Browser Launch API

Before Cypress launches a browser, it gives you the opportunity to modify the browser preferences, install extensions, add and remove command-line arguments, and modify other options from your pluginsFile.

Syntax

on('before:browser:launch', (browser = {}, launchOptions) => {
  /* ... */
})

browser (object)

An object describing the browser being launched, with the following properties:

PropertyTypeDescription
namestringMachine-friendly name, like chrome, electron, edge, or firefox.
familystringRendering engine being used. chromium or firefox.
channelstringRelease channel of the browser, such as stable, dev, or canary.
displayNamestringHuman-readable display name for the browser.
versionstringFull version.
pathstringPath to the browser on disk. Blank for Electron.
infostring(Optional) Extra information about the browser (used for display in the browser selector of the Test Runner)
majorVersionnumberThe major version number of the browser.
isHeadlessbooleanWhether the browser is running headlessly.
isHeadedbooleanWhether the browser displays headed.

launchOptions (object)

Options that can be modified to control how the browser is launched, with the following properties:

PropertyTypeDescription
preferencesobjectAn object describing browser preferences. Differs between browsers. See Changing browser preferences for details.
argsstring[]An array of strings that will be passed as command-line args when the browser is launched. Has no effect on Electron. See Modify browser launch arguments for details.
extensionsstring[]An array of paths to folders containing unpacked WebExtensions to be loaded before the browser starts. Note: Electron currently only supports Chrome DevTools extensions. See Add browser extensions for details.

Usage

Modify browser launch arguments, preferences, and extensions

Using your pluginsFile you can tap into the before:browser:launch event and modify how Cypress launches the browser (e.g. modify arguments, user preferences, and extensions).

This event will yield you the browser object, which gives you information about the browser, and the launchOptions object, which allows you to modify how the browser is launched.

The returned launchOptions object will become the new launch options for the browser.

Modify browser launch arguments:

Here are args available for the currently supported browsers:

Open devtools by default
// cypress/plugins/index.js
module.exports = (on, config) => {
  on('before:browser:launch', (browser = {}, launchOptions) => {
    // `args` is an array of all the arguments that will
    // be passed to browsers when it launches
    console.log(launchOptions.args) // print all current args

    if (browser.family === 'chromium' && browser.name !== 'electron') {
      // auto open devtools
      launchOptions.args.push('--auto-open-devtools-for-tabs')
    }

    if (browser.family === 'firefox') {
      // auto open devtools
      launchOptions.args.push('-devtools')
    }

    if (browser.name === 'electron') {
      // auto open devtools
      launchOptions.preferences.devTools = true
    }

    // whatever you return here becomes the launchOptions
    return launchOptions
  })
}

Add browser extensions:

// cypress/plugins/index.js
module.exports = (on, config) => {
  on('before:browser:launch', (browser, launchOptions) => {
    // supply the absolute path to an unpacked extension's folder
    // NOTE: extensions cannot be loaded in headless Chrome
    launchOptions.extensions.push('Users/jane/path/to/extension')

    return launchOptions
  })
}

Changing browser preferences:

Here are preferences available for the currently supported browsers:

// cypress/plugins/index.js
module.exports = (on, config) => {
  on('before:browser:launch', (browser, launchOptions) => {
    if (browser.family === 'chromium' && browser.name !== 'electron') {
      // in Chromium, preferences can exist in Local State, Preferences, or Secure Preferences
      // via launchOptions.preferences, these can be acccssed as `localState`, `default`, and `secureDefault`

      // for example, to set `somePreference: true` in Preferences:
      launchOptions.preferences.default['preference'] = true

      return launchOptions
    }

    if (browser.family === 'firefox') {
      // launchOptions.preferences is a map of preference names to values
      launchOptions.preferences['some.preference'] = true

      return launchOptions
    }

    if (browser.name === 'electron') {
      // launchOptions.preferences is a `BrowserWindow` `options` object
      launchOptions.preferences.darkTheme = true

      return launchOptions
    }
  })
}

Modify Electron app switches

Cypress Test Runner is an Electron application, and its behavior (and the behavior of the bundled-in Electron browser) can be customized using command line switches. The supported switches depend on the Electron version, see Electron documentation.

