Teardown

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So far, we've been writing just one test using a single it() block. However, in most situations, we will need to write many tests for a particular feature that get executed in succession.

Running multiple test can introduce issues if the tests make changes to the testing environment: changes to the environment in one test might affect the next test. Some common changes to an environment include:

• altering files and directory structure
• changing read and write permissions on a file
• editing records in a database

To address this issue, we often add a teardown step to the end of our tests. The teardown phase makes our tests isolated by resetting the environment before the next test runs. This provides two key benefits:

• Changes of the environment caused by one test do not affect the other tests.
• Isolated tests can be executed in any order.

Note: In some cases -including the previous exercises- the teardown phase isn't needed because there are no conditions to reset.

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This exercise uses Node's filesystem library fs in addition to assert. It's okay if you're not familiar with fs -we will briefly explain the key methods we are using:

• fs.appendFileSync(path, str) creates a new file at path with the string str as content. If a file at path exist, the string str will be appended to the end.
• fs.readFileSync(path) returns the contents of the file found at path.
  The first test found in this exercise verifies that we can use fs.appendFileSync() to create a file called ./message.txt with the string 'Hello Node.js'. The second test verifies we can do the same but with an empty string.
  If you open up your file system, you'll see that the file message.txt doesn't exist yet. Run the test in the terminal. Why does the first test pass but the second test fails?

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Note the new file message.txt created in your directory. It should have the text Hello Node.js.
In both tests, we are modifying the enviroment by creating a new message.txt file. However, the current tests do not have a teardown phase. The test fails because the second test appends an empty string to the file created from the first test instead of creating a new file with just an empty string.
Furthermore, if you were to run the test file again, the first test would now fail. Run the test again and see for yourself - the message.txt file will now have the text Hello Node.jsHelloNode.js.

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You got different output because the test was not isolated.

• In the first run the file didn't exist, so fs.appendFileSync() created a message.txt file containing the string.
• In the second run, the method added the string to the end of message.txt because it already existed.
• The second test run failed because the string Hello Node.js was added twice to the same file!
  In the terminal enter rm message.txt to delete the file.

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To ensure that each test is isolated from each other, and that each run of the test file isolated, we will add a teardown step to the tests. In this case, we want to make sure that after each test, the file found at path is deleted.