Spies, Stubs, and Clocks

First and foremost, why do we need to control some of the behaviors of our application? Wouldn’t we want to test it from a user’s perspective without modifying anything programmatically? There can be several reasons for controlling the original behavior of our app. Let’s take a look at some.

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Reasons for controlling behavior

When we want to modify the behavior of our application, here are a couple of scenarios that will make our test suite more robust:

• Avoid side effects, such as a bad network connection
• We are dealing with Promises, and we want to automatically resolve or reject them.
• We want to force things to fail to test a failure path.
• We want

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