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- Hello again,

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and welcome to Lesson 2.

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This lesson is all about Jest matchers.

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Jest matchers are functions
in the Jest library

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that enable you to write tests
concisely and expressively.

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We'll begin with a look at matchers

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for test inequality, range, and null,

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undefined, not a number,
and true and false.

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Then we'll see how to use Jest matchers

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to test properties in objects

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and how to test the
elements in a collection.

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We'll also see how to test for errors.

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If we call a function

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and expect an error to occur,

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we can test that the
function did actually cause

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the error to occur.

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At the end of the lesson,

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we'll see how to extend
the Jest matcher library

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by writing our own custom matchers,

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for example, to test if a
string is a valid postcode

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or something like that.