babel-plugin-tester/README.md

Utilities for testing babel plugins

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Utilities for testing babel plugins 🧪
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# babel-plugin-tester

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This is a fairly simple abstraction to help you write tests for your babel
plugin or preset. It was built to work with [Jest][4], but most of the
functionality will work with [Mocha][5], [Jasmine][6], [`node:test`][7],
[Vitest][8], and any other test runner that defines standard `describe` and `it`
globals with async support (see [appendix][9]).

This package is tested on both Windows and nix (Ubuntu) environments.

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---

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- [Install](#install)
- [Usage](#usage)
  - [Import](#import)
  - [Invoke](#invoke)
  - [Execute](#execute)
  - [Configure](#configure)
- [Examples](#examples)
  - [Simple Example](#simple-example)
  - [Full Example](#full-example)
  - [Fixtures Examples](#fixtures-examples)
- [Appendix](#appendix)
  - [Testing Framework Compatibility](#testing-framework-compatibility)
  - [Using Babel for Configuration Loading](#using-babel-for-configuration-loading)
  - [`pluginName` Inference Caveat](#pluginname-inference-caveat)
  - [Custom Snapshot Serialization](#custom-snapshot-serialization)
  - [Formatting Output with Prettier](#formatting-output-with-prettier)
  - [Built-In Debugging Support](#built-in-debugging-support)
  - [`TEST_ONLY`/`TEST_NUM_ONLY` and `TEST_SKIP`/`TEST_NUM_SKIP` Environment Variables](#test_onlytest_num_only-and-test_skiptest_num_skip-environment-variables)
  - [`setup` and `teardown` Run Order](#setup-and-teardown-run-order)
  - [Published Package Details](#published-package-details)
  - [License](#license)
- [Contributing and Support](#contributing-and-support)
  - [Inspiration](#inspiration)
  - [Contributors](#contributors)

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<br />

## Install

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```shell
npm install --save-dev babel-plugin-tester
```

## Usage

To use babel-plugin-tester:

1. Import babel-plugin-tester into your test file.
2. Invoke `pluginTester` in your test file.
3. Execute your test file.

### Import

ESM:

```javascript
import { pluginTester } from 'babel-plugin-tester';
```

CJS:

```javascript
const { pluginTester } = require('babel-plugin-tester');
```

### Invoke

```javascript
/* file: test/unit.test.js */

import { pluginTester } from 'babel-plugin-tester';
import yourPlugin from 'universe:your-plugin';

pluginTester({
  plugin: yourPlugin,
  tests: {
    /* Your test objects */
  }
});
```

> [!TIP]
>
> Note how `pluginTester` does not appear inside any `test`/`it` block nor
> within any [hook functions][10]. For advanced use cases, `pluginTester` may
> appear within one or more `describe` blocks, though this is discouraged.

### Execute

In your terminal of choice:

```shell
# Prettier@3 requires --experimental-vm-modules for older Node versions
NODE_OPTIONS='--no-warnings --experimental-vm-modules' npx jest
```

### Configure

This section lists the options you can pass to babel-plugin-tester. They are all
optional with respect to the following:

- When testing a preset, the [`preset`][11] option is required.
- When testing a plugin, the [`plugin`][12] option is required.
- You must test either a preset or a plugin.
- You cannot use preset-specific options ([`preset`][11], [`presetName`][13],
  [`presetOptions`][14]) and plugin-specific options ([`plugin`][12],
  [`pluginName`][15], [`pluginOptions`][16]) at the same time.

#### `plugin`

This is used to provide the babel plugin under test. For example:

```javascript
/* file: test/unit.test.js */

import { pluginTester } from 'babel-plugin-tester';
import identifierReversePlugin from 'universe:identifier-reverse-plugin';

pluginTester({
  plugin: identifierReversePlugin,
  tests: {
    /* Your test objects */
  }
});

/* file: src/identifier-reverse-plugin.js */

// Normally you would import this from your plugin module
function identifierReversePlugin() {
  return {
    name: 'identifier reverse',
    visitor: {
      Identifier(idPath) {
        idPath.node.name = idPath.node.name.split('').reverse().join('');
      }
    }
  };
}
```

#### `pluginName`

This is used as the [describe block name][17] and in your [tests' names][18]. If
`pluginName` can be inferred from the [`plugin`][12]'s [name][19], then it will
be and you do not need to provide this option. If it cannot be inferred for
whatever reason, `pluginName` defaults to `"unknown plugin"`.

Note that there is a small [caveat][20] when relying on `pluginName` inference.

#### `pluginOptions`

This is used to pass options into your plugin at transform time. If provided,
the object will be [`lodash.mergeWith`][lodash.mergewith]'d with each [test
object's `pluginOptions`][21]/[fixture's `pluginOptions`][22], with the latter
taking precedence. Note that arrays will be concatenated and explicitly
undefined values will unset previously defined values during merging.

#### `preset`

This is used to provide the babel preset under test. For example:

```javascript
/* file: cool-new-babel-preset.test.js */

import path from 'node:path';
import { pluginTester } from 'babel-plugin-tester';
import coolNewBabelPreset from './cool-new-babel-preset.js';

pluginTester({
  preset: coolNewBabelPreset,
  // A path to a directory containing your test fixtures
  fixtures: path.join(__dirname, 'fixtures')
});

