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# How to write a test for the Node.js project

## What is a test?

Most tests in Node.js core are JavaScript programs that exercise a functionality
provided by Node.js and check that it behaves as expected. Tests should exit
with code `0` on success. A test will fail if:

* It exits by setting `process.exitCode` to a non-zero number.
  * This is usually done by having an assertion throw an uncaught Error.
  * Occasionally, using `process.exit(code)` may be appropriate.
* It never exits. In this case, the test runner will terminate the test because
  it sets a maximum time limit.

Add tests when:

* Adding new functionality.
* Fixing regressions and bugs.
* Expanding test coverage.

## Test directory structure

See [directory structure overview][] for outline of existing test and locations.

## How to write a good test

A good test should be written in a style that is optimal for debugging
when it fails.

In principle, when adding a new test, it should be placed in a new file.
Unless there is strong motivation to do so, refrain from appending
new test cases to an existing file. Similar to the reproductions we ask
for in the issue tracker, a good test should be as minimal and isolated as
possible to facilitate debugging.

A good test should come with comments explaining what it tries to test,
so that when it fails, other contributors can fix it with the full context
of its intention, and be able to modify it in good confidence when the context
changes.

## Test structure

Let's analyze this basic test from the Node.js test suite:

```js
'use strict';                                                          // 1
const common = require('../common');                                   // 2
const fixtures = require('../common/fixtures');                        // 3

// This test ensures that the http-parser can handle UTF-8 characters  // 5
// in the http header.                                                 // 6

const assert = require('node:assert');                                 // 8
const http = require('node:http');                                     // 9

const server = http.createServer(common.mustCall((req, res) => {       // 11
  res.end('ok');                                                       // 12
}));                                                                   // 13
server.listen(0, () => {                                               // 14
  http.get({                                                           // 15
    port: server.address().port,                                       // 16
    headers: { 'Test': 'Düsseldorf' },                                 // 17
  }, common.mustCall((res) => {                                        // 18
    assert.strictEqual(res.statusCode, 200);                           // 19
    server.close();                                                    // 20
  }));                                                                 // 21
});                                                                    // 22
// ...                                                                 // 23
```

### **Lines 1-3**

```js
'use strict';
const common = require('../common');
const fixtures = require('../common/fixtures');
```

The first line enables strict mode. All tests should be in strict mode unless
the nature of the test requires that the test run without it.

The second line loads the `common` module. The [`common` module][] is a helper
module that provides useful tools for the tests. Some common functionality has
been extracted into submodules, which are required separately like the fixtures
module here.

Even if a test uses no functions or other properties exported by `common`,
the test should still include the `common` module before any other modules. This
is because the `common` module includes code that will cause a test to fail if
the test leaks variables into the global space. In situations where a test uses
no functions or other properties exported by `common`, include it without
assigning it to an identifier:

```js
require('../common');
```

### **Lines 5-6**

```js
// This test ensures that the http-parser can handle UTF-8 characters
// in the http header.
```

A test should start with a comment containing a brief description of what it is
designed to test.

### **Lines 8-9**

```js
const assert = require('node:assert');
const http = require('node:http');
```

The test checks functionality in the `node:http` module.

Most tests use the `node:assert` module to confirm expectations of the test.

The require statements are sorted in
[ASCII][] order (digits, upper
case, `_`, lower case).

### **Lines 11-22**

```js
const server = http.createServer(common.mustCall((req, res) => {
  res.end('ok');
}));
server.listen(0, () => {
  http.get({
    port: server.address().port,
    headers: { 'Test': 'Düsseldorf' },
  }, common.mustCall((res) => {
    assert.strictEqual(res.statusCode, 200);
    server.close();
  }));
});
```

This is the body of the test. This test is simple, it just tests that an
HTTP server accepts `non-ASCII` characters in the headers of an incoming
request. Interesting things to notice:

* If the test doesn't depend on a specific port number, then always use 0
  instead of an arbitrary value, as it allows tests to run in parallel safely,
  as the operating system will assign a random port. If the test requires a
  specific port, for example if the test checks that assigning a specific port
  works as expected, then it is ok to assign a specific port number.
* The use of `common.mustCall` to check that some callbacks/listeners are
  called.
* The HTTP server closes once all the checks have run. This way, the test can
  exit gracefully. Remember that for a test to succeed, it must exit with a
  status code of 0.

## General recommendations

### Timers

Avoid timers unless the test is specifically testing timers. There are multiple
reasons for this. Mainly, they are a source of flakiness. For a thorough
explanation go [here](https://github.com/nodejs/testing/issues/27).

In the event a test needs a timer, consider using the
`common.platformTimeout()` method. It allows setting specific timeouts
depending on the platform:

```js
const timer = setTimeout(fail, common.platformTimeout(4000));
```

will create a 4-second timeout on most platforms but a longer timeout on slower
platforms.

