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// Rookie mistake #2: WTF, how do I use forEach() with promises?
// I want to remove() all docs
db.allDocs({include_docs: true}).then(function (result) {
  result.rows.forEach(function (row) {
    db.remove(row.doc);  
  });
}).then(function () {
  // I naively believe all docs have been removed() now!
});
// right way
db.allDocs({include_docs: true}).then(function (result) {
  return Promise.all(result.rows.map(function (row) {
    return db.remove(row.doc);
  }));
}).then(function (arrayOfResults) {
  // All docs have really been removed() now!
});


// Rookie mistake #3: forgetting to add .catch()
somePromise().then(function () {
  return anotherPromise();
}).then(function () {
  return yetAnotherPromise();
}).catch(console.log.bind(console));



// Rookie mistake #4: using "deferred"
$q.when(db.put(doc)).then(/* ... */); // <-- this is all the code you need

// Another strategy is to use the revealing constructor pattern, 
// which is useful for wrapping non-promise APIs. For instance, 
// to wrap a callback-based API like Node's fs.readFile(), you can simply do:

new Promise(function (resolve, reject) {
  fs.readFile('myfile.txt', function (err, file) {
    if (err) {
      return reject(err);
    }
    resolve(file);
  });
}).then(/* ... */)


// Rookie mistake #5: using side effects instead of returning
//BAD
somePromise().then(function () {
  someOtherPromise();
}).then(function () {
  // Gee, I hope someOtherPromise() has resolved!
  // Spoiler alert: it hasn't.
});


// Okay, this is a good point to talk about everything you ever need to know about promises.
// Seriously, this is the one weird trick that, once you understand it, will prevent all of the errors I've been talking about. You ready?
// As I said before, the magic of promises is that they give us back our precious return and throw. But what does this actually look like in practice?
// Every promise gives you a then() method (or catch(), which is just sugar for then(null, ...)). Here we are inside of a then() function:
somePromise().then(function () {
  // I'm inside a then() function!
});
// What can we do here? There are three things:
// 1. return another promise
// 2. return a synchronous value (or undefined)
// 3. throw a synchronous error
// That's it. Once you understand this trick, you understand promises. So let's go through each point one at a time.
// 1. Return another promise
getUserByName('nolan').then(function (user) {
  return getUserAccountById(user.id);//return a promise
}).then(function (userAccount) {
  // I got a user account!
});
// 2. Return a synchronous value (or undefined)
getUserByName('nolan').then(function (user) {
  if (inMemoryCache[user.id]) {
    return inMemoryCache[user.id];    // returning a synchronous value!
  }
  return getUserAccountById(user.id); // returning a promise!
}).then(function (userAccount) {
  // I got a user account!
});
//3. Throw a synchronous error
getUserByName('nolan').then(function (user) {
  if (user.isLoggedOut()) {
    throw new Error('user logged out!'); // throwing a synchronous error!
  }
  if (inMemoryCache[user.id]) {
    return inMemoryCache[user.id];       // returning a synchronous value!
  }
  return getUserAccountById(user.id);    // returning a promise!
}).then(function (userAccount) {
  // I got a user account!
}).catch(function (err) {
  // Boo, I got an error!
});
// Advanced mistake #1: not knowing about Promise.resolve()
new Promise(function (resolve, reject) {
  resolve(someSynchronousValue);
}).then(/* ... */);
// You can express this more succinctly using Promise.resolve():
Promise.resolve(someSynchronousValue).then(/* ... */);
// This is also incredibly useful for catching any synchronous errors. 
// It's so useful, that I've gotten in the habit of beginning nearly all of my promise-returning API methods like this:
function somePromiseAPI() {
  return Promise.resolve().then(function () {
    doSomethingThatMayThrow();
    return 'foo';
  }).then(/* ... */);
}
// Advanced mistake #2: catch() isn't exactly like then(null, ...)

