promises.js

'use strict';

var promise = {
  state: 'pending',

  value: null,

  dependencies: [],

  then: function(expression) {
    var result = createPromise();

    if( this.state === 'pending' ) {
      var self = this;
      this.dependencies.push( function(value) {
        expression(value).then(function(newValue) {
          result.fulfil(newValue);

          return createPromise();
        });
      });

    }
    else {

      expression(this.value).then(function(newValue) {
        result.fulfil(newValue);

        return createPromise();
      })

    }

    return result;
  },

  fulfil: function(value) {
    if (this.state !== 'pending') {
      throw new Error('Trying to fulfil an already fulfilled promise!');
    }
    else {
      this.state = 'fulfilled';
      this.value = value;

      var dependencies = this.dependencies;
      this.dependencies = [];

      dependencies.forEach(function(expression) {
        expression(value);
      });
    }
  }
};

var squareAreaAbstraction = function(side) {
  var result = createPromise();

  result.fulfil(side * side);
  return result;
};

var printAbstraction = function(squareArea) {
  var result = createPromise();
  result.fulfil(console.log(squareArea));
  return result;
};

var sidePromise = createPromise();
var squareAreaPromise = sidePromise.then(squareAreaAbstraction);
sidePromise.fulfil(10);

var printPromise = squareAreaPromise.then(printAbstraction);



function createPromise() {
  var p = Object.create(promise);
  p.value = null;
  p.dependencies = [];
  p.state = 'pending';
  return p;
}