cypress-devserver-esbuild

cypress-devserver-esbuild

Minimal esbuild dev server for usage with cypress component tests. Tests are packaged as esm-bundles and delivered via a tiny express server. Written in TypeScript with full type definitions included.

Installation

Add the library to your devDependencies

npm install -D cypress-devserver-esbuild

Compatibility

cypress-devserver-esbuild Version	Cypress Version	Notes
1.0.x - 1.4.x	^12.0.0 - ^13.x.x	Initial releases supporting Cypress 12 and 13
1.5.x+	^12.0.0 - ^14.x.x	Added support for Cypress 14 while maintaining backward compatibility

Requirements:

• Cypress: Version 12 or higher is required for all versions
• esbuild: Version 0.17.0 or higher is required as a peer dependency
  • Minimum supported: ^0.17.0
  • Tested up to: ^0.25.5 (latest as of 2025)
• Node.js: Version 16 or higher recommended
• Starting from version 1.5.0, Cypress 14 is also supported

Usage

Import the createEsbuildDevServer-function from cypress-devserver-esbuild. As the first parameter pass your esbuild-configuration. The dev-server will set the entry points and build the tests as esm-bundles with codesplitting. If no outDir is set the devserver will compile the tests into /dist. If you use css-modules and compile them into separate css-files, you will need to set hasCssFiles: true. If your tests result in too many chunks and the MaxConnectionsPerHost-limit is slowing your tests down you can first try to explicitly set splitting: false in your esbuild config. If this is too slow you can also try to set singleBundle: true. This will bundle all the related scripts on demand, however if your test's scope is very wide this might be slow as well. This lib assumes the cypress-config is placed in the project's root-folder.

const { defineConfig } = require('cypress')
const { join } = require('path')
const { cssModulesPlugin } = require('@asn.aeb/esbuild-css-modules-plugin')
const { createEsbuildDevServer } = require('./src/dev-server')

const testConfig = {
  outdir: './dist-test',
  plugins: [
    cssModulesPlugin({
      emitCssBundle: {
        filename: 'css-bundle.css'
      }
    })
  ]
}

module.exports = defineConfig({
  component: {
    specPattern: './cypress/component/*.cy.jsx',
    devServer: createEsbuildDevServer(testConfig, {
      singleBundle: true,
      getCssFilePath: (spec, outdir) => join(outdir, './css-bundle.css'),
      port: 1234
    })
  }
})

Dev Server Parameters

Esbuild Config

The first parameter is your esbuild config. Please note that spec-pattern is resolved using globby which accepts different patterns compared to esbuild, so only glob-patterns supported by globby will work. Also the following parameters are overwritten by the dev-server:

Option	Value	Note
entryPoints	supportFile, test1, test2, ...	List of paths to all tests + the support file
bundle	true	bundle needs to be true to be able to load tests with an import
publicPath	null	public paths are not required for cypress tests and setting one will cause the test to break
format	'esm'	format needs to be esm to be able to use code splitting and for imports to work
splitting	true	splitting defaults to true, as it's much faster during build-time
outbase	'./'	outbase should be set so the folder structure is preserved and tests can be consistenly accessed
outdir	'./dist'	outdir is required for splitting

Note: Italic values are overwriteable

Options

additionalEntryPoints

Default: undefined

You can add a list of additional entry points to be build if needed. The array will be directly merged to the esbuild config building your tests.

getCssFilePath

Default: undefined

If a function is passed, the function will be called to get the path(s) of css files to be included with the test. The first parameter is the Cypress spec object containing the relative and absolute paths of the test. The second parameter is the output dir. You can return either a single full path to a css file, or an Array of paths. The content of the css files will be injected in a style-tag at the end of the head of the test's index.html file.

singleBundle

Default: false

If set to true esbuild will bundle your test and all it's dependencies into a single file. This might slow down the startup of your tests, so check the output for the build-times.

singleBundleConfig

Default: undefined

Additional esbuild-config parameters for the esbuild-job used to merge files into a single bundle.

port

Default: 0

logFunction

You can provide a log function. The first parameter of the function is the loglevel, ranging from 1 to 6 with 1 being critical errors and 6 debug information. A reasonable default might be to log output <= 3. All following parameters are part of the log message.