// Advanced mistake #3: promises vs promise factories
function executeSequentially(promiseFactories) {
  var result = Promise.resolve();
  promiseFactories.forEach(function (promiseFactory) {
    result = result.then(promiseFactory);
  });
  return result;
}
function myPromiseFactory() {
  return somethingThatCreatesAPromise();
}

// Advanced mistake #4: okay, what if I want the result of two promises?
// Wanting to be good JavaScript developers and avoid the pyramid of doom, we might just store the user object in a higher-scoped variable:
var user;
getUserByName('nolan').then(function (result) {
  user = result;
  return getUserAccountById(user.id);
}).then(function (userAccount) {
  // okay, I have both the "user" and the "userAccount"
});
// This works, but I personally find it a bit kludgey. My recommended strategy: just let go of your preconceptions and embrace the pyramid:
getUserByName('nolan').then(function (user) {
  return getUserAccountById(user.id).then(function (userAccount) {
    // okay, I have both the "user" and the "userAccount"
  });
});
// ...at least, temporarily. If the indentation ever becomes an issue, 
// then you can do what JavaScript developers have been doing since time immemorial, and extract the function into a named function:
function onGetUserAndUserAccount(user, userAccount) {
  return doSomething(user, userAccount);
}

function onGetUser(user) {
  return getUserAccountById(user.id).then(function (userAccount) {
    return onGetUserAndUserAccount(user, userAccount);
  });
}

getUserByName('nolan')
  .then(onGetUser)
  .then(function () {
  // at this point, doSomething() is done, and we are back to indentation 0
});

putYourRightFootIn()
.then(putYourRightFootOut)
.then(putYourRightFootIn)  
.then(shakeItAllAbout);

// Advanced mistake #5: promises fall through
// What do you think this code prints out?
Promise.resolve('foo').then(Promise.resolve('bar')).then(function (result) {
  console.log(result);
});
// If you think it prints out bar, you're mistaken. It actually prints out foo!

// The reason this happens is because when you pass then() a non-function (such as a promise),
//  it actually interprets it as then(null), which causes the previous promise's result to fall through. 
//  You can test this yourself:
Promise.resolve('foo').then(null).then(function (result) {
  console.log(result);
});
// so most likely you meant to do:
Promise.resolve('foo').then(function () {
  return Promise.resolve('bar');
}).then(function (result) {
  console.log(result);
});






//BASICS
/*================================================================
=            JavaScript Promises There and back again            =
================================================================*/
var promise = new Promise(function(resolve, reject) {
  // do a thing, possibly async, then…

  if (/* everything turned out fine */) {
    resolve("Stuff worked!");
  }
  else {
    reject(Error("It broke"));
  }
});

promise.then(function(result) {
  console.log(result); // "Stuff worked!"
}, function(err) {
  console.log(err); // Error: "It broke"
});

//Promisifying XMLHttpRequest
function get(url) {
  // Return a new promise.
  return new Promise(function(resolve, reject) {
    // Do the usual XHR stuff
    var req = new XMLHttpRequest();
    req.open('GET', url);

    req.onload = function() {
      // This is called even on 404 etc
      // so check the status
      if (req.status == 200) {
        // Resolve the promise with the response text
        resolve(req.response);
      }
      else {
        // Otherwise reject with the status text
        // which will hopefully be a meaningful error
        reject(Error(req.statusText));
      }
    };

    // Handle network errors
    req.onerror = function() {
      reject(Error("Network Error"));
    };

    // Make the request
    req.send();
  });
}

//Queuing asynchronous actions
getJSON('story.json').then(function(story) {
  return getJSON(story.chapterUrls[0]);
}).then(function(chapter1) {
  console.log("Got chapter 1!", chapter1);
});

var storyPromise;

function getChapter(i) {
  storyPromise = storyPromise || getJSON('story.json');

  return storyPromise.then(function(story) {
    return getJSON(story.chapterUrls[i]);
  })
}

// and using it is simple:
getChapter(0).then(function(chapter) {
  console.log(chapter);
  return getChapter(1);
}).then(function(chapter) {
  console.log(chapter);
});


// Creating a sequence
// Start off with a promise that always resolves
var sequence = Promise.resolve();

// Loop through our chapter urls
story.chapterUrls.forEach(function(chapterUrl) {
  // Add these actions to the end of the sequence
  sequence = sequence.then(function() {
    return getJSON(chapterUrl);
  }).then(function(chapter) {
    addHtmlToPage(chapter.html);
  });
});

