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03-assertions.md

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Assertions

Translations: Français

Assertions are mixed into the execution object provided to each test implementation:

test('unicorns are truthy', t => {
	t.truthy('unicorn'); // Assertion
});

Assertions are bound to their test so you can assign them to a variable or pass them around:

test('unicorns are truthy', t => {
	const truthy = t.truthy;
	truthy('unicorn');
});

If multiple assertion failures are encountered within a single test, AVA will only display the first one.

Assertion planning

Assertion plans ensure tests only pass when a specific number of assertions have been executed. They'll help you catch cases where tests exit too early. They'll also cause tests to fail if too many assertions are executed, which can be useful if you have assertions inside callbacks or loops.

If you do not specify an assertion plan, your test will still fail if no assertions are executed. Set the failWithoutAssertions option to false in AVA's package.json configuration to disable this behavior.

Note that, unlike tap and tape, AVA does not automatically end a test when the planned assertion count is reached.

These examples will result in a passed test:

test('resolves with 3', t => {
	t.plan(1);

	return Promise.resolve(3).then(n => {
		t.is(n, 3);
	});
});

test.cb('invokes callback', t => {
	t.plan(1);

	someAsyncFunction(() => {
		t.pass();
		t.end();
	});
});

These won't:

test('loops twice', t => {
	t.plan(2);

	for (let i = 0; i < 3; i++) {
		t.true(i < 3);
	}
}); // Fails, 3 assertions are executed which is too many

test('invokes callback synchronously', t => {
	t.plan(1);

	someAsyncFunction(() => {
		t.pass();
	});
}); // Fails, the test ends synchronously before the assertion is executed

Skipping assertions

Any assertion can be skipped using the skip modifier. Skipped assertions are still counted, so there is no need to change your planned assertion count.

test('skip assertion', t => {
	t.plan(2);
	t.is.skip(foo(), 5); // No need to change your plan count when skipping
	t.is(1, 1);
});

Enhanced assertion messages

Enabling Babel will also enable power-assert, giving you more descriptive assertion messages.

Let's take this example, using Node's standard assert library:

const a = /foo/;
const b = 'bar';
const c = 'baz';
require('assert').ok(a.test(b) || b === c);

If you paste that into a Node REPL it'll return:

AssertionError: false == true

With AVA's assert assertion however, this test:

test('enhanced assertions', t => {
	const a = /foo/;
	const b = 'bar';
	const c = 'baz';
	t.assert(a.test(b) || b === c);
});

Will output:

6:   const c = 'baz';
7:   t.assert(a.test(b) || b === c);
8: });

Value is not truthy:

false

a.test(b) || b === c
=> false

b === c
=> false

c
=> 'baz'

b
=> 'bar'

a.test(b)
=> false

b
=> 'bar'

a
=> /foo/

Custom assertions

You can use any assertion library instead of or in addition to the built-in one, provided it throws exceptions when the assertion fails.

This won't give you as nice an experience as you'd get with the built-in assertions though, and you won't be able to use the assertion planning (see #25).

You'll have to configure AVA to not fail tests if no assertions are executed, because AVA can't tell if custom assertions pass. Set the failWithoutAssertions option to false in AVA's package.json configuration.

const assert = require('assert');

test('custom assertion', t => {
	assert(true);
});

Built-in assertions

.pass(message?)

Passing assertion.

.fail(message?)

Failing assertion.

.assert(value, message?)

Asserts that value is truthy. This is power-assert enabled.

.truthy(value, message?)

Assert that value is truthy.

.falsy(value, message?)

Assert that value is falsy.

.true(value, message?)

Assert that value is true.

.false(value, message?)

Assert that value is false.

.is(value, expected, message?)

Assert that value is the same as expected. This is based on Object.is().

.not(value, expected, message?)

Assert that value is not the same as expected. This is based on Object.is().

.deepEqual(value, expected, message?)

Assert that value is deeply equal to expected. See Concordance for details. Works with React elements and react-test-renderer.

.notDeepEqual(value, expected, message?)

Assert that value is not deeply equal to expected. The inverse of .deepEqual().

.throws(fn, expectation?, message?)

