What happens when a test case fails?

This is what the console will display:

A reference error

Only the first failing assertion should be displayed within eachitblock. Moving from failing an earlier assertion to failing a later assertion means progress!

Here, we see that there is aReference Error. This isn’t actually a failing assertion, but instead suggests that the code being tested is throwing an error before the assertion is even being run.

A reference error typically indicates that the code is trying to use a variable that has never been declared.

Check out theMDN’s JavaScript docsif you see error types that you are not familiar with.

The end of the log for a failed test case provides a number like 12:18. This indicates that the failed assertion started at line 12, character 18. The line number is the best place to look to identify the assertion that failed. You should generally ignore the character number, there’s usually no reason to count the characters in a line!

So the failing test tells us that we need a global variable calledplayerOneMoveOneType, and it should be initialized as undefined.

Let’s write some code that will give as an assertion error. Here’s some code that will end the reference error that we saw before, by defining theplayerOneMoveOneTypevariable, but won’t pass the assertion that it should contain the value undefined.

// this code exists within the code that we are testing, not the test filelet playerOneMoveOneType = "lol gotcha";
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// this code exists within the code that we are testing, not the test filelet playerOneMoveOneType = "lol gotcha";

When you run the tests again, usingnpm test, you’ll get output like this:

An assertion error

Now, chai can give us more information about what we need to fix, because it got to the assertion before an error was thrown. We can see from the screenshot that anAssertionErrorwas raised. The test expected ‘lol gotcha’ to equal undefined. Getting the actual value and the expected value helps us to pinpoint exactly the changes we need to make to pass this test.

Now, let’s fix the code, initializingplayerOneMoveOneTypeto undefined as the test instructs .

// this code exists within the code that we are testing, not the test filelet playerOneMoveOneType = undefined;
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// this code exists within the code that we are testing, not the test filelet playerOneMoveOneType = undefined;

Finally, our code passes the test! We can move on to the next test.

A reference error

Edge Cases

Tests are often written for various edge cases. This is common, because poor handling of edge cases is responsible for a lot of bugs!

An example of a common edge case is: how does a function handle weird input? What happens if a function that expects to get a number is passed a string, or is passed no argument at all? Do we want to throw an error? Return undefined? Regardless, we want the decision to be consistent and well-documented.

Planning and testing for these edge cases is a common use of tests, and it can make the difference between code that will work 80% of the time, and code that will work 100% of the time.

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