Check out a free preview of the full Testing JavaScript Applications (feat. React and Redux) course:
The "Assertions" Lesson is part of the full, Testing JavaScript Applications (feat. React and Redux) course featured in this preview video. Here's what you'd learn in this lesson:

Kent introduces Jest's expect() function. When you're writing tests, you often need to check that values meet certain conditions and expect() gives you access to some "matchers" that let you validate different things depending on what needs to be tested. - https://facebook.github.io/jest/docs/expect.html

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