/* file: cool-new-babel-preset.js */

function identifierReversePlugin() {
  return {
    name: 'identifier reverse',
    visitor: {
      Identifier(idPath) {
        idPath.node.name = idPath.node.name.split('').reverse().join('');
      }
    }
  };
}

function identifierAppendPlugin() {
  return {
    name: 'identifier append',
    visitor: {
      Identifier(idPath) {
        idPath.node.name = `${idPath.node.name}_appended`;
      }
    }
  };
}

export function coolNewBabelPreset() {
  return { plugins: [identifierReversePlugin, identifierAppendPlugin] };
}
```

#### `presetName`

This is used as the [describe block name][17] and in your [tests' names][18].
Defaults to `"unknown preset"`.

#### `presetOptions`

This is used to pass options into your preset at transform time. If provided,
the object will be [`lodash.mergeWith`][lodash.mergewith]'d with each [test
object's `presetOptions`][23]/[fixture's `presetOptions`][24], with the latter
taking precedence. Note that arrays will be concatenated and explicitly
undefined values will unset previously defined values during merging.

#### `babel`

This is used to provide your own implementation of babel. This is particularly
useful if you want to use a different version of babel than what's required by
this package.

#### `babelOptions`

This is used to configure babel. If provided, the object will be
[`lodash.mergeWith`][lodash.mergewith]'d with the [defaults][25] and each [test
object's `babelOptions`][26]/[fixture's `babelOptions`][27], with the latter
taking precedence.

Be aware that arrays will be concatenated and explicitly undefined values will
unset previously defined values during merging.

> [!IMPORTANT]
>
> For `babel-plugin-tester@>=12`, [duplicate entries][2] in
> [`babelOptions.plugins`][55] and [`babelOptions.presets`][73] are reduced,
> with latter entries _completely overwriting_ any that came before. In other
> words: the last duplicate plugin or preset configuration wins. **They are not
> merged.** This makes it easy to provide an alternative one-off configuration
> for a plugin or preset that is also used elsewhere, such as a project's root
> `babel.config.js` file.
>
> Attempting the same with `babel-plugin-tester@<12` will cause babel [to
> throw][2] since duplicate entries are technically not allowed.

Also note that [`babelOptions.babelrc`][28] and [`babelOptions.configFile`][29]
are set to `false` by default, which disables automatic babel configuration
loading. [This can be re-enabled if desired][30].

To simply reuse your project's [`babel.config.js`][31] or some other
configuration file, set `babelOptions` like so:

```javascript
// file: /repos/my-project/tests/unit-plugin.test.ts

import path from 'node:path';
import { pluginTester } from 'babel-plugin-tester';

pluginTester({
  plugin: yourPlugin,
  // ...
  babelOptions: require(path.join('..', 'babel.config.js')),
  // ...
  tests: {
    /* Your test objects */
  }
});
```

##### Custom Plugin and Preset Run Order

By default, when you include a custom list of [plugins][32] or [presets][3] in
`babelOptions`, the plugin or preset under test will always be the final plugin
or preset to run.

For example, consider the `myPlugin` plugin:

```javascript
import { pluginTester } from 'babel-plugin-tester';

pluginTester({
  plugin: myPlugin,
  pluginName: 'my-plugin',
  babelOptions: {
    plugins: [
      ['@babel/plugin-syntax-decorators', { legacy: true }],
      ['@babel/plugin-proposal-class-properties', { loose: true }]
    ]
  }
});
```

By default, `myPlugin` will be invoked _after_ @babel/plugin-syntax-decorators
and @babel/plugin-proposal-class-properties (i.e. `myPlugin` is _appended_ by
default).

It is possible to specify a custom ordering using the exported
`runPluginUnderTestHere` symbol. For instance, to run `myPlugin` _after_
@babel/plugin-syntax-decorators but _before_
@babel/plugin-proposal-class-properties:

```javascript
import { pluginTester, runPluginUnderTestHere } from 'babel-plugin-tester';

pluginTester({
  plugin: myPlugin,
  pluginName: 'my-plugin',
  babelOptions: {
    plugins: [
      ['@babel/plugin-syntax-decorators', { legacy: true }],
      runPluginUnderTestHere,
      ['@babel/plugin-proposal-class-properties', { loose: true }]
    ]
  }
});
```

Or to run `myPlugin` _before_ both @babel/plugin-syntax-decorators and
@babel/plugin-proposal-class-properties:

```javascript
import { pluginTester, runPluginUnderTestHere } from 'babel-plugin-tester';

pluginTester({
  plugin: myPlugin,
  pluginName: 'my-plugin',
  babelOptions: {
    plugins: [
      runPluginUnderTestHere,
      ['@babel/plugin-syntax-decorators', { legacy: true }],
      ['@babel/plugin-proposal-class-properties', { loose: true }]
    ]
  }
});
```

The same can be done when testing presets. Note that `myPreset` is normally
_prepended_ by default since, unlike plugins, [presets are run in reverse
order][33]:

```javascript
import { pluginTester, runPresetUnderTestHere } from 'babel-plugin-tester';

