Notes about promises

native_promise_sample.js

function updateTask(oldTask, newTask) {
  var promise = new Promise(function (resolve, reject) {
    resolve({status: 'success'});
    // resolve(Error('some error'));
  });
  return promise;
}  

promise_angular.js

var postFile = function(name, file) {
  var deferred = $q.defer();

  var form = new FormData();
  form.append('file', file);

  var xhr = new XMLHttpRequest();
  xhr.open('POST', apiUrl + name, true);
  xhr.onload = function(e) {
      if (e.target.status == 200) {
          deferred.resolve();
      } else {
          deferred.reject(e.target.status);
      }
      if (!$rootScope.$$phase) $rootScope.$apply();
  };
  xhr.send(form);

  return deferred.promise;
};

rules.md

API

Based on stackoverlow

We have a problem with promises

У нас проблема с промисами

Tips

Асинхронный JavaScript. Прошлое, настоящее и будущее

It's pretty straightforward with some simple rules:

• Whenever you create a promise in a then, return it - any promise you don't return will not be waited for outside.
• Whenever you create multiple promises, .all them - that way it waits for all the promises and no errors from any of them are silenced.
• Whenever you nest thens, you can typically return in the middle - then chains are usually at most 1 level deep.
• Whenever you perform IO, it should be with a promise - either it should be in a promise or it should use a promise to signal its completion.

And some tips:

• Mapping is better done with .map than with for/push - if you're mapping values with a function, map lets you concisely express the notion of applying actions one by one and aggregating the result.
• Concurrency is better than sequential execution if it's free - it's better to execute things concurrently and wait for them Promise.all then to execute things one after the other - each waiting before the next.