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[MUSIC]

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Hey, I'm Treasure, web developer and
teacher here at Treehouse.

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Writing asynchronous code
in Express often involves

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acting upon data whenever
you happen to receive it.

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For example, say I want to retrieve
a list of movies from a database and

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render that movie data into a template.

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Because of the asynchronous nature of
working with the database, there's no

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guarantee I'll get that movie information
back before Express renders the template.

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The result, an empty HTML
template sent to the browser.

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To keep that from happening,
you need a way to make sure you have

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all the data you need before
the template is rendered.

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Traditionally, Express uses call backs
to deal with asynchronous tasks.

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You'd call a function that
retrieves a list of movies, and

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that function would take
a call back that runs when and

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only when the list of movies has returned,
or an error has occurred.

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This is fine if the actions
you're performing are simple.

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It's when you have several asynchronous
tasks to perform before you send something

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to the browser that things can get messy.

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For example, if you needed to retrieve a
certain subset of movies from a database,

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then the ratings for

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those movies, then a list of actors
who starred in those movies.

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You'd end up with a call back, nested
in a call back, nested in a call back.

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We'll see an example of
this in a later video.

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This is one reason that approaches to
asynchronous programming in JavaScript

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have evolved and
changed a lot over the years.

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Making the async landscape sometimes
difficult and overwhelming to navigate.

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Recently, new approaches have become
available natively in JavaScript.

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In this course we'll look more
closely at two other approaches and

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how you can use them in Express,
Promises and async/await.

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These approaches are used by
popular libraries like Mongoose and

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Sequelize that help you
interact with databases.

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As a developer, you're likely to encounter
and will need to understand how these

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three approaches, callbacks, Promises,
and async/await are used in Express.

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This course assumes that you're
familiar with the basics

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of building a server side
application with Express.

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You should understand what
asynchronous programming is,

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feel comfortable with callbacks, and
have some familiarity with Promises.

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If you've never heard of
asynchronous programing or Promises,

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see the teacher's notes for resources
that should help you get up to speed.

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While asynchronous programming can be
a big and confounding topic in JavaScript,

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I hope you'll finish this workshop feeling
more confident in your knowledge about how

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each of these approaches are used
to build applications in Express.