# Assertions

Translations: [Français](https://github.com/avajs/ava-docs/blob/master/fr_FR/docs/03-assertions.md)

Assertions are mixed into the [execution object](./02-execution-context.md) provided to each test implementation:

```js
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:

```js
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.

Assertions return a boolean indicating whether they passed. You can use this to return early from a test. Note that this does not apply to the "throws" and `snapshot()` assertions.

## 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](./06-configuration.md) to disable this behavior.

Note that, unlike [`tap`](https://www.npmjs.com/package/tap) and [`tape`](https://www.npmjs.com/package/tape), AVA does *not* automatically end a test when the planned assertion count is reached.

These examples will result in a passed test:

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

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

These won't:

```js
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.

```js
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);
});
```

## 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](#built-in-assertions) though, and you won't be able to use the [assertion planning](#assertion-planning) ([see #25](https://github.com/avajs/ava/issues/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](./06-configuration.md).

```js
import assert from 'assert';

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

## Built-in assertions

### `.pass(message?)`

Passing assertion. Returns a boolean indicating whether the assertion passed.

### `.fail(message?)`

Failing assertion. Returns a boolean indicating whether the assertion passed.

### `.assert(actual, message?)`

Asserts that `actual` is truthy. Returns a boolean indicating whether the assertion passed.

### `.truthy(actual, message?)`

Assert that `actual` is truthy. Returns a boolean indicating whether the assertion passed.

### `.falsy(actual, message?)`

Assert that `actual` is falsy. Returns a boolean indicating whether the assertion passed.

### `.true(actual, message?)`

Assert that `actual` is `true`. Returns a boolean indicating whether the assertion passed.

### `.false(actual, message?)`

Assert that `actual` is `false`. Returns a boolean indicating whether the assertion passed.

### `.is(actual, expected, message?)`

Assert that `actual` is the same as `expected`. This is based on [`Object.is()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is). Returns a boolean indicating whether the assertion passed.

### `.not(actual, expected, message?)`

Assert that `actual` is not the same as `expected`. This is based on [`Object.is()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is). Returns a boolean indicating whether the assertion passed.

### `.deepEqual(actual, expected, message?)`

Assert that `actual` is deeply equal to `expected`. See [Concordance](https://github.com/concordancejs/concordance) for details.

### `.notDeepEqual(actual, expected, message?)`

Assert that `actual` is not deeply equal to `expected`. The inverse of `.deepEqual()`. Returns a boolean indicating whether the assertion passed.

### `.like(actual, selector, message?)`

Assert that `actual` is like `selector`. This is a variant of `.deepEqual()`, however `selector` does not need to have the same enumerable properties as `actual` does.

Instead AVA derives a *comparable* object from `actual`, based on the deeply-nested properties of `selector`. This object is then compared to `selector` using `.deepEqual()`.

Any values in `selector` that are not regular objects should be deeply equal to the corresponding values in `actual`.

In the following example, the `map` property of `actual` must be deeply equal to that of `selector`. However `nested.qux` is ignored, because it's not in `selector`.

```js
t.like({
	map: new Map([['foo', 'bar']]),
	nested: {
		baz: 'thud',
		qux: 'quux'
	}
}, {
	map: new Map([['foo', 'bar']]),
	nested: {
		baz: 'thud',
	}
})
```

Finally, this returns a boolean indicating whether the assertion passed.

### `.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. If the assertion fails then `undefined` is returned.

`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 `undefined`.

Example:

```js
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. If the assertion fails then `undefined` is returned.

`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 `undefined`.

Example:

```js
test('throws', async t => {
	await t.throwsAsync(async () => {
		throw new TypeError('🦄');
	}, {instanceOf: TypeError, message: '🦄'});
});
```

```js
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. Does not return anything.

### `.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:

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

Does not return anything.

### `.regex(contents, regex, message?)`

Assert that `contents` matches `regex`. Returns a boolean indicating whether the assertion passed.

### `.notRegex(contents, regex, message?)`

Assert that `contents` does not match `regex`. Returns a boolean indicating whether the assertion passed.

### `.snapshot(expected, message?)`

Compares the `expected` value with a previously recorded snapshot. Snapshots are stored for each test, so ensure you give your tests unique titles.

### `.try(title?, implementation | 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 implementation 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](./02-execution-context.md), 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:

```js
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);
	}, ...twoRandomIntegers());

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

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

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

Returns a boolean indicating whether the assertion passed.