### The _common_ API

Make use of the helpers from the `common` module as much as possible. Please
refer to the [common file documentation](https://github.com/nodejs/node/tree/HEAD/test/common)
for the full details of the helpers.

#### common.mustCall

One interesting case is `common.mustCall`. The use of `common.mustCall` may
avoid the use of extra variables and the corresponding assertions. Let's
explain this with a real test from the test suite.

```js
'use strict';
require('../common');
const assert = require('node:assert');
const http = require('node:http');

let request = 0;
let listening = 0;
let response = 0;
process.on('exit', () => {
  assert.equal(request, 1, 'http server "request" callback was not called');
  assert.equal(listening, 1, 'http server "listening" callback was not called');
  assert.equal(response, 1, 'http request "response" callback was not called');
});

const server = http.createServer((req, res) => {
  request++;
  res.end();
}).listen(0, () => {
  listening++;
  const options = {
    agent: null,
    port: server.address().port,
  };
  http.get(options, (res) => {
    response++;
    res.resume();
    server.close();
  });
});
```

This test could be greatly simplified by using `common.mustCall` like this:

```js
'use strict';
const common = require('../common');
const http = require('node:http');

const server = http.createServer(common.mustCall((req, res) => {
  res.end();
})).listen(0, common.mustCall(() => {
  const options = {
    agent: null,
    port: server.address().port,
  };
  http.get(options, common.mustCall((res) => {
    res.resume();
    server.close();
  }));
}));
```

**Note:** Many functions invoke their callback with an `err` value as the first
argument. It is not a good idea to simply pass `common.mustCall()` to those
because `common.mustCall()` will ignore the error. Use `common.mustSucceed()`
instead.

#### Countdown module

The common [Countdown module](https://github.com/nodejs/node/tree/HEAD/test/common#countdown-module)
provides a simple countdown mechanism for tests that require a particular
action to be taken after a given number of completed tasks (for instance,
shutting down an HTTP server after a specific number of requests).

```js
const Countdown = require('../common/countdown');

const countdown = new Countdown(2, () => {
  console.log('.');
});

countdown.dec();
countdown.dec(); // The countdown callback will be invoked now.
```

#### Testing promises

When writing tests involving promises, it is generally good to wrap the
`onFulfilled` handler, otherwise the test could successfully finish if the
promise never resolves (pending promises do not keep the event loop alive).
Node.js automatically crashes - and hence, the test fails - in the case of an
`unhandledRejection` event.

```js
const common = require('../common');
const assert = require('node:assert');
const fs = require('node:fs').promises;

// Wrap the `onFulfilled` handler in `common.mustCall()`.
fs.readFile('test-file').then(
  common.mustCall(
    (content) => assert.strictEqual(content.toString(), 'test2'),
  ));
```

### Flags

Some tests will require running Node.js with specific command line flags set. To
accomplish this, add a `// Flags:` comment in the preamble of the
test followed by the flags. For example, to allow a test to require some of the
`internal/*` modules, add the `--expose-internals` flag.
A test that would require `internal/freelist` could start like this:

```js
'use strict';

// Flags: --expose-internals

require('../common');
const assert = require('node:assert');
const freelist = require('node:internal/freelist');
```

In specific scenarios it may be useful to get a hold of `primordials` or
`internalBinding()`. You can do so using

```bash
node --expose-internals -r internal/test/binding lib/fs.js
```

This only works if you preload `node:internal/test/binding` by command line
flag.

### Assertions

When writing assertions, prefer the strict versions:

* `assert.strictEqual()` over `assert.equal()`
* `assert.deepStrictEqual()` over `assert.deepEqual()`

When using `assert.throws()`, if possible, provide the full error message:

```js
assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /^Error: Wrong value$/, // Instead of something like /Wrong value/
);
```

In the case of internal errors, prefer checking only the `code` property:

```js
assert.throws(
  () => {
    throw new ERR_FS_FILE_TOO_LARGE(`${sizeKiB} Kb`);
  },
  { code: 'ERR_FS_FILE_TOO_LARGE' },
  // Do not include message: /^File size ([0-9]+ Kb) is greater than 2 GiB$/
);
```

### Console output

Output written by tests to stdout or stderr, such as with `console.log()` or
`console.error()`, can be useful when writing tests, as well as for debugging
them during later maintenance. The output will be suppressed by the test runner
(`./tools/test.py`) unless the test fails, but will always be displayed when
running tests directly with `node`. For failing tests, the test runner will
include the output along with the failed test assertion in the test report.

Some output can help debugging by giving context to test failures. For example,
when troubleshooting tests that timeout in CI. With no log statements, we have
no idea where the test got hung up.

