rxjs.js

const { of } = require('rxjs');
const { map } = require('rxjs/operators');

const append = (text) => map(x => x + text);
const removeTheLetter = (letter) => map(x => 
  x.split('')
  .filter(c => c !== letter)
  .join(''));

const makeLetterUpperCase = (letter) => map(x => 
  x.split('')
  .map(c => c === letter ? c.toUpperCase() : c)
  .join(''));

const removeWordsWithLessThanFourLetters = map(x => 
  x.split(' ')
  .filter(word => word.length > 4)
  .join(' '));