Promise Composition

https://github.com/eu81273/jsfiddle-console

by dimitrs_papadimitriou

HTML

<script src="https://rawgit.com/eu81273/jsfiddle-console/master/console.js"></script>
<script src="https://code.jquery.com/jquery-3.4.0.js"></script>

JavaScript

//Rediscovering Promises in Javascript
  //https://medium.com/@dimpapadim3/promises-made-simple-in-javascript-db9e3bc39537
  //https://youtu.be/uPpTOwA2vXU

  Promise.prototype.map = function(mapping) {
    var initialPromise = this;
    return new Promise(function(resolve) {
      initialPromise.then(result => resolve(mapping(result)))
    });
  }

  var EitherCoyoAsync = function(actions, g) {
    this.g = g;

    this.map = (f) => new EitherCoyoAsync(actions, x => f(this.g(x)));
    this.cata = alg => actions(x => alg.ok(this.lower(x)), alg.error);
    this.lower = (v) => this.g(v);
  }

  Promise.prototype.toEither = function() {
    var initialPromise = this;
    var either = new EitherCoyoAsync(function(resolve, reject) {
      initialPromise.then(resolve).catch(reject)
    }, x => x);
    return either;
  }

  
  fetch("https://api.github.com/users")
   .map(response => response.json()) //this is promise map 
   .toEither()  
   .map(users=>users.map(u=>u.login))//this is EitherCoyoAsync map 
   .cata({
      ok: v => console.log(v),
      error: v => console.log("left" + v)
    });