What is the process for retrieving the result of a promise at a later time?

Question

What is the process for retrieving the result of a promise at a later time?

var screencastId = 'abc'

var a = youtube.get(screencastId);
a.then(function(screencast) {
  // Great, the screencast information is now available.
  console.log(screencast);
});

// How can I access the `screencast` variable below?
connection.beginTransactionAsync().then(function() {
  return connection.queryAsync('INSERT IGNORE INTO Channels SET ?', screencast.channel);
}).then(function() {
  return connection.queryAsync('INSERT INTO Screencasts SET ?', screencast);
}).then(function() {
  var values = tags.map(function(tag) { return [tag]; });
  return connection.queryAsync('INSERT IGNORE INTO Tags VALUES ?', [values])
}).then(function() {
  var values = tags.map(function(tag) { return [screencast.screencastId, tag]; });
  return connection.queryAsync('INSERT INTO ScreencastTags VALUES ?', [values])
}).then(function() {
  return connection.commit();
}).error(function(e) {
  connection.rollback();
  winston.error(e);
});

This code breaks down into two main steps:

Retrieve data about a particular screencast from YouTube via the youtube module.
Save details regarding that screencast in the database.

Both of these processes are asynchronous by nature.

My inquiry is, how do I access the screencast parameter during the second step?

I came across this detailed response, but as someone with limited experience in JavaScript, I'm unsure how to implement it in this scenario.

javascript promise bluebird

Answer 1

Answer №1

There are two approaches to solving this issue:

You can create a global variable called screencast, similar to how you have screencastId, which is accessible across all .then calls.
If using a global variable is not feasible, you can also return a new Promise in each .then statement. In the resolve function of the promise, pass the parameter screencast.

Here's an example of the first approach (without being able to test it):

var screencastId = 'abc', screencast

youtube.get(screencastId)
.then(function(sc) {
  screencast = sc;
})

// To maintain the chain
.then(function(){
  return connection.beginTransactionAsync()
})

.then(function() {
  return connection.queryAsync('INSERT IGNORE INTO Channels SET ?', screencast.channel);
}).then(function() {
  return connection.queryAsync('INSERT INTO Screencasts SET ?', screencast);
}).then(function() {
  var values = tags.map(function(tag) { return [tag]; });
  return connection.queryAsync('INSERT IGNORE INTO Tags VALUES ?', [values])
}).then(function() {
  var values = tags.map(function(tag) { return [screencast.screencastId, tag]; });
  return connection.queryAsync('INSERT INTO ScreencastTags VALUES ?', [values])
}).then(function() {
  return connection.commit();
}).error(function(e) {
  connection.rollback();
  winston.error(e);
});

Using a new Promise in the example:

youtube.get(screencastId)
.then(function(sc) {
  return new Promise(function(resolve,reject){
    resolve(sc)
  })
})
.then(function(sc){
  console.log(sc)
})

Answer 2

There are two approaches to solving this issue:

You can create a global variable called screencast, similar to how you have screencastId, which is accessible across all .then calls.
If using a global variable is not feasible, you can also return a new Promise in each .then statement. In the resolve function of the promise, pass the parameter screencast.

Here's an example of the first approach (without being able to test it):

var screencastId = 'abc', screencast

youtube.get(screencastId)
.then(function(sc) {
  screencast = sc;
})

// To maintain the chain
.then(function(){
  return connection.beginTransactionAsync()
})

.then(function() {
  return connection.queryAsync('INSERT IGNORE INTO Channels SET ?', screencast.channel);
}).then(function() {
  return connection.queryAsync('INSERT INTO Screencasts SET ?', screencast);
}).then(function() {
  var values = tags.map(function(tag) { return [tag]; });
  return connection.queryAsync('INSERT IGNORE INTO Tags VALUES ?', [values])
}).then(function() {
  var values = tags.map(function(tag) { return [screencast.screencastId, tag]; });
  return connection.queryAsync('INSERT INTO ScreencastTags VALUES ?', [values])
}).then(function() {
  return connection.commit();
}).error(function(e) {
  connection.rollback();
  winston.error(e);
});

Using a new Promise in the example:

youtube.get(screencastId)
.then(function(sc) {
  return new Promise(function(resolve,reject){
    resolve(sc)
  })
})
.then(function(sc){
  console.log(sc)
})

Answer 3

Answer №2

In my personal view, a good approach would be to declare a global variable called `globalScreencast` and assign the value of the `screencast` in the first step like this:

a.then(function(screencast) {
    // Great! I now have access to screencast information.
    console.log(screencast);
    globalScreencast = screencast;
});

Now, in the subsequent steps, you can easily use the `globalScreencast` variable.

Answer 4

In my personal view, a good approach would be to declare a global variable called `globalScreencast` and assign the value of the `screencast` in the first step like this:

a.then(function(screencast) {
    // Great! I now have access to screencast information.
    console.log(screencast);
    globalScreencast = screencast;
});

Now, in the subsequent steps, you can easily use the `globalScreencast` variable.

What is the process for retrieving the result of a promise at a later time?

Answer №1

Answer №2

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