There have been cases where tests fail without `console.log()`, and then pass
when its added, so be cautious about its use, particularly in tests of the I/O
and streaming APIs.

Excessive use of console output is discouraged as it can overwhelm the display,
including the Jenkins console and test report displays. Be particularly
cautious of output in loops, or other contexts where output may be repeated many
times in the case of failure.

In some tests, it can be unclear whether a `console.log()` statement is required
as part of the test (message tests, tests that check output from child
processes, etc.), or is there as a debug aide. If there is any chance of
confusion, use comments to make the purpose clear.

### ES.Next features

For performance considerations, we only use a selected subset of ES.Next
features in JavaScript code in the `lib` directory. However, when writing
tests, for the ease of backporting, it is encouraged to use those ES.Next
features that can be used directly without a flag in
[all maintained branches][]. [node.green][] lists available features
in each release, such as:

* `let` and `const` over `var`
* Template literals over string concatenation
* Arrow functions when appropriate

## Naming test files

Test files are named using kebab casing. The first component of the name is
`test`. The second is the module or subsystem being tested. The third is usually
the method or event name being tested. Subsequent components of the name add
more information about what is being tested.

For example, a test for the `beforeExit` event on the `process` object might be
named `test-process-before-exit.js`. If the test specifically checked that arrow
functions worked correctly with the `beforeExit` event, then it might be named
`test-process-before-exit-arrow-functions.js`.

## Imported tests

### Web platform tests

See [`test/wpt`](../../test/wpt/README.md) for more information.

## C++ unit test

C++ code can be tested using [Google Test][]. Most features in Node.js can be
tested using the methods described previously in this document. But there are
cases where these might not be enough, for example writing code for Node.js
that will only be called when Node.js is embedded.

### Adding a new test

The unit test should be placed in `test/cctest` and be named with the prefix
`test` followed by the name of unit being tested. For example, the code below
would be placed in `test/cctest/test_env.cc`:

```cpp
#include "gtest/gtest.h"
#include "node_test_fixture.h"
#include "env.h"
#include "node.h"
#include "v8.h"

static bool called_cb = false;
static void at_exit_callback(void* arg);

class EnvTest : public NodeTestFixture { };

TEST_F(EnvTest, RunAtExit) {
  v8::HandleScope handle_scope(isolate_);
  v8::Local<v8::Context> context = v8::Context::New(isolate_);
  node::IsolateData* isolateData = node::CreateIsolateData(isolate_, uv_default_loop());
  Argv argv{"node", "-e", ";"};
  auto env = node::CreateEnvironment(isolateData, context, 1, *argv, 2, *argv);
  node::AtExit(env, at_exit_callback);
  node::RunAtExit(env);
  EXPECT_TRUE(called_cb);
}

static void at_exit_callback(void* arg) {
  called_cb = true;
}
```

Next add the test to the `sources` in the `cctest` target in node.gyp:

```console
'sources': [
  'test/cctest/test_env.cc',
  ...
],
```

The only sources that should be included in the cctest target are
actual test or helper source files. There might be a need to include specific
object files that are compiled by the `node` target and this can be done by
adding them to the `libraries` section in the cctest target.

The test can be executed by running the `cctest` target:

```bash
make cctest
```

A filter can be applied to run single/multiple test cases:

```bash
make cctest GTEST_FILTER=EnvironmentTest.AtExitWithArgument
```

`cctest` can also be run directly which can be useful when debugging:

```bash
out/Release/cctest --gtest_filter=EnvironmentTest.AtExit\*
```

### Node.js test fixture

There is a [test fixture][] named `node_test_fixture.h` which can be included by
unit tests. The fixture takes care of setting up the Node.js environment
and tearing it down after the tests have finished.

It also contains a helper to create arguments to be passed into Node.js. It
will depend on what is being tested if this is required or not.

### Test coverage

To generate a test coverage report, see the
[Test Coverage section of the Building guide][].

Nightly coverage reports for the Node.js `main` branch are available at
<https://coverage.nodejs.org/>.

[ASCII]: https://man7.org/linux/man-pages/man7/ascii.7.html
[Google Test]: https://github.com/google/googletest
[Test Coverage section of the Building guide]: https://github.com/nodejs/node/blob/HEAD/BUILDING.md#running-coverage
[`common` module]: https://github.com/nodejs/node/blob/HEAD/test/common/README.md
[all maintained branches]: https://github.com/nodejs/lts
[directory structure overview]: https://github.com/nodejs/node/blob/HEAD/test/README.md#test-directories
[node.green]: https://node.green/
[test fixture]: https://github.com/google/googletest/blob/HEAD/docs/primer.md#test-fixtures-using-the-same-data-configuration-for-multiple-tests-same-data-multiple-tests