Next: ["thing","name","owner","loc\r"]
Next: ["chair","sitty","billy","san fran\r"]
Next: ["table","setty","bryan","new oak"]
Completed
Epilogue
If you set objectMode and columns to true in the csv config object, and then project this sideEffect function with the map operator like so:
function sideEffect (v){
console.log(v)
return v;
};
let subscription = Rx.Node.fromTransformStream(parseCsv)
.map(sideEffect)
.subscribe(onNext, onError, onComplete);
You'll get this output:
{ thing: 'chair',
name: 'sitty',
owner: 'billy',
'loc\r': 'san fran\r' }
Next: [object Object]
{ thing: 'table',
name: 'setty',
owner: 'bryan',
'loc\r': 'new oak' }
Next: [object Object]
Completed