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Utilities for testing babel plugins

Build Status Code Coverage version downloads MIT License

All Contributors

PRs Welcome Code of Conduct

The problem

You're writing a babel plugin and want to write tests for it.

This solution

This is a fairly simple abstraction to help you write tests for your babel plugin. It works with jest (my personal favorite) and most of it should also work with mocha and jasmine.


This module is distributed via npm which is bundled with node and should be installed as one of your project's devDependencies:

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



import pluginTester from 'babel-plugin-tester'
// or
const pluginTester = require('babel-plugin-tester').default


import yourPlugin from '../your-plugin'

  plugin: yourPlugin,
  tests: {
    /* your test objects */



Your babel plugin. For example:

  plugin: identifierReversePlugin,
  tests: {
    /* your test objects */

// normally you would import this from your plugin module
function identifierReversePlugin() {
  return {
    name: 'identifier reverse',
    visitor: {
      Identifier(idPath) { ='').reverse().join('')


This is used for the describe title as well as the test titles.


This can be used to pass options into your plugin at transform time. This option can be overwritten using the test object.


To use babel.config.js instead of .babelrc, set babelOptions to the config object:

  plugin: yourPlugin,
  babelOptions: require('./babel.config.js'),
  tests: {
    /* your test objects */


This can be used to specify a title for the describe block (rather than using the pluginName).


Relative paths from the other options will be relative to this. Normally you'll provide this as filename: __filename. The only options property affected by this value is fixtures. Test Object properties affected by this value are: fixture and outputFixture. If those properties are not absolute paths, then they will be path.joined with path.dirname of the filename.


This is used to control which line endings the output from babel should have

Options Description
lf Unix - default
crlf Windows
auto Use the system default
preserve Use the line ending from the input


This is a path to a directory with this format:

├── first-test # test title will be: "first test"
│   ├── code.js # required
│   └── output.js # required
└── second-test
    ├── .babelrc # optional
    ├── options.json # optional
    ├── code.js
    └── output.js

With this you could make your test config like so:

  fixtures: path.join(__dirname, '__fixtures__'),

And it would run two tests. One for each directory in __fixtures__, with plugin options set to the content of options.json

Options are inherited, placing a options.json file in __fixtures__ would add those options to all fixtures.


You provide test objects as the tests option to babel-plugin-tester. You can either provide the tests as an object of test objects or an array of test objects.

If you provide the tests as an object, the key will be used as the title of the test.

If you provide an array, the title will be derived from it's index and a specified title property or the pluginName.

Read more about test objects below.


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


Use this to provide your own fixture output file extension. This is particularly useful if you are testing typescript input. If ommited fixture input file extension will be used.

The rest of the options you provide will be [lodash.merge][lodash.merge]d with each test object. Read more about those next!

Test Objects

A minimal test object can be:

  1. A string representing code
  2. An object with a code property

Here are the available properties if you provide an object:


The code that you want to run through your babel plugin. This must be provided unless you provide a fixture instead. If there's no output or outputFixture and snapshot is not true, then the assertion is that this code is unchanged by the plugin.


If provided, this will be used instead of the pluginName. If you're using the object API, then the key of this object will be the title (see example below).


If this is provided, the result of the plugin will be compared with this output for the assertion. It will have any indentation stripped and will be trimmed as a convenience for template literals.


If you'd rather put your code in a separate file, you can specify a filename here. If it's an absolute path, that's the file that will be loaded, otherwise, this will be path.joined with the filename path.


If you'd rather put your output in a separate file, you can specify this instead (works the same as fixture).


To run only this test. Useful while developing to help focus on a single test. Can be used on multiple tests.


To skip running this test. Useful for when you're working on a feature that is not yet supported.


If you'd prefer to take a snapshot of your output rather than compare it to something you hard-code, then specify snapshot: true. This will take a snapshot with both the source code and the output, making the snapshot easier to understand.


If a particular test case should be throwing an error, you can that using one of the following:

  // ...
  error: true,
  error: 'should have this exact message',
  error: /should pass this regex/,
  error: SyntaxError, // should be instance of this constructor
  error: err => {
    if (err instanceof SyntaxError && /message/.test(err.message)) {
      return true; // test will fail if function doesn't return `true`


If you need something set up before a particular test is run, you can do this with setup. This function will be run before the test runs. It can return a function which will be treated as a teardown function. It can also return a promise. If that promise resolves to a function, that will be treated as a teardown function.


If you set up some state, it's quite possible you want to tear it down. You can either define this as its own property, or you can return it from the setup function. This can likewise return a promise if it's asynchronous.


This defaults to a function which formats your code output with prettier. If you have prettier configured, then it will use your configuration. If you don't then it will be default configuration.

If you'd like to specify your own, then feel free to do so. Here's the API:

function customFormatter(code, {filename}) {
  // format the code
  return formattedCode

Learn more about the built-in formatter below.

The use case for this originally was for testing codemods and formatting their result with prettier-eslint.