pluginTester({
  preset: myPreset,
  presetName: 'my-preset',
  babelOptions: {
    presets: [
      '@babel/preset-typescript',
      ['@babel/preset-react', { pragma: 'dom' }],
      runPresetUnderTestHere
    ]
  }
});
```

In this example, `myPreset` will run first instead of last.

#### `title`

This is used to specify a custom title for the two top-level [describe
blocks][17], the first enclosing all [tests][34] (i.e. `describe(title, ...)`)
and the second enclosing all [fixtures][35] (i.e.
``describe(`${title} fixtures`, ...)``).

Explicitly setting this option will override any defaults or inferred values.
Set to `false` to prevent the creation of these enclosing describe blocks.
Otherwise, the title defaults to using [`pluginName`][15]/[`presetName`][13].

#### `filepath`

This is used to resolve relative paths provided by the [`fixtures`][35] option;
the test object properties [`codeFixture`][36], [`outputFixture`][37], and
[`execFixture`][38]; and [during configuration resolution for prettier][39].
That is: if the aforesaid properties are not absolute paths, they will be
[`path.join`][40]'d with the [directory name][41] of `filepath`.

`filepath` is also passed to `formatResult` if a more specific path is not
available, and it is used as the default value for `babelOptions.filename` in
[test objects][42].

This option defaults to the absolute path of the file that [invoked the
`pluginTester` function][43].

> [!NOTE]
>
> For backwards compatibility reasons, `filepath` is synonymous with `filename`.
> They can be used interchangeably, though care must be taken not to confuse the
> babel-plugin-tester option `filename` with `babelOptions.filename`. They are
> NOT the same!

#### `endOfLine`

This is used to control which line endings both the actual output from babel and
the expected output will be converted to. Defaults to `"lf"`.

| Options      | Description                             |
| ------------ | --------------------------------------- |
| `"lf"`       | Use Unix-style line endings             |
| `"crlf"`     | Use Windows-style line endings          |
| `"auto"`     | Use the system default line endings     |
| `"preserve"` | Use the line endings from the input     |
| `false`      | Disable line ending conversion entirely |

> [!NOTE]
>
> When disabling line ending conversion, note that [Babel will always output
> LF][44] even if the input is CRLF.

#### `setup`

This function will be run before every test runs, including fixtures. It can
return a function which will be treated as a [`teardown`][45] function. It can
also return a promise. If that promise resolves to a function, that will be
treated as a [`teardown`][45] function.

See [here][46] for the complete run order.

#### `teardown`

This function will be run after every test runs, including fixtures. You can
define this via `teardown` or you can return it from the [`setup`][47] function.
This can likewise return a promise if it is asynchronous.

This function, if provided, will be run _after_ any teardown function returned
by [`setup`][47]. See [here][46] for the complete run order.

#### `formatResult`

This function is used to format all babel outputs, and defaults to a function
that invokes [prettier][48]. If a prettier configuration file is [found][49],
then that will be used. Otherwise, prettier will use its own default
configuration.

You can also [override or entirely disable formatting][39].

#### `snapshot`

Equivalent to [`snapshot`][50] but applied globally across all [test
objects][42].

#### `fixtureOutputName`

Equivalent to [`fixtureOutputName`][51] but applied globally across all
[fixtures][35].

#### `fixtureOutputExt`

Equivalent to [`fixtureOutputExt`][52] but applied globally across all
[fixtures][35].

#### `titleNumbering`

Determines which test titles are prefixed with a number when registering [test
blocks][18] (e.g. `` `1. ${title}` ``, `` `2. ${title}` ``, etc). Defaults to
`"all"`.

| Options           | Description                                         |
| ----------------- | --------------------------------------------------- |
| `"all"`           | All test object and fixtures tests will be numbered |
| `"tests-only"`    | Only test object tests will be numbered             |
| `"fixtures-only"` | Only fixtures tests will be numbered                |
| `false`           | Disable automatic numbering in titles entirely      |

#### `restartTitleNumbering`

Normally, multiple [invocations][43] of babel-plugin-tester in the same test
file will share the same [test title numbering][53]. For example:

```javascript
/* file: test/unit.test.js */

import { pluginTester } from 'babel-plugin-tester';
import yourPlugin from 'universe:your-plugin';

pluginTester({
  plugin: yourPlugin,
  tests: { 'test one': testOne, 'test two': testTwo }
});

pluginTester({
  plugin: yourPlugin,
  tests: { 'test one': testOne, 'test x': testTwo }
});

pluginTester({
  plugin: yourPlugin,
  tests: { 'test five': testOne }
});
```

Will result in [test blocks][18] with names like:

```text
1. Test one
2. Test two
3. Test one
4. Test x
5. Test five
```

However, setting this option to `true` will restart the numbering:

```javascript
/* file: test/unit.test.js */

import { pluginTester } from 'babel-plugin-tester';
import yourPlugin from 'universe:your-plugin';

pluginTester({
  plugin: yourPlugin,
  tests: { 'test one': testOne, 'test two': testTwo }
});

pluginTester({
  plugin: yourPlugin,
  restartTitleNumbering: true,
  tests: { 'test one': testOne, 'test x': testTwo }
});

pluginTester({
  plugin: yourPlugin,
  tests: { 'test five': testOne }
});
```

Which will result in [test blocks][18] with names like:

```text
1. Test one
2. Test two
1. Test one
2. Test x
3. Test five
```

This option is `false` by default.

#### `fixtures`

There are two ways to create tests: using the [`tests`][34] option to provide
one or more [test objects][42] or using the `fixtures` option described here.
Both can be used simultaneously.

The `fixtures` option must be a path to a directory with a structure similar to
the following:

```text
fixtures
├── first-test         # test title will be: "1. first test"
│   ├── code.js        # required
│   └── output.js      # required (unless using the `throws` option)
├── second-test        # test title will be: "2. second test"
│   ├── .babelrc.js    # optional
│   ├── options.json   # optional
│   ├── code.ts        # required (other file extensions are allowed too)
│   └── output.js      # required (unless using the `throws` option)
└── nested
    ├── options.json   # optional
    ├── third-test     # test title will be: "3. nested > third test"
    │   ├── code.mjs   # required (other file extensions are allowed too)
    │   ├── output.js  # required (unless using the `throws` option)
    │   └── options.js # optional (overrides props in nested/options.json)
    └── x-fourth-test  # test title will be: "4. nested > x fourth test"
        └── exec.js    # required (alternative to code/output structure)
```

> [!TIP]
>
> `.babelrc`, `.babelrc.json`, `.babelrc.js`, `.babelrc.cjs`, and `.babelrc.mjs`
> config files in fixture directories are supported out-of-the-box.

Assuming the `fixtures` directory is in the same directory as your test file,
you could use it with the following configuration:

