A preprocessor is the plugin responsible for preparing a support file or a test file for the browser.

A preprocessor could transpile your file from another language (CoffeeScript or ClojureScript) or from a newer version of JavaScript (ES2017).

A preprocessor also typically watches the source files for changes, processes them again, and then notifies Cypress to re-run the tests.

Examples

We’ve created three preprocessors as examples for you to look at. These are fully functioning preprocessors.

The code contains comments that explain how it utilizes the preprocessor API.

Defaults

By default, Cypress comes packaged with the webpack preprocessor already installed.

The webpack preprocessor handles:

ES2015 and JSX via Babel
TypeScript
CoffeeScript 1.x.x
Watching and caching files

Are you looking to change the default options for webpack?

If you already use webpack in your project, you can pass in your webpack config as shown here.

If you don’t use webpack in your project or would like to keep the majority of the default options, you can modify the default options. Editing the options allows you to do things like:

Add your own Babel plugins
Modify options for TypeScript compilation
Add support for CoffeeScript 2.x.x

Usage

To use a preprocessor, you should bind to the file:preprocessor event in your pluginsFile:

// plugins file
module.exports = (on, config) => {
  on('file:preprocessor', (file) => {
    // ...
  })
}

The callback function should return one of the following:

A promise* that eventually resolves the path to the built file**.
A promise* that eventually rejects with an error that occurred during processing.

* The promise should resolve only after the file has completed writing to disk. The promise resolving is a signal that the file is ready to be served to the browser.

** The built file is the file that is created by the preprocessor that will eventually be served to the browser.

If, for example, the source file is spec.coffee, the preprocessor should:

Compile the CoffeeScript into JavaScript spec.js

Write that JavaScript file to disk (example: /Users/foo/tmp/spec.js)

Resolve with the absolute path to that file: /Users/foo/tmp/spec.js

This callback function can and will be called multiple times with the same filePath.

The callback function is called any time a file is requested by the browser. This happens on each run of the tests.

Make sure not to start a new watcher each time it is called. Instead, cache the watcher and, on subsequent calls, return a promise that resolves when the latest version of the file has been processed.

File object

The file object passed to the callback function has the following properties:

Property	Description
`filePath`	The full path to the source file.
`outputPath`	The suggested path for saving the preprocessed file to disk. This is unique to the source file. A preprocessor can choose to write the file elsewhere, but Cypress automatically provides you this value as a convenient default.
`shouldWatch`	A boolean indicating whether the preprocessor should watch for file changes or not.

File events

The file object passed to the callback function is an Event Emitter.

Receiving ‘close’ event

When the running spec, the project, or the browser is closed while running tests, the close event will be emitted. The preprocessor should do any necessary cleanup in this function, like closing the watcher when watching.

// example
const watcher = fs.watch(filePath, /* ... */)

file.on('close', () => {
  watcher.close()
})

Sending ‘rerun’ event

If watching for file changes, emit rerun after a file has finished being processed to let Cypress know to rerun the tests.

// example
fs.watch(filePath, () => {
  file.emit('rerun')
})

Publishing

Publish preprocessors to npm with the naming convention cypress-*-preprocessor (e.g. cypress-clojurescript-preprocessor).

Use the following npm keywords:

"keywords": [
  "cypress",
  "cypress-plugin",
  "cypress-preprocessor"
]

Feel free to submit your published plugins to our list of plugins.

W3cubDocs

Preprocessors API