Full Example + Docs

import pluginTester from 'babel-plugin-tester'
import identifierReversePlugin from '../identifier-reverse-plugin'

// NOTE: you can use beforeAll, afterAll, beforeEach, and afterEach
// right here if you need

  // required
  plugin: identifierReversePlugin,

  // will default to 'unknown plugin'
  pluginName: 'identifier reverse',

  // defaults to the plugin name
  title: 'describe block title',

  // used to test specific plugin options
  pluginOptions: {
    optionA: true,

  // only necessary if you use fixture or outputFixture in your tests
  filename: __filename,

  // these will be `lodash.mergeWith`d with the test objects
  // below are the defaults:
  babelOptions: {
    parserOpts: {},
    generatorOpts: {},
    babelrc: false,
    configFile: false,

  // use jest snapshots (only works with jest)
  snapshot: false,

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

  // tests as objects
  tests: {
    // the key is the title
    // the value is the code that is unchanged (because `snapshot: false`)
    // test title will be: `1. does not change code with no identifiers`
    'does not change code with no identifiers': '"hello";',

    // test title will be: `2. changes this code`
    'changes this code': {
      // input to the plugin
      code: 'var hello = "hi";',
      // expected output
      output: 'var olleh = "hi";',

  // tests as an array
  tests: [
    // should be unchanged by the plugin (because `snapshot: false`)
    // test title will be: `1. identifier reverse`
      // test title will be: `2. identifier reverse`
      code: 'var hello = "hi";',
      output: 'var olleh = "hi";',
      // test title will be: `3. unchanged code`
      title: 'unchanged code',
      // because this is an absolute path, the `fixtures` above will not be
      // used to resolve this path.
      fixture: path.join(__dirname, 'some-path', 'unchanged.js'),
      // no output, outputFixture, or snapshot, so the assertion will be that
      // the plugin does not change this code.
      // because these are not absolute paths, they will be joined with the
      // `fixtures` path provided above
      fixture: '__fixtures__/changed.js',
      // because outputFixture is provided, the assertion will be that the
      // plugin will change the contents of `changed.js` to the contents of
      // `changed-output.js`
      outputFixture: '__fixtures__/changed-output.js',
      // as a convenience, this will have the indentation striped and it will
      // be trimmed.
      code: `
        function sayHi(person) {
          return 'Hello ' + person + '!'
      // this will take a jest snapshot. The snapshot will have both the
      // source code and the transformed version to make the snapshot file
      // easier to understand.
      snapshot: true,
      code: 'var hello = "hi";',
      output: 'var olleh = "hi";',
      // this can be used to overwrite the setting set above
      pluginOptions: {
        optionA: false,
      title: 'unchanged code',
      setup() {
        // runs before this test
        return function teardown() {
          // runs after this tests
        // can also return a promise
      teardown() {
        // runs after this test
        // can return a promise

Simple Example

import pluginTester from 'babel-plugin-tester'
import identifierReversePlugin from '../identifier-reverse-plugin'

  plugin: identifierReversePlugin,
  snapshot: true,
  tests: [
    {code: '"hello";', snapshot: false},
      code: 'var hello = "hi";',
      output: 'var olleh = "hi";',
      function sayHi(person) {
        return 'Hello ' + person + '!'


Un-string snapshot serializer

If you're using jest and snapshots, then the snapshot output could have a bunch of bothersome \" to escape quotes because when Jest serializes a string, it will wrap everything in double quotes. This isn't a huge deal, but it makes the snapshots harder to read. So we automatically add a snapshot serializer for you to remove those.

If you don't like that, then do this:

- import pluginTester from 'babel-plugin-tester'
+ import pluginTester from 'babel-plugin-tester/pure'

Prettier formatter

By default, a formatter is included which formats your results with prettier. It will look for a prettier configuration relative to the file that's being tested or the current working directory. If it can't find one, then it uses the default configuration for prettier.

This makes your snapshots easier to read. But if you'd like to not have that, then you can either import the pure file (as shown above) or you can override the formatResult option:

  // ... other options
  formatResult: r => r,
  // ... more options


I've been thinking about this for a while. The API was inspired by:

Other Solutions


Looking to contribute? Look for the [Good First Issue][good-first-issue] label.

🐛 Bugs

Please file an issue for bugs, missing documentation, or unexpected behavior.

[See Bugs][bugs]

💡 Feature Requests

Please file an issue to suggest new features. Vote on feature requests by adding a 👍. This helps maintainers prioritize what to work on.

[See Feature Requests][requests]


Thanks goes to these people (emoji key):

Kent C. Dodds

💻 📖 🚇 ⚠️

james kyle

💻 📖 👀 ⚠️

Brad Bohen


Kyle Welch

💻 📖 ⚠️



Rubén Norte

💻 ⚠️

André Neves

💻 ⚠️

Kristoffer K.

💻 ⚠️

Alex Kanunnikov

💻 ⚠️

Sebastian Silbermann


Andrey Los


Charles Bodman


Michaël De Boey




Marek Buchar

💻 ⚠️ 📖

Jay Phelps




joe moon

💻 ⚠️

This project follows the all-contributors specification. Contributions of any kind welcome!