RxJS: How may I treat bindNodeCallback() as an Observable that can have operators chained to it?

Question

For RxJS learning purposes, I'm building a small node.js app that puts everything in a stream where possible.

As a first step, I'm working to load an external configuration yaml file. I've seen examples where fs.readFile is wrapped in a bindNodeCallback, which returns an observable when called. However, the readFile variable set below is still a typeof of 'function'. As a result, I'm unable to chain flatMap to it.

How may I reorganize the code below to chain bindNodeCallback() and yaml.safeLoad() (which doesn't offer a callback) before initializing my app?

const yaml = require('js-yaml');
const fs = require('fs');
const { bindNodeCallback, of } = require('rxjs');
const { mergeMap, map } = require('rxjs/operators');

const configPath = './config/config.yml';
const configEncoding = 'utf8';

const readFile = bindNodeCallback(fs.readFile);

readFile(configPath, configEncoding)
.flatMap(yamlString => of(yaml.safeLoad(yamlString))) // error: .flatMap is not a funciton
.subscribe(
    config => {
        console.log(config);
        // launch the app when config is loaded
    }, 
    err => console.error(err)
);

Answer 1

readFile should still be a function, and it does return an Observable when executed, but flatMap is not a method on the Observable that's returned. If you get rid of the call to .flatMap() and jump straight to .subscribe(), that works.

Were you trying to use flatMap from rxjs/operators, or the chaining interface?

---

This works, but I had to yarn add rxjs-compat for some reason. Also note that I swapped to using Observable#pipe() with flatMap as a pipeable operator.

const fs = require('fs')
const yaml = require('js-yaml')
const { bindNodeCallback } = require('rxjs')
const { flatMap } = require('rxjs/operators')
const { of } = require('rxjs/observable/of')

const configPath = './config.yml'
const configEncoding = 'utf8'

const readFile = bindNodeCallback(fs.readFile)

readFile(configPath, configEncoding)
  .pipe(flatMap(yamlString => of(yaml.safeLoad(yamlString))))
  .subscribe(
    config => {
      console.log(config)
    },
    err => console.error(err),
  )