AVA is a test runner for Node.js that runs tests concurrently and in parallel to improve performance over test runners that run tests sequentially like Mocha. It runs each test file as a separate process, utilizing multiple CPU cores. AVA also bundles tools like Babel and Power Assert to support modern JavaScript features and provide better assertion errors. It helps developers write isolated atomic tests and avoids issues caused by shared global state between tests.

1. AVA - a futuristic test runner
What it is, what problems it solves and when to use it

2. About me
• Several production-grade node apps in the wild
• muuuf
• Up & Front
• EthPort (blockchain 2.0)
• Several big sites where I freelanced
• Maintainer of jade/pug and named-routes, help out at
CoffeeScript, bluebird and sequelize
• Run London & Berlin Node.js meetup groups
• Actually a Maths PhD

3. My problem
I implemented something that was essentially a HTTP
proxy. I wanted a lot of integrations tests using real
HTTP requests. Once you study the HTTP protocol,
you’ll want to test a lot of cases incl. different transfer
encodings, cookies, proxy caches and tcp keep-alives.
Especially the cache/max-age tests were wasteful since
you need to wait at least 2 full seconds for a cached
resource to be invalided.
My test suite quickly took multiple minutes to run even
though the CPU was not utilized.

4. Introducing AVA
AVA is a new test runner for node.js that
• takes advantage of node’s async nature and runs your
tests concurrently
• runs test files in parallel as separate processes, for
better performance and an isolated environment
• comes with batteries included; it bundles babel,
power-assert and others to hit the ground running

5. Concurrency
Simple test example in mocha:
These 500 tests are run serially, or “blocking”. Each test
must finish completely before the next one can start,
even if the CPU is at 0%, waiting for I/O.

6. Concurrency
The same test example in ava:
As soon as the first test yields to the event queue (`await
request`), the second one is started. When it yields to
the event queue, the third one is started, and so on.
This achieves concurrency on 1 cpu core without using
threads.
Also forces you to isolate your tests from each other.

7. Parallelism
Mocha runs a parent process with the tests running in
one child process, utilizing at most 1 cpu core:
AVA runs a parent process with every test file running in
a separate child process, utilizing as many cpu cores as
there are test files:
This forces you to avoid implicit globals. You can use the
`require` property to load configurations into each process.

8. Batteries included
AVA comes with babel and power-assert to support:
• ES2015 language features
• Enhanced assertion messages
• Promise support
• Generator function support
• Async function support
• Observable support
• Optional TAP output
• Clean stack traces

9. Batteries included
In particular, power-assert gives error messages like
these:

10. Simple configuration
AVA can be configured right inside your package.json:

11. Demo time
Or, how I stopped worrying and learned to love live-coding
See the code at https://github.com/alubbe/avatalk

12. Demo time
Or, how I stopped worrying and learned to love live-coding
Watch at https://raw.githubusercontent.com/alubbe/avatalk/master/video%20mocha.gif

13. Demo time
Or, how I stopped worrying and learned to love live-coding
Watch at https://raw.githubusercontent.com/alubbe/avatalk/master/video%20ava.gif

14. To wrap up
• AVA is great for test suites with a lot of async
operations
• It helps you write atomic tests that do not depend on
global state or the state of other tests
• If you do have tests that depend on each other, you can
ava to execute them serially
• AVA supports the .only & .skip syntax and can run any
TAP-compatible reporter for the output
• But, it will not make your unit tests run faster on a
machine with 1 cpu core
• It can potentially DDOS the thing you are testing and it
won’t speed up karma processes or database queries