Example using switchMap

package.json

{
  "name": "extra",
  "version": "1.0.0",
  "description": "",
  "main": "test.js",
  "scripts": {
    "test": "mocha"
  },
  "author": "Mike Harris",
  "license": "ISC",
  "dependencies": {
    "mocha": "^4.0.1",
    "rxjs": "^5.4.3"
  }
}

test.js

const assert = require('assert');
const Rx = require('rxjs/Rx');
const BehaviorSubject = require('rxjs/BehaviorSubject');

require('rxjs/add/operator/map');
require('rxjs/add/operator/switchMap');
require('rxjs/add/operator/first');
require('rxjs/add/operator/filter');

describe('switchMap', () => {
  it('should pass value on', () => {
    const value = 42;
    let actual;
    
    Rx.Observable.from([value])
      .map(x => x + 1)
      .switchMap(x => Rx.Observable.from([x + 1]))
      .first()
      .subscribe(x => actual = x);

    assert.equal(actual, value + 1 + 1);
  }),
  it('should call next observable on change of first', () => {
    const change$ = new Rx.BehaviorSubject(0);
    let actual = null;

    change$
      .filter(x => x !== 0)
      .switchMap(x => Rx.Observable.from([1, 2, 3, 4]))
      .subscribe(x => actual = x);

    // default is 0
    assert.equal(actual, null);

    // every 0 is filtered
    change$.next(0);
    assert.equal(actual, null);

    change$.next(1);
    assert.equal(actual, 4);
  });
});