babel-plugin-tester
Utilities for testing babel plugins
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.
Installation
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
Usage
import
import pluginTester from 'babel-plugin-tester'
// or
const pluginTester = require('babel-plugin-tester')
Invoke
import yourPlugin from '../your-plugin'
pluginTester({
plugin: yourPlugin,
tests: {
/* your test objects */
},
})
options
plugin
Your babel plugin. For example:
pluginTester({
plugin: identifierReversePlugin,
tests: {
/* your test objects */
},
})
// 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 for the describe title as well as the test titles. If it can be
inferred from the plugin's name then it will be and you don't need to
provide this option.
pluginOptions
This can be used to pass options into your plugin at transform time. This option
can be overwritten using the test object.
babel.config.js
To use babel.config.js instead of
.babelrc, set babelOptions to the config object:
pluginTester({
plugin: yourPlugin,
...
babelOptions: require('./babel.config.js'),
...
tests: {
/* your test objects */
},
});
title
This can be used to specify a title for the describe block (rather than using
the pluginName).
filename
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.
endOfLine
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, #### fixtures
This is a path to a directory with this format:
__fixtures__
├── 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:
pluginTester({
plugin,
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.
tests
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.
babel
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.
...rest
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:
- A
stringrepresenting code - An
objectwith acodeproperty
Here are the available properties if you provide an object:
code
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.
title
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).
output
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.
fixture
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.
outputFixture
If you'd rather put your output in a separate file, you can specify this
instead (works the same as fixture).
only
To run only this test. Useful while developing to help focus on a single test.
Can be used on multiple tests.
skip
To skip running this test. Useful for when you're working on a feature that is
not yet supported.
snapshot
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.
error
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`
}
},
}
setup
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.
teardown
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.
formatResult
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.
Examples
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
pluginTester({
// required
plugin: identifierReversePlugin,
// unnecessary if it's returned with the 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`
'"hello";',
{
// 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'
pluginTester({
plugin: identifierReversePlugin,
snapshot: true,
tests: [
{code: '"hello";', snapshot: false},
{
code: 'var hello = "hi";',
output: 'var olleh = "hi";',
},
`
function sayHi(person) {
return 'Hello ' + person + '!'
}
console.log(sayHi('Jenny'))
`,
],
})
Defaults
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:
pluginTester({
// ... other options
formatResult: r => r,
// ... more options
})
Inspiration
I've been thinking about this for a while. The API was inspired by:
- ESLint's RuleTester
- @thejameskyle's tweet
Other Solutions
Contributors
Thanks goes to these people (emoji key):
This project follows the all-contributors specification.
Contributions of any kind welcome!
LICENSE
MIT