Preprocessors API
A preprocessor is the plugin responsible for preparing a support file or a spec 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
⚠️ This code is part of the
setupNodeEvents function and
thus executes in the Node environment. You cannot call Cypress
or cy
commands in this function, but you do have the direct access to the file system
and the rest of the operating system.
To use a preprocessor, you should bind to the file:preprocessor
event in your
setupNodeEvents function:
- cypress.config.js
- cypress.config.ts
const { defineConfig } = require('cypress')
module.exports = defineConfig({
// setupNodeEvents can be defined in either
// the e2e or component configuration
e2e: {
setupNodeEvents(on, config) {
on('file:preprocessor', (file) => {
// ...
})
},
},
})
import { defineConfig } from 'cypress'
export default defineConfig({
// setupNodeEvents can be defined in either
// the e2e or component configuration
e2e: {
setupNodeEvents(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')
})
Source maps
Cypress utilizes source maps to enhance the error experience. Stack traces are translated so that your source files are shown instead of the generated file that is loaded by the browser. This also enables displaying code frames. Without inline source maps, you will not see code frames.
By default, Cypress will include an inline source map in your spec file, so you will get the most out of the error experience. If you modify the preprocessor, ensure that inline source maps are enabled to get the same experience. Some examples of this include:
- With webpack and the
webpack preprocessor, set
the
devtool
option toinline-source-map
. - With esbuild and the
esbuild preprocessor, set
the
sourcemap
option toinline
when creating the bundler. - With cucumber and the
cucumber preprocessor
third party bundlers are used to bundle the code.
- For esbuild and the
esbuild preprocessor,
set the
sourcemap
option toinline
when creating the bundler. Note: the cucumber preprocessor documentation for the esbuild bundler describes an experimental option:prettySourceMaps
, but this option is buggy and setting source maps on the esbuild bundler directly works better. - The other bundlers all default their source maps appropriately.
- For esbuild and the
esbuild preprocessor,
set the
If using TypeScript with any custom preprocessor, you will want to make sure that the TypeScript
compiler is generating source maps. This can be done by setting the sourceMap
option in your
tsconfig.json
to true
. The inlineSourceMap
option is not recommended as it does not provide an accurate code frame.
Please see our recommended tsconfig.json as a reference.
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.