Even though JavaScript is single-threaded, IO in Node.js can happen in parallel due to its async nature. AVA takes advantage of this and runs your tests concurrently, which is especially beneficial for IO heavy tests. In addition, test files are run in parallel as separate processes, giving you even better performance and a isolated environment for each test file. [Switching](https://github.com/sindresorhus/pageres/commit/663be15acb3dd2eb0f71b1956ef28c2cd3fdeed0) from Mocha to AVA in Pageres brought the test time down from 31 sec to 11 sec. Having tests run concurrently forces you to write atomic tests, meaning tests that don't depend on global state or the state of other tests, which is a great thing!
Tests are run async and require you to either set planned assertions `t.plan(1)`, explicitly end the test when done `t.end()`, or return a promise. [Async functions](#async-function-support) already returns a promise implicitly, so no need for you to explicitly return a promise in that case.
Test files are run from their current directory, so [`process.cwd()`](https://nodejs.org/api/process.html#process_process_cwd) is always the same as [`__dirname`](https://nodejs.org/api/globals.html#globals_dirname). You can just use relative paths instead of doing `path.join(__dirname, 'relative/path')`.
To create a test, you call the `test` function you require'd from AVA and pass in an optional test name and a function containing the test execution. The passed function is given the context as the first argument, where you can call the different AVA methods and [assertions](#assertions).
Planned assertions are useful for being able to assert that all async actions happened. It also comes with the benefit of not having to manually end the test.
While concurrency is awesome, there are some things that can't be done concurrently. In these rare cases, you can call `test.serial`, which will force those tests to run serially before the concurrent ones.
used in the same manner as `test()`. The test function given to `test.before()` and `test.after()` is called before/after all tests. You can also use `test.beforeEach()` and `test.afterEach()` if you need setup/teardown for each test. Hooks are run serially in the test file. Add as many of these as you want.
You can use any assertion module instead or in addition to the one that comes with AVA, but you won't be able to use the `.plan()` method, [yet](https://github.com/sindresorhus/ava/issues/25).
Plan how many assertion there are in the test. The test will fail if the actual assertion count doesn't match planned assertions. When planned assertions are used you don't need to explicitly end the test.
Be aware that this doesn't work with custom assert modules. You must then call `.end()` explicitly.
AVA comes with [`power-assert`](https://github.com/power-assert-js/power-assert) builtin, giving you more descriptive assertion messages. It reads your test and tries to infer more information from the code.
Each test file is run in a separate Node.js process. This comes with a lot of benefits. Different test files can no longer affect each other. Like test files mocking with the global environment, overriding builtins, etc. However, it's mainly done for performance reasons. Even though Node.js can run async IO concurrently, that doesn't help much when tests are heavy on synchronous operations, which blocks the main thread. By running tests concurrently and test files in parallel we take full advantage of modern systems.
Running tests concurrently comes with some challenges, doing IO is one. Usually, serial tests just create temp directories in the current test directory and cleans it up at the end. This won't work when you run tests concurrently as tests will conflict with each other. The correct way to do it is to use a new temp directory for each test. The [`tempfile`](https://github.com/sindresorhus/tempfile) and [`temp-write`](https://github.com/sindresorhus/temp-write) modules can be helpful.
You can't use [`istanbul`](https://github.com/gotwarlost/istanbul) for code coverage as AVA [spawns the test files](#isolated-environment), but you can use [`nyc`](https://github.com/bcoe/nyc) instead, which is basically `istanbul` with support for subprocesses.
Mocha requires you to use implicit globals like `describe` and `it` with the default interface (which most people use), too unopinionated, bloated, synchronous by default, serial test execution, and slow. Tape and node-tap are pretty good. AVA is highly inspired by their syntax. However, they both execute tests serially and they've made [TAP](https://testanything.org) a first-class citizen which has IMHO made their codebases a bit convoluted and coupled. TAP output is hard to read so you always end up using an external tap reporter. AVA is highly opinionated and concurrent. It comes with a default simple reporter and will in the future support TAP through a reporter.
