Custom error messages with Jest for assertions

While writing an application in Node, I was trying to test whether an object had certain properties (by lack of TypeScript, because this was a a very simple application).

The (now simplified) test looked like this.

At a certain point, this test failed, and this was my error message:

A pretty non-helpful message

This doesn’t seem too bad, but there’s no way for me to know which property throws the error, as it’s looping over the required properties. So what is a poor developer to do? Adding a custom message, of course — with the faulty prop displayed inside! I want to see something like “ Doesn’t contain prop ‘version’ “. That shouldn’t be too hard.

However, as discussed in these posts, it apparently is:

• Custom error messages for assertions · Issue #3293 … — GitHub
• How to add custom message to Jest expect? — Stack Overflow
• Print message on expect() assert failure — Stack Overflow
• … and a whole bunch more.

It seems to be something that Jest’s creators overlooked.

The solution

First, you need to know that Jest’s `expect`-function throws an error when things don’t turn out as expected. This means that you can catch this error and do something with it.

And this works as we want! You can see the text which identifies the culprit. A great thing.

However, the test is passing, which is a bad thing for CI/CD, where tests are automated and prevent malformed code to reach the customer. Because of this, you need to let Jest know that something went wrong, and that the test should fail instead.

This is easy though. You can simply throw another error 👍🏈

Throw the error 🏈

…which works like a charm. You both get a nice message and the test fails as well.

The message is showing and the test is FAILING :)

I hope this helped you, leave a message if you want!

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