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 an 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 don't depend on global state or the state of other tests, which is a great thing!
*WARNING: NON-STANDARD BEHAVIOR:* The AVA CLI will always try to find and use your projects local install of AVA. This is true even when you run the global `ava` command. This non-standard behavior solves an important [issue](https://github.com/sindresorhus/ava/issues/157), and should have no impact on everyday use.
Tests are run asynchronously and require you to return a supported async object (a promise, or [observable](https://github.com/zenparsing/zen-observable)). We *highly* recommend the use of [async functions](#async-function-support); They make async code concise and readable, and they implicitly return a promise, so you don't have to.
If you're unable to use promises or other supported async objects, you may enable "callback mode" by defining your test with `test.cb([title', fn)`. Tests declared this way **must** be manually ended with `t.end()`. This mode is mainly intended for testing callback-style APIs.
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).
An assertion plan can be used to ensure a specific number of assertions are made. In the most common scenario, it validates that the test didn't exit before executing the expected number of assertions. It also fails the test if too many assertions are executed, which can be useful if you have assertions inside callbacks or loops.
AVA no longer supports automatically ending tests via `t.plan(...)`. This helps prevent false positives if you add assertions, but forget to increase your plan count.
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 optionally specify a title that is shown on failure.
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).
We don't yet [support Babel 6](https://github.com/sindresorhus/ava/pull/221), but you can use any Babel version in your project. We use our own bundled one.
AVA currently only transpiles the tests you ask it to run. *It will not transpile modules you ```import``` from outside of the test.* While there are valid reasons for taking this approach, it may not be what you expect!
As a simple workaround, you can use [Babel's require hook](https://babeljs.io/docs/usage/require/) in order to do on-the-fly transpiling of modules that are subsequently imported. Because AVA supports ES2015 module syntax, you can use it to import the require hook itself:
AVA supports using `t.end` as the final callback when using node-style error-first callback APIs. AVA will consider any truthy value passed as the first argument to `t.end` to be an error. Note that `t.end` requires "callback mode", which can be enabled by using the `test.cb` chain.
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.
As of version `5.0.0` it uses source maps to report coverage for your actual code, regardless of transpilation. Make sure that the code you're testing includes an inline source map or references a source map file. If you use `babel/register` you can set the `sourceMaps` option in your `.babelrc` to `inline`.
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.
