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a82fee5 Nov 11, 2018
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Common Pitfalls

Translations: Français

ESLint plugin

If you use ESLint, you can install eslint-plugin-ava. It will help you use AVA correctly and avoid some common pitfalls.

Transpiling imported modules

AVA currently only transpiles the tests you ask it to run, as well as test helpers (files starting with _ or in helpers directory) inside the test directory. It will not transpile modules you import from outside of the test. This may be unexpected but there are workarounds.

If you use Babel you can use its require hook to transpile imported modules on-the-fly. To add it, configure it in your package.json.

You can also transpile your modules in a separate process and refer to the transpiled files rather than the sources from your tests.

AVA in Docker

If you run AVA in Docker as part of your CI, you need to fix the appropriate environment variables. Specifically, adding -e CI=true in the docker exec command. See #751.

AVA uses is-ci to decide if it's in a CI environment or not using these variables.

AVA and connected client limits

You may be using a service that only allows a limited number of concurrent connections. For example, many database-as-a-service businesses offer a free plan with a limit on how many clients can be using it at the same time. AVA can hit those limits as it runs multiple processes, but well-written services should emit an error or throttle in those cases. If the one you're using doesn't, the tests will hang.

By default, AVA will use as many processes as there are logical cores on your machine. This is capped at two in a CI environment.

Use the concurrency flag to limit the number of processes ran. For example, if your service plan allows 5 clients, you should run AVA with concurrency=5 or less.

Asynchronous operations

You may be running an asynchronous operation inside a test and wondering why it's not finishing. If your asynchronous operation uses promises, you should return the promise:

test('fetches foo', t => {
	return fetch().then(data => {
		t.is(data, 'foo');
	});
});

Better yet, use async / await:

test('fetches foo', async t => {
	const data = await fetch();
	t.is(data, 'foo');
});

If you're using callbacks, use test.cb:

test.cb('fetches foo', t => {
	fetch((err, data) => {
		t.is(data, 'foo');
		t.end();
	});
});

Alternatively, promisify the callback function using something like pify:

test('fetches foo', async t => {
	const data = await pify(fetch)();
	t.is(data, 'foo');
});

Attributing uncaught exceptions to tests

AVA can't trace uncaught exceptions back to the test that triggered them. Callback-taking functions may lead to uncaught exceptions that can then be hard to debug. Consider promisifying and using async/await, as in the above example. This should allow AVA to catch the exception and attribute it to the correct test.

Why are the enhanced assertion messages not shown?

Ensure that the first parameter passed into your test is named t. This is a requirement of power-assert, the library that provides the enhanced messages.

test('one is one', t => {
	t.is(1, 1);
});

Helpers are not compiled when using a non-default test folder

This is a known issue. You should put your tests in a folder called test or __tests__.


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