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- Hello again, and welcome to Lesson 6.

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In this lesson, we'll take
a closer look at mocking

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in Jest.

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We'll begin by discussing
the concept of spies.

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A spy is a just construct
that enables you to observe

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how function is invoked by your code.

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Next, we'll see how to mock timers.

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This enables you to
emulate the passage of time

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so your tests can run more
quickly when they involve timers.

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The last few sections in this lesson

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are all about how to
test ECMAScript classes.

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So first, we'll begin with a quick recap

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of how to define ECMAScript classes

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and then we'll see how to mock
a class using automatic mocks

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and using manual mocks.