Examples of using Promise.

promise-example-parallel.js

// Start from a Promise.
var foo = new Promise(function (resolve, reject) {
  // Has to resolve to run callbacks in `.then()`.
  resolve(1);
});

foo.then(function (result) {
  // Received final resolve value from the Promise.
  // result === 1;
  console.log('result 1', result);
  // Returned value is passed to the callback in the next `.then()`.
  return result + 1;
}).then(function (result) {
  // Received final resolve value from the Promise in the callback in the previous `.then()`.
  // result === 2;
  console.log('result 2', result);
});

foo.then(function (result) {
  // Received final resolve value from the Promise.
  // result === 1;
  console.log('result 3', result);
  // Returned value is passed to the callback in the next `.then()`.
  return result + 1;
}).then(function (result) {
  // Received final resolve value from the Promise in the callback in the previous `.then()`.
  // result === 2;
  console.log('result 4', result);
});

// Output:
// result 1 1
// result 3 1
// result 2 2
// result 4 2

promise-example-serial.js

// Start from a Promise.
(new Promise(function (resolve, reject) {
  // Has to resolve to run callbacks in `.then()`.
  resolve(
    // Pass another Promise as the resolve value to wait for the fulfillment of that Promise.
    new Promise(function (resolve, reject) {
      // Final resolve value will be passed to the callback in the next `.then()`.
      resolve(1);
    })
  );
})).then(function (result) {
  // Received final resolve value from the Promise.
  // result === 1;
  console.log('result', result);
  // Returned value is passed to the callback in the next `.then()`.
  return result + 1;
}).then(function (result) {
  // Received return value from the previous callback in `.then()`.
  // result === 2;
  console.log('result', result);
  return new Promise(function (resolve, reject) {
    // Final resolve value will be passed to the callback in the next `.then()`.
    resolve(result + 1);
  });
}).then(function (result) {
  // Received final resolve value from the Promise in the callback in the previous `.then()`.
  // result === 3;
  console.log('result', result);
});

// Output:
// result 1
// result 2
// result 3