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- Hello again and welcome to lesson four.

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This lesson is all about
testing asynchronous code.

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This is a really important aspect

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in industrial-strength web applications.

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We'll begin by testing asynchronous code

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that invokes a callback
when it's finished.

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We'll see how to verify the
callback is invoked correctly.

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We'll also see how to specify
an extended time mode,

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which is important if your
code is particularly slow.

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Next up, we'll see how to test promises.

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We'll recap how promises work.

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We'll see how to test promises,

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and we'll see how to use the
async and the await keywords

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to simplify how we handle promises

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in our code and in our tests.

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To finish off the
lesson, we'll take a look

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at Rxjs, reactive
extensions for JavaScript.

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This library introduces the
concept of observable objects.

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We'll explain how to use these

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and how to test them in
our Jest application tests.