```javascript
pluginTester({
  plugin,
  fixtures: path.join(__dirname, 'fixtures')
});
```

> [!NOTE]
>
> If `fixtures` is not an absolute path, it will be [`path.join`][40]'d with the
> [directory name][41] of [`filepath`][54].

And it would run four tests, one for each directory in `fixtures` containing a
file starting with "code" or "exec".

##### `code.js`

This file's contents will be used as the source code input into babel at
transform time. Any file extension can be used, even a multi-part extension
(e.g. `.test.js` in `code.test.js`) as long as the file name starts with
`code.`; the [expected output file][56] will have the same file extension suffix
(i.e. `.js` in `code.test.js`) as this file unless changed with the
[`fixtureOutputExt`][52] option.

After being transformed by babel, the resulting output will have whitespace
trimmed, line endings [converted][57], and then get [formatted by prettier][39].

Note that this file cannot appear in the same directory as [`exec.js`][58]. If
more than one `code.*` file exists in a directory, the first one will be used
and the rest will be silently ignored.

##### `output.js`

This file, if provided, will have its contents compared with babel's output,
which is [`code.js`][59] transformed by babel and [formatted with prettier][39].
If this file is missing and neither [`throws`][60] nor [`exec.js`][58] are being
used, this file will be automatically generated from babel's output.
Additionally, the name and extension of this file can be changed with the
[`fixtureOutputName`][51] and [`fixtureOutputExt`][52] options.

Before being compared to babel's output, this file's contents will have
whitespace trimmed and line endings [converted][57].

Note that this file cannot appear in the same directory as [`exec.js`][58].

##### `exec.js`

This file's contents will be used as the input into babel at transform time just
like the [`code.js`][59] file, except the output will be _evaluated_ in the
[same _CJS_ context][61] as the test runner itself, meaning it supports features
like a/sync IIFEs, debugging breakpoints (!), and has access to mocked modules,
`expect`, `require`, `__dirname` and `__filename` (derived from this file's
path), and other globals/features provided by your test framework. However, the
context does not support _`import`, top-level await, or any other ESM syntax_.
Hence, while any file extension can be used (e.g. `.ts`, `.vue`, `.jsx`), this
file will always be evaluated as CJS.

The test will always pass unless an exception is thrown (e.g. when an `expect()`
fails).

Use this to make advanced assertions on the output. For example, to test that
[babel-plugin-proposal-throw-expressions][62] actually throws, your `exec.js`
file might contain:

```javascript
expect(() => throw new Error('throw expression')).toThrow('throw expression');
```

> [!CAUTION]
>
> Keep in mind that, despite sharing a global context, execution will occur in a
> [separate realm][63], which means native/intrinsic types will be different.
> This can lead to unexpectedly failing tests. For example:
>
> ```javascript
> expect(require(`${__dirname}/imported-file.json`)).toStrictEqual({
>   data: 'imported'
> });
> ```
>
> This may fail in some test frameworks with the message "serializes to the same
> string". This is because the former object's `Object` prototype comes from a
> different realm than the second object's `Object` prototype, meaning the two
> objects are not technically _strictly_ equal. However, something like the
> following, which creates two objects in the same realm, will pass:
>
> ```javascript
> expect(
>   Object.fromEntries(
>     Object.entries(require(`${__dirname}/imported-file.json`))
>   )
> ).toStrictEqual({ data: 'imported' });
> ```
>
> Or use `JSON.stringify` + `toBe` (or your testing framework's equivalent):
>
> ```javascript
> expect(JSON.stringify(require(`${__dirname}/imported-file.json`))).toBe(
>   JSON.stringify({ data: 'imported' })
> );
> ```
>
> Or use `isEqual` (or your testing framework's equivalent):
>
> ```javascript
> expect(require(`${__dirname}/imported-file.json`)).toEqual({
>   data: 'imported'
> });
> ```

After being transformed by babel but before being evaluated, the babel output
will have whitespace trimmed, line endings [converted][57], and then get
[formatted by prettier][39].

Note that this file cannot appear in the same directory as [`code.js`][59] or
[`output.js`][56]. If more than one `exec.*` file exists in a directory, the
first one will be used and the rest will be silently ignored.

##### `options.json` (Or `options.js`)

For each fixture, the contents of the entirely optional `options.json` file are
[`lodash.mergeWith`][lodash.mergewith]'d with the options provided to
babel-plugin-tester, with the former taking precedence. Note that arrays will be
concatenated and explicitly undefined values will unset previously defined
values during merging.

For added flexibility, `options.json` can be specified as `options.js` instead
so long as a JSON object is exported via [`module.exports`][64]. If both files
exist in the same directory, `options.js` will take precedence and
`options.json` will be ignored entirely.

Fixtures support deeply nested directory structures as well as shared or "root"
`options.json` files. For example, placing an `options.json` file in the
`fixtures/nested` directory would make its contents the "global configuration"
for all fixtures under `fixtures/nested`. That is: each fixture would
[`lodash.mergeWith`][lodash.mergewith] the options provided to
babel-plugin-tester, `fixtures/nested/options.json`, and the contents of their
local `options.json` file as described above.

What follows are the properties you may use if you provide an options file, all
of which are optional:

###### `babelOptions`

This is used to configure babel. Properties specified here override
([`lodash.mergeWith`][lodash.mergewith]) those from the [`babelOptions`][65]
option provided to babel-plugin-tester.

Note that arrays will be concatenated, explicitly undefined values will unset
previously defined values, and (as of `babel-plugin-tester@>=12`) duplicate
plugin/preset configurations will override each other (last configuration wins)
during merging.

###### `pluginOptions`

This is used to pass options into your plugin at transform time. Properties
specified here override ([`lodash.mergeWith`][lodash.mergewith]) those from the
[`pluginOptions`][16] option provided to babel-plugin-tester. Note that arrays
will be concatenated and explicitly undefined values will unset previously
defined values during merging.

Unlike with babel-plugin-tester's options, you can safely mix plugin-specific
properties (like `pluginOptions`) with preset-specific properties (like
[`presetOptions`][24]) in your options files.

###### `presetOptions`

This is used to pass options into your preset at transform time. Properties
specified here override ([`lodash.mergeWith`][lodash.mergewith]) those from the
[`presetOptions`][14] option provided to babel-plugin-tester. Note that arrays
will be concatenated and explicitly undefined values will unset previously
defined values during merging.

Unlike with babel-plugin-tester's options, you can safely mix plugin-specific
properties (like [`pluginOptions`][22]) with preset-specific properties (like
`presetOptions`) in your options files.

###### `title`

If provided, this will be used as the title of the test. Otherwise, the
directory name will be used as the title by default (with spaces replacing
dashes).

###### `only`

Use this to run only the specified fixture. Useful while developing to help
focus on a small number of fixtures. Can be used in multiple `options.json`
files.

> [!IMPORTANT]
>
> Requires [Jest][66], an equivalent interface (like [Vitest][8]), or a
> manually-defined `it` object exposing an appropriate [`only`][67] method.

###### `skip`

Use this to skip running the specified fixture. Useful for when you are working
on a feature that is not yet supported. Can be used in multiple `options.json`
files.

> [!IMPORTANT]
>
> Requires [Jest][66], an equivalent interface (like [Vitest][8]), or a
> manually-defined `it` object exposing an appropriate [`skip`][68] method.

###### `throws`

> [!IMPORTANT]
>
> When using certain values, this property must be used in `options.js` instead
> of `options.json`.

Use this to assert that a particular `code.js` file should cause babel to throw
an error during transformation. For example:

```javascript
{
  // ...
  throws: true,
  throws: 'should have this exact message',
  throws: /should pass this regex/,
  throws: SyntaxError, // Should be an instance of this class
  throws: err => {
    if (err instanceof SyntaxError && /message/.test(err.message)) {
      return true; // Test will fail if this function's return value !== true
    }
  },
}
```

> [!CAUTION]
>
> Be careful using `instanceof` [across realms][69] as it can lead to [strange
> behavior][70] with [frontend frames/windows][71] and with tools that rely on
> [Node's VM module][72] (like Jest).

If the value of `throws` is a class, that class must [be a subtype of
`Error`][77] or the behavior of babel-plugin-tester is undefined.

Note that this property cannot be present when using an [`exec.js`][58] or
[`output.js`][56] file or when using the [`outputRaw`][78] option.

> [!NOTE]
>
> For backwards compatibility reasons, `throws` is synonymous with `error`. They
> can be used interchangeably, with `throws` taking precedence.

###### `setup`

> [!IMPORTANT]
>
> As it requires a function value, this property must be used in `options.js`
> instead of `options.json`.

This function will be run before a particular fixture's tests are run. It can
return a function which will be treated as a [`teardown`][79] function. It can
also return a promise. If that promise resolves to a function, that will be
treated as a [`teardown`][79] function.

This function, if provided, will run _after_ any [`setup`][47] function provided
as a babel-plugin-tester option. See [here][46] for the complete run order.

###### `teardown`

> [!IMPORTANT]
>
> As it requires a function value, this property must be used in `options.js`
> instead of `options.json`.

This function will be run after a fixture's tests finish running. You can define
this via `teardown` or you can return it from the [`setup`][80] function. This
can likewise return a promise if it is asynchronous.

This function, if provided, will be run _after_ any teardown function returned
by the [`setup`][80] property, both of which will run _before_ any
[`teardown`][45] function provided as a babel-plugin-tester option. See
[here][46] for the complete run order.

###### `formatResult`

> [!IMPORTANT]
>
> As it requires a function value, this property must be used in `options.js`
> instead of `options.json`.

This function is used to format all babel outputs, and defaults to a function
that invokes [prettier][48]. If a prettier configuration file is [found][49],
then that will be used. Otherwise, prettier will use its own default
configuration.

You can also [entirely disable formatting][39].

This will override the [`formatResult`][81] function provided to
babel-plugin-tester.

###### `outputRaw`

> [!WARNING]
>
> This feature is only available in `babel-plugin-tester@>=12`.

> [!IMPORTANT]
>
> As it requires a function value, this property must be used in `options.js`
> instead of `options.json`.

This option is similar in intent to [`output.js`][56] except it tests against
the _entire [`BabelFileResult`][82] object_ returned by [babel's `transform`
function][83] instead of only the `code` property of [`BabelFileResult`][82].

`outputRaw` must be a function with the following signature:

```typescript
outputRaw: (output: BabelFileResult) => void
```

Where the `output` parameter is an instance of [`BabelFileResult`][82]:

```typescript
interface BabelFileResult {
  ast?: Node | undefined;
  code?: string | undefined;
  ignored?: boolean | undefined;
  map?: object | undefined;
  metadata?: BabelFileMetadata | undefined;
}
```

So long as the `outputRaw` function does not throw, it will never cause the test
to fail. On the other hand, if the `outputRaw` function throws, such as when
`expect(output.metadata).toStrictEqual({ ... })` fails, the test will fail
regardless of other options.

The `output` parameter is not trimmed, converted, stripped, or modified at all.

Note that `outputRaw` does not _replace_ [`output.js`][56] etc, it only adds
additional (custom) expectations to your test. Further note that this option
_can_ appear alongside any other [`fixtures`][35] option except [`throws`][84].

###### `fixtureOutputName`

Use this to provide your own fixture output file name. Defaults to `"output"`.

###### `fixtureOutputExt`

Use this to provide your own fixture output file extension. Including the
leading period is optional; that is: if you want `output.jsx`,
`fixtureOutputExt` can be set to either `"jsx"` or `".jsx"`. If omitted, the
[input fixture][59]'s file extension will be used instead.

This is particularly useful if you are testing TypeScript input.

#### `tests`

There are two ways to create tests: using the [`fixtures`][35] option that
leverages the filesystem or using the `tests` option described here. Both can be
used simultaneously.

Using the `tests` option, you can provide [test objects][42] describing your
expected transformations. You can provide `tests` as an object of test objects
or an array of test objects. If you provide an object, the object's keys will be
used as the default title of each test. If you provide an array, each test's
default title will be derived from its index and
[`pluginName`][15]/[`presetName`][13].

See [the example][85] for more details.

##### Test Objects

A minimal test object can be:

1. A `string` representing [code][86].
2. An `object` with a [`code`][86] property.

What follows are the properties you may use if you provide an object, most of
which are optional:

###### `babelOptions`

This is used to configure babel. Properties specified here override
([`lodash.mergeWith`][lodash.mergewith]) those from the [`babelOptions`][65]
option provided to babel-plugin-tester.

Note that arrays will be concatenated, explicitly undefined values will unset
previously defined values, and (as of `babel-plugin-tester@>=12`) duplicate
plugin/preset configurations will override each other (last configuration wins)
during merging.

###### `pluginOptions`

This is used to pass options into your plugin at transform time. Properties
specified here override ([`lodash.mergeWith`][lodash.mergewith]) those from the
[`pluginOptions`][16] option provided to babel-plugin-tester. Note that arrays
will be concatenated and explicitly undefined values will unset previously
defined values during merging.

Unlike with babel-plugin-tester's options, you can safely mix plugin-specific
properties (like `pluginOptions`) with preset-specific properties (like
[`presetOptions`][23]) in your test objects.

###### `presetOptions`

This is used to pass options into your preset at transform time. Properties
specified here override ([`lodash.mergeWith`][lodash.mergewith]) those from the
[`presetOptions`][14] option provided to babel-plugin-tester. Note that arrays
will be concatenated and explicitly undefined values will unset previously
defined values during merging.

Unlike with babel-plugin-tester's options, you can safely mix plugin-specific
properties (like [`pluginOptions`][21]) with preset-specific properties (like
`presetOptions`) in your test objects.

###### `title`

If provided, this will be used as the title of the test. Otherwise, the title
will be determined from test object by default.

###### `only`

Use this to run only the specified test. Useful while developing to help focus
on a small number of tests. Can be used on multiple tests.

> [!IMPORTANT]
>
> Requires [Jest][66], an equivalent interface (like [Vitest][8]), or a
> manually-defined `it` object exposing an appropriate [`only`][67] method.

###### `skip`

Use this to skip running the specified test. Useful for when you are working on
a feature that is not yet supported. Can be used on multiple tests.

> [!IMPORTANT]
>
> Requires [Jest][66], an equivalent interface (like [Vitest][8]), or a
> manually-defined `it` object exposing an appropriate [`skip`][68] method.

###### `throws`

Use this to assert that a particular test object should cause babel to throw an
error during transformation. For example:

```javascript
{
  // ...
  throws: true,
  throws: 'should have this exact message',
  throws: /should pass this regex/,
  throws: SyntaxError, // Should be an instance of this class
  throws: err => {
    if (err instanceof SyntaxError && /message/.test(err.message)) {
      return true; // Test will fail if this function's return value !== true
    }
  },
}
```

> [!CAUTION]
>
> Be careful using `instanceof` [across realms][69] as it can lead to [strange
> behavior][70] with [frontend frames/windows][71] and with tools that rely on
> [Node's VM module][72] (like Jest).

If the value of `throws` is a class, that class must [be a subtype of
`Error`][77] or the behavior of babel-plugin-tester is undefined.

Note that this property cannot be present when using the [`output`][87],
[`outputRaw`][88], [`outputFixture`][37], [`exec`][89], [`execFixture`][38], or
[`snapshot`][50] properties.

> [!NOTE]
>
> For backwards compatibility reasons, `throws` is synonymous with `error`. They
> can be used interchangeably, with `throws` taking precedence.

###### `setup`

This function will be run before a particular test is run. It can return a
function which will be treated as a [`teardown`][90] function. It can also
return a promise. If that promise resolves to a function, that will be treated
as a [`teardown`][90] function.

This function, if provided, will run _after_ any [`setup`][47] function provided
as a babel-plugin-tester option. See [here][46] for the complete run order.

###### `teardown`

This function will be run after a test finishes running. You can define this via
`teardown` or you can return it from the [`setup`][91] function. This can
likewise return a promise if it is asynchronous.

This function, if provided, will be run _after_ any teardown function returned
by the [`setup`][91] property, both of which will run _before_ any
[`teardown`][45] function provided as a babel-plugin-tester option. See
[here][46] for the complete run order.

###### `formatResult`

This function is used to format all babel outputs, and defaults to a function
that invokes [prettier][48]. If a prettier configuration file is [found][49],
then that will be used. Otherwise, prettier will use its own default
configuration.

You can also [entirely disable formatting][39].

This will override the [`formatResult`][81] function provided to
babel-plugin-tester.

###### `snapshot`

If you would prefer to take a snapshot of babel's output rather than compare it
to something you provide manually, specify `snapshot: true`. This will cause
babel-plugin-tester to generate a snapshot containing both the [source code][86]
and babel's output.

Defaults to `false`.

Note that this property cannot appear in the same test object as the
[`output`][87], [`outputFixture`][37], [`exec`][89], [`execFixture`][38], or
[`throws`][84] properties. However, it _can_ be used with [`outputRaw`][88].

> [!IMPORTANT]
>
> Requires [Jest][66], an [appropriate shim][92] or equivalent interface (like
> [Vitest][8]), or a manually-defined `expect` object exposing an appropriate
> [`toMatchSnapshot`][93] method.

###### `code`

The code that you want babel to transform using your plugin or preset. This must
be provided unless you are using the [`codeFixture`][36] or [`exec`][89]
properties instead. If you do not provide the [`output`][87] or
[`outputFixture`][37] properties, and [`snapshot`][50] is not truthy, then the
assertion is that this code is unchanged by the transformation.

Before being transformed by babel, any indentation will be stripped as a
convenience for template literals. After being transformed, the resulting output
will have whitespace trimmed, line endings [converted][57], and then get
[formatted by prettier][39].

Note that this property cannot appear in the same test object as the
[`codeFixture`][36], [`exec`][89], or [`execFixture`][38] properties.

###### `output`

The value of this property will be compared with the output from [babel's
`transform` function][83].

Before being compared to babel's output, this value will have whitespace
trimmed, line endings [converted][57], and any indentation stripped as a
convenience for template literals.

Note that this property cannot appear in the same test object as the
[`outputFixture`][37], [`exec`][89], [`execFixture`][38], [`throws`][84], or
[`snapshot`][50] properties. However, it _can_ be used with [`outputRaw`][88].

###### `outputRaw`

> [!WARNING]
>
> This feature is only available in `babel-plugin-tester@>=12`.

This property is similar to [`output`][87] and related properties except it
tests against the _entire [`BabelFileResult`][82] object_ returned by [babel's
`transform` function][83] instead of only the `code` property of
[`BabelFileResult`][82].

`outputRaw` must be a function with the following signature:

```typescript
outputRaw: (output: BabelFileResult) => void
```

Where the `output` parameter is an instance of [`BabelFileResult`][82]:

```typescript
interface BabelFileResult {
  ast?: Node | undefined;
  code?: string | undefined;
  ignored?: boolean | undefined;
  map?: object | undefined;
  metadata?: BabelFileMetadata | undefined;
}
```

So long as the `outputRaw` function does not throw, this property will never
cause the test to fail. On the other hand, if the `outputRaw` function throws,
such as when `expect(output.metadata).toStrictEqual({ ... })` fails, the test
will fail regardless of other properties.

The `output` parameter is not trimmed, converted, stripped, or modified at all.

Note that `outputRaw` does not _replace_ [`output`][87] etc, it only adds
additional (custom) expectations to your test. Further note that `outputRaw`
_can_ appear in the same test object as any other property except
[`throws`][84].

###### `exec`

The provided source will be transformed just like the [`code`][86] property,
except the output will be _evaluated_ in the [same _CJS_ context][61] as the
test runner itself, meaning it supports features like a/sync IIFEs, debugging
breakpoints (!), and has access to mocked modules, `expect`, `require`,
`__dirname` and `__filename` (derived from available path info and falling back
on [`filepath`][54]), and other globals/features provided by your test
framework. However, the context does not support _`import`, top-level await, or
any other ESM syntax_. Hence, while any file extension can be used (e.g. `.ts`,
`.vue`, `.jsx`), this file will always be evaluated as CJS.

The test will always pass unless an exception is thrown (e.g. when an `expect()`
fails).

Use this to make advanced assertions on the output. For example, you can test
that [babel-plugin-proposal-throw-expressions][62] actually throws using the
following:

```javascript
{
  // ...
  exec: `
    expect(() => throw new Error('throw expression')).toThrow('throw expression');
  `;
}
```

> [!CAUTION]
>
> Keep in mind that, despite sharing a global context, execution will occur in a
> [separate realm][63], which means native/intrinsic types will be different.
> This can lead to unexpectedly failing tests. For example:
>
> ```javascript
> expect(require(`${__dirname}/imported-file.json`)).toStrictEqual({
>   data: 'imported'
> });
> ```
>
> This may fail in some test frameworks with the message "serializes to the same
> string". This is because the former object's `Object` prototype comes from a
> different realm than the second object's `Object` prototype, meaning the two
> objects are not technically _strictly_ equal. However, something like the
> following, which creates two objects in the same realm, will pass:
>
> ```javascript
> expect(
>   Object.fromEntries(
>     Object.entries(require(`${__dirname}/imported-file.json`))
>   )
> ).toStrictEqual({ data: 'imported' });
> ```
>
> Or use `JSON.stringify` + `toBe` (or your testing framework's equivalent):
>
> ```javascript
> expect(JSON.stringify(require(`${__dirname}/imported-file.json`))).toBe(
>   JSON.stringify({ data: 'imported' })
> );
> ```
>
> Or use `isEqual` (or your testing framework's equivalent):
>
> ```javascript
> expect(require(`${__dirname}/imported-file.json`)).toEqual({
>   data: 'imported'
> });
> ```

After being transformed by babel but before being evaluated, the babel output
will have whitespace trimmed, line endings [converted][57], and then get
[formatted by prettier][39].

Note that this property cannot appear in the same test object as the
[`execFixture`][38], [`code`][86], [`codeFixture`][36], [`output`][87],
[`outputFixture`][37], [`throws`][84], or [`snapshot`][50] properties. However,
it _can_ be used with [`outputRaw`][88].

###### `codeFixture`

If you would rather put your [`code`][86] in a separate file, you can specify a
file path here instead. If it is an absolute path, then that's the file that
will be loaded. Otherwise, `codeFixture` will be [`path.join`][40]'d with the
[directory name][41] of [`filepath`][54].

After being transformed by babel, the resulting output will have whitespace
trimmed, line endings [converted][57], and then get [formatted by prettier][39].

Like [`code`][86], this property cannot appear in the same test object as the
[`exec`][89] or [`execFixture`][38] properties, nor the [`code`][86] property.

> [!TIP]
>
> If you find you are using this property more than a couple of times, consider
> using [`fixtures`][35] instead.

> [!NOTE]
>
> For backwards compatibility reasons, `codeFixture` is synonymous with
> `fixture`. They can be used interchangeably, though care must be taken not to
> confuse the test object property `fixture` with the babel-plugin-tester option
> [_`fixtures`_][35], the latter being plural.

###### `outputFixture`

If you would rather put your [`output`][87] in a separate file, you can specify
a file path here instead. If it is an absolute path, then that's the file that
will be loaded. Otherwise, `outputFixture` will be [`path.join`][40]'d with the
[directory name][41] of [`filepath`][54].

Before being compared to babel's output, this file's contents will have
whitespace trimmed and line endings [converted][57].

Like [`output`][87], this property cannot appear in the same test object as the
[`exec`][89], [`execFixture`][38], [`throws`][84], or [`snapshot`][50]
properties, nor the [`output`][87] property. However, it _can_ be used with
[`outputRaw`][88].

> [!TIP]
>
> If you find you are using this property more than a couple of times, consider
> using [`fixtures`][35] instead.

###### `execFixture`

If you would rather put your [`exec`][89] in a separate file, you can specify a
file path here instead. If it is an absolute path, then that's the file that
will be loaded. Otherwise, `execFixture` will be [`path.join`][40]'d with the
[directory name][41] of [`filepath`][54].

After being transformed by babel but before being evaluated, the babel output
will have whitespace trimmed, line endings [converted][57], and then get
[formatted by prettier][39].

Like [`exec`][89], this property cannot appear in the same test object as the
[`code`][86], [`codeFixture`][36], [`output`][87], [`outputFixture`][37],
[`throws`][84], or [`snapshot`][50] properties, nor the [`exec`][89] property.
However, it _can_ be used with [`outputRaw`][88].

> [!TIP]
>
> If you find you are using this property more than a couple of times, consider
> using [`fixtures`][35] instead.

## Examples

### Simple Example

```javascript
import { pluginTester } from 'babel-plugin-tester';
import identifierReversePlugin from '../identifier-reverse-plugin';

// NOTE: you can use beforeAll, afterAll, beforeEach, and afterEach as usual,
// but initial configuration tasks, like loading content from fixture files,
// will complete *at the point the pluginTester function is called* which means
// BEFORE beforeAll and other Jest hooks are run.

pluginTester({
  plugin: identifierReversePlugin,
  // Defaults to false, but with this line we set the default to true across
  // *all* tests.
  snapshot: true,
  tests: [
    {
      code: "'hello';"
      // Snapshot should show that prettier has changed the single quotes to
      // double quotes (using prettier's default configuration).
    },
    {
      // This test will pass if and only if code has not changed.
      code: '"hello";'
      // To prevent false negatives (like with reckless use of `npx jest -u`),
      // snapshots of code that does not change are forbidden. Snapshots
      // succeed only when babel output !== code input.
      snapshot: false;
    },
    {
      code: 'var hello = "hi";',
      output: 'var olleh = "hi";',
      // You can't take a snapshot and also manually specify an output string.
      // It's either one or the other.
      snapshot: false
    },
    // A valid test can be a test object or a simple string.
    `
      function sayHi(person) {
        return 'Hello ' + person + '!'
      }
      console.log(sayHi('Jenny'))
    `
  ]
});
```

### Full Example

```javascript
import path from 'node:path';
import { pluginTester } from 'babel-plugin-tester';
import identifierReversePlugin from '../identifier-reverse-plugin';

pluginTester({
  // One (and ONLY ONE) of the two following lines MUST be included.
  plugin: identifierReversePlugin,
  //preset: coolNewBabelPreset,

  // Usually unnecessary if it is returned by the plugin. This will default to
  // 'unknown plugin' if a name cannot otherwise be inferred.
  pluginName: 'identifier reverse',
  // Unlike with pluginName, there is no presetName inference. This will default
  // to 'unknown preset' if a name is not provided.
  //presetName: 'cool-new-babel-preset',

  // Used to test specific plugin options.
  pluginOptions: {
    optionA: true
  },
  //presetOptions: {
  //  optionB: false,
  //}

  // Defaults to the plugin name.
  title: 'describe block title',

  // Only useful if you are using fixtures, codeFixture, outputFixture, or
  // execFixture options. Defaults to the absolute path of the file the
  // pluginTester function was invoked from, which in this case  is equivalent
  // to the following line:
  filepath: __filename,

  // These are the defaults that will be lodash.mergeWith'd with the provided
  // babelOptions option.
  babelOptions: {
    parserOpts: {},
    generatorOpts: {},
    babelrc: false,
    configFile: false
  },

  // Defaults to false but we're being explicit here: do not use snapshots
  // across all tests. Note that snapshots are only guaranteed to work with
  // Jest.
  snapshot: false,

  // Defaults to a function that formats with prettier.
  formatResult: customFormatFunction,

  // You can provide tests as an object:
  tests: {
    // The key is the title. The value is the code that is unchanged (because
    // snapshot === f