Runthrough of Promise control flow

promise.js

var a = new Promise(function (resolve, reject) {
  reject(1)
})

function b_success() {
  console.log('b: success', err);
  // not called...
  return new Promise(function (resolve, reject) {
    reject(2)
  })
}

function b_fail(err) {
  console.log('b: fail', err);
  throw err;
}

function c_success(value) {
  // called
  console.log('c: success', value);
}

function c_fail(err) {
  // not called
  console.log('c: fail', err);
  return err
}

function d_success(value) {
  // called
  console.log('d: success', value);

  // new promise will update the success value
  // when it completes, vs. just returning value
  return new Promise(function(resolve, reject) {
    resolve(2);
  })
}

function d_fail(err) {
  // not called
  console.log('d: fail', err);
}

function e_success(value) {
  // called
  console.log('e: success', value);
}

function e_fail(err) {
  // not called
  console.log('e: fail', err);
}

a
  .then(b_success, b_fail)
  .then(c_success, c_fail)
  .then(d_success, d_fail)
  .then(e_success, e_fail)

// Yields:
//
// b: fail 1
// c: fail 1
// d: success 1
// e: success 2