// We can tidy up the above code using array.reduce:
// Loop through our chapter urls
story.chapterUrls.reduce(function(sequence, chapterUrl) {
  // Add these actions to the end of the sequence
  return sequence.then(function() {
    return getJSON(chapterUrl);
  }).then(function(chapter) {
    addHtmlToPage(chapter.html);
  });
}, Promise.resolve());
//put it all together
getJSON('story.json').then(function(story) {
  addHtmlToPage(story.heading);

  return story.chapterUrls.reduce(function(sequence, chapterUrl) {
    // Once the last chapter's promise is done…
    return sequence.then(function() {
      // …fetch the next chapter
      return getJSON(chapterUrl);
    }).then(function(chapter) {
      // and add it to the page
      addHtmlToPage(chapter.html);
    });
  }, Promise.resolve());
}).then(function() {
  // And we're all done!
  addTextToPage("All done");
}).catch(function(err) {
  // Catch any error that happened along the way
  addTextToPage("Argh, broken: " + err.message);
}).then(function() {
  // Always hide the spinner
  document.querySelector('.spinner').style.display = 'none';
});

//all
getJSON('story.json').then(function(story) {
  addHtmlToPage(story.heading);

  // Take an array of promises and wait on them all
  return Promise.all(
    // Map our array of chapter urls to
    // an array of chapter json promises
    story.chapterUrls.map(getJSON)
  );
}).then(function(chapters) {
  // Now we have the chapters jsons in order! Loop through…
  chapters.forEach(function(chapter) {
    // …and add to the page
    addHtmlToPage(chapter.html);
  });
  addTextToPage("All done");
}).catch(function(err) {
  // catch any error that happened so far
  addTextToPage("Argh, broken: " + err.message);
}).then(function() {
  document.querySelector('.spinner').style.display = 'none';
});

// best of all.
getJSON('story.json').then(function(story) {
  addHtmlToPage(story.heading);

  // Map our array of chapter urls to
  // an array of chapter json promises.
  // This makes sure they all download parallel.
  return story.chapterUrls.map(getJSON)
    .reduce(function(sequence, chapterPromise) {
      // Use reduce to chain the promises together,
      // adding content to the page for each chapter
      return sequence.then(function() {
        // Wait for everything in the sequence so far,
        // then wait for this chapter to arrive.
        return chapterPromise;
      }).then(function(chapter) {
        addHtmlToPage(chapter.html);
      });
    }, Promise.resolve());
}).then(function() {
  addTextToPage("All done");
}).catch(function(err) {
  // catch any error that happened along the way
  addTextToPage("Argh, broken: " + err.message);
}).then(function() {
  document.querySelector('.spinner').style.display = 'none';
});

// Generators
function spawn(generatorFunc) {
  function continuer(verb, arg) {
    var result;
    try {
      result = generator[verb](arg);
    } catch (err) {
      return Promise.reject(err);
    }
    if (result.done) {
      return result.value;
    } else {
      return Promise.resolve(result.value).then(onFulfilled, onRejected);
    }
  }
  var generator = generatorFunc();
  var onFulfilled = continuer.bind(continuer, "next");
  var onRejected = continuer.bind(continuer, "throw");
  return onFulfilled();
}
//
spawn(function *() {
  try {
    // 'yield' effectively does an async wait,
    // returning the result of the promise
    let story = yield getJSON('story.json');
    addHtmlToPage(story.heading);

    // Map our array of chapter urls to
    // an array of chapter json promises.
    // This makes sure they all download parallel.
    let chapterPromises = story.chapterUrls.map(getJSON);

    for (let chapterPromise of chapterPromises) {
      // Wait for each chapter to be ready, then add it to the page
      let chapter = yield chapterPromise;
      addHtmlToPage(chapter.html);
    }

    addTextToPage("All done");
  }
  catch (err) {
    // try/catch just works, rejected promises are thrown here
    addTextToPage("Argh, broken: " + err.message);
  }
  document.querySelector('.spinner').style.display = 'none';
});

/*-----  JavaScript Promises There and back again  ------*/