Set rate using RxJS5 - node.js

Question

I have this code which just reads in data from a .csv file and converts it to json and logs the data:
const fs = require('fs');
const path = require('path');
const sd = path.resolve(__dirname + '/fixtures/SampleData.csv');
const strm = fs.createReadStream(sd).setEncoding('utf8');
const Rx = require('rxjs/Rx');
const csv2json = require('csv2json');
const dest = strm
.pipe(csv2json({
separator: ','
}));
dest.on('error', function(e){
console.error(e.stack || e);
})
const obs = Rx.Observable.fromEvent(dest, 'data')
.flatMap(d => Rx.Observable.timer(100).mapTo(d))
obs.subscribe(v => {
console.log(String(v));
})
What the code is doing is logging all the data after a 100 ms delay. I actually want to delay on each line of data and log each line after a small delay.
The above code doesn't achieve that - what is the best way to control the rate at which the data is logged?
Hypothesis: All the lines of data come in approximately at the same time, so all are delayed 100 ms, so they end up getting printed at pretty much the same time. I need to only start delaying the next line after the previous as been logged.
the following code seems to do the same thing as using the timer above:
const obs = Rx.Observable.fromEvent(dest, 'data')
.delay(100)

Answer 1

Hypothesis: All the lines of data come in approximately at the same
time, so all are delayed 100 ms, so they end up getting printed at
pretty much the same time. I need to only start delaying the next line
after the previous as been logged.
Your hypothesis is correct
Solution
Swap out the .flatMap() in your original solution with .concatMap()
Rx.Observable.from([1,2,3,4])
.mergeMap(i => Rx.Observable.timer(500).mapTo(i))
.subscribe(val => console.log('mergeMap value: ' + val));
Rx.Observable.from([1,2,3,4])
.concatMap(i => Rx.Observable.timer(500).mapTo(i))
.subscribe(val => console.log('concatMap value: ' + val));
<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/5.0.3/Rx.js"></script>
This will ensure that every emission completes before the next emission is subscribed to and starts delaying its value.

Answer 2

I couldn't find the functionality I needed in the RxJS library (although it might be there, I just couldn't find it, let me know if there is a better, more idiomatic, way).
So I wrote this, which seems to do the job:
const fs = require('fs');
const path = require('path');
const sd = path.resolve(__dirname + '/fixtures/SampleData.csv');
const strm = fs.createReadStream(sd).setEncoding('utf8');
const Rx = require('rxjs/Rx');
const csv2json = require('csv2json');
const p = Rx.Observable.prototype;
p.eachWait = function(timeout){
const source = this;
const values = [];
let flipped = true;
const onNext = function (sub){
flipped = false;
setTimeout(() => {
var c = values.pop();
if(c) sub.next(c);
if(values.length > 0){
onNext(sub);
}
else{
flipped = true;
}
}, timeout);
}
return Rx.Observable.create(sub => {
return source.subscribe(
function next(v){
values.unshift(v);
if(flipped){
onNext(sub);
}
},
sub.error.bind(sub),
sub.complete.bind(sub)
);
});
}
const dest = strm
.pipe(csv2json({
separator: ','
}));
dest.on('error', function(e){
console.error(e.stack || e);
});
const obs = Rx.Observable.fromEvent(dest, 'data')
.eachWait(1000)
obs.subscribe(v => {
console.log(String(v));
});
I assume this is as about as performant as you can make it - only one timer should be running at any given moment.