Assert that an error is thrown. fn must be a function which should throw. The thrown value must be an error. It is returned so you can run more assertions against it.

expectation can be an object with one or more of the following properties:

  • instanceOf: a constructor, the thrown error must be an instance of
  • is: the thrown error must be strictly equal to expectation.is
  • message: either a string, which is compared against the thrown error's message, or a regular expression, which is matched against this message
  • name: the expected .name value of the thrown error
  • code: the expected .code value of the thrown error

expectation does not need to be specified. If you don't need it but do want to set an assertion message you have to specify null.

Example:

const fn = () => {
	throw new TypeError('🦄');
};

test('throws', t => {
	const error = t.throws(() => {
		fn();
	}, {instanceOf: TypeError});

	t.is(error.message, '🦄');
});

.throwsAsync(thrower, expectation?, message?)

Assert that an error is thrown. thrower can be an async function which should throw, or a promise that should reject. This assertion must be awaited.

The thrown value must be an error. It is returned so you can run more assertions against it.

expectation can be an object with one or more of the following properties:

  • instanceOf: a constructor, the thrown error must be an instance of
  • is: the thrown error must be strictly equal to expectation.is
  • message: either a string, which is compared against the thrown error's message, or a regular expression, which is matched against this message
  • name: the expected .name value of the thrown error
  • code: the expected .code value of the thrown error

expectation does not need to be specified. If you don't need it but do want to set an assertion message you have to specify null.

Example:

test('throws', async t => {
	await t.throwsAsync(async () => {
		throw new TypeError('🦄');
	}, {instanceOf: TypeError, message: '🦄'});
});
const promise = Promise.reject(new TypeError('🦄'));

test('rejects', async t => {
	const error = await t.throwsAsync(promise);
	t.is(error.message, '🦄');
});

.notThrows(fn, message?)

Assert that no error is thrown. fn must be a function which shouldn't throw.

.notThrowsAsync(nonThrower, message?)

Assert that no error is thrown. nonThrower can be an async function which shouldn't throw, or a promise that should resolve.

Like the .throwsAsync() assertion, you must wait for the assertion to complete:

test('resolves', async t => {
	await t.notThrowsAsync(promise);
});

.regex(contents, regex, message?)

Assert that contents matches regex.

.notRegex(contents, regex, message?)

Assert that contents does not match regex.

.snapshot(expected, message?)

.snapshot(expected, options?, message?)

Compares the expected value with a previously recorded snapshot. Snapshots are stored for each test, so ensure you give your tests unique titles. Alternatively pass an options object to select a specific snapshot, for instance {id: 'my snapshot'}.

Snapshot assertions cannot be skipped when snapshots are being updated.

.try(title?, implementation | macro | macro[], ...args?)

.try() allows you to try assertions without causing the test to fail.

The implementation function behaves the same as any other test function. You can even use macros. The first title argument is always optional. Additional arguments are passed to the implemetation or macro function.

.try() is an asynchronous function. You must await it. The result object has commit() and discard() methods. You must decide whether to commit or discard the result. If you commit a failed result, your test will fail.

You can check whether the attempt passed using the passed property. Any assertion errors are available through the errors property. The attempt title is available through the title property.

Logs from t.log() are available through the logs property. You can choose to retain these logs as part of your test by passing {retainLogs: true} to the commit() and discard() methods.

The implementation function receives its own execution context, just like a test function. You must be careful to only perform assertions using the attempt's execution context. At least one assertion must pass for your attempt to pass.

You may run multiple attempts concurrently, within a single test. However you can't use snapshots when you do so.

Example:

const twoRandomIntegers = () => {
	const rnd = Math.round(Math.random() * 100);
	const x = rnd % 10;
	const y = Math.floor(rnd / 10);
	return [x, y];
};

test('flaky macro', async t => {
	const firstTry = await t.try((tt, a, b) => {
		tt.is(a, b);
	}, ...randomIntegers());

	if (firstTry.passed) {
		firstTry.commit();
		return;
	}

	firstTry.discard();
	t.log(firstTry.errors);

	const secondTry = await t.try((tt, a, b) => {
		tt.is(a, b);
	}, ...randomIntegers());
	secondTry.commit();
});