You can pass Electron-specific launch arguments using the ELECTRON_EXTRA_LAUNCH_ARGS environment variable. For example, to disable HTTP browser cache and ignore certificate errors, you can set the environment variables before running Cypress like below:

Linux/OSX

export ELECTRON_EXTRA_LAUNCH_ARGS=--disable-http-cache --lang=es

Windows

set ELECTRON_EXTRA_LAUNCH_ARGS=--disable-http-cache --lang=es

Cypress already sets some the Electron command line switches internally. See file packages/server/lib/environment.js. There is no way to see all currently set switches after Electron's launch.

See all Chrome browser switches

If you are running Cypress tests using a Chromium-based browser, you can see ALL currently set command line switches and the browser version information by opening a new tab and typing chrome://version url.

See all Chrome switches

Examples

Set screen size when running headless

When a browser runs headless, there is no physical display. You can override the default screen size of 1280x720 when running headless as shown below. This will affect the size of screenshots and videos taken during the run.

// cypress/plugins/index.js
module.exports = (on, config) => {
  on('before:browser:launch', (browser, launchOptions) => {
    if (browser.name === 'chrome' && browser.isHeadless) {
      // fullPage screenshot size is 1400x1200 on non-retina screens
      // and 2800x2400 on retina screens
      launchOptions.args.push('--window-size=1400,1200')

      // force screen to be non-retina (1400x1200 size)
      launchOptions.args.push('--force-device-scale-factor=1')

      // force screen to be retina (2800x2400 size)
      // launchOptions.args.push('--force-device-scale-factor=2')
    }

    if (browser.name === 'electron' && browser.isHeadless) {
      // fullPage screenshot size is 1400x1200
      launchOptions.preferences.width = 1400
      launchOptions.preferences.height = 1200
    }

    if (browser.name === 'firefox' && browser.isHeadless) {
      // menubars take up height on the screen
      // so fullPage screenshot size is 1400x1126
      launchOptions.args.push('--width=1400')
      launchOptions.args.push('--height=1200')
    }

    return launchOptions
  })
}

Start fullscreen

// cypress/plugins/index.js
module.exports = (on, config) => {
  on('before:browser:launch', (browser = {}, launchOptions) => {
    if (browser.family === 'chromium' && browser.name !== 'electron') {
      launchOptions.args.push('--start-fullscreen')

      return launchOptions
    }

    if (browser.name === 'electron') {
      launchOptions.preferences.fullscreen = true

      return launchOptions
    }
  })
}

Use fake video for webcam testing

By default, Cypress passes the Chrome command line switch to enable a fake video for a media stream. This is to better enable testing webcam functionality without having to have the necessary hardware to test.

Enable fake video for testing

You can however send your own video file for testing by passing a Chrome command line switch pointing to a video file.

// cypress/plugins/index.js
module.exports = (on, config) => {
  on('before:browser:launch', (browser = {}, launchOptions) => {
    if (browser.family === 'chromium' && browser.name !== 'electron') {
      // Mac/Linux
      launchOptions.args.push(
        '--use-file-for-fake-video-capture=cypress/fixtures/my-video.y4m'
      )

      // Windows
      // launchOptions.args.push('--use-file-for-fake-video-capture=c:\\path\\to\\video\\my-video.y4m')
    }

    return launchOptions
  })
}

Support unique file download mime types

Cypress supports a myriad of mime types when testing file downloads, but in case you have a unique one, you can add support for it.

module.exports = (on) => {
  on('before:browser:launch', (browser, options) => {
    // only Firefox requires all mime types to be listed
    if (browser.family === 'firefox') {
      const existingMimeTypes =
        options.preferences['browser.helperApps.neverAsk.saveToDisk']
      const myMimeType = 'my/mimetype'

      // prevents the browser download prompt
      options.preferences[
        'browser.helperApps.neverAsk.saveToDisk'
      ] = `${existingMimeTypes},${myMimeType}`

      return options
    }
  })
}

Set a Firefox flag

If we need to set a particular Firefox flag, like browser.send_pings we can do it via preferences

module.exports = (on) => {
  on('before:browser:launch', (browser = {}, launchOptions) => {
    if (browser.family === 'firefox') {
      launchOptions.preferences['browser.send_pings'] = true
    }

    return launchOptions
  })
}

The above example comes from the blog post How to Test Anchor Ping.

History

VersionChanges
4.0.0New options object replaces old args as second argument to before:browser:launch
4.0.0All Chromium-based browsers, including Electron, have chromium set as their family property.
4.0.0Added channel property to browser.