Table of Contents

• Why not just use x?
• Installation
  • Manual installation
    • ESM version
    • CJS (CommonJS) version
  • Babel (only for CJS)
  • TypeScript
    • ESM version
    • CJS version
  • Preprocess
• Options
  • CJS mode options
• Testing Library
• Credits
• LICENSE

Why not just use x?

Seems like other libraries won't be working with preprocessors, won't be maintained actively or didn't have proper licensing.

Installation

If you're using SvelteKit, you can set it up and install it with svelte-add-jest by running:

npx apply rossyman/svelte-add-jest

Manual installation

This library has peerDependencies listings for svelte >= 3 and jest >= 27.

npm install svelte-jester -D

ESM version

Add the following to your Jest config:

{
  "transform": {
    "^.+\\.svelte$": "svelte-jester"
  },
  "moduleFileExtensions": ["js", "svelte"],
  "extensionsToTreatAsEsm": [".svelte"]
}

Run your tests with NODE_OPTIONS=--experimental-vm-modules. For Windows you need to add cross-env as well.

  "test": "cross-env NODE_OPTIONS=--experimental-vm-modules jest src",

CJS (CommonJS) version

Add the following to your Jest config:

{
  "transform": {
    "^.+\\.svelte$": "./node_modules/svelte-jester/dist/transformer.cjs"
  },
  "moduleFileExtensions": ["js", "svelte"]
}

Babel (only for CJS)

npm install @babel/core @babel/preset-env babel-jest -D

Add the following to your Jest config

"transform": {
  "^.+\\.js$": "babel-jest",
  "^.+\\.svelte$": "svelte-jester"
}

Create a .babelrc and add the following

{
  "presets": [["@babel/preset-env", { "targets": { "node": "current" } }]]
}

TypeScript

To enable TypeScript support you'll need to setup svelte-preprocess and ts-jest.

1. Install typescript, svelte-preprocess, and ts-jest:

   npm install typescript svelte-preprocess ts-jest -D

ESM version

1. Create a svelte.config.js at the root of your project:

   import preprocess from 'svelte-preprocess'
   
   /** @type {import('@sveltejs/kit').Config} */
   export default config = {
     preprocess: preprocess(),
     // ...
   };

   To learn what options you can pass to sveltePreprocess, please refer to the documentation.

2. In your Jest config, enable preprocessing for svelte-jester, and add ts-jest as a transform:

   "transform": {
     "^.+\\.svelte$": [
       "svelte-jester",
       {
         "preprocess": true
       }
     ],
     "^.+\\.ts$": [
       "ts-jest",
       {
         "useESM": true
         // optional: seperate tsconfig for tests
         //"tsconfig": "tsconfig.spec.json",
       }
     ],
   },
   "moduleFileExtensions": [
     "js",
     "ts",
     "svelte"
   ],
   "extensionsToTreatAsEsm": [
     ".svelte",
     ".ts"
   ],

However if you do not want to create a svelte.config.js at the root of your project or you wish to use a custom config just for tests, you may pass the path to the config file to the preprocess option thus:

  "transform": {
    "^.+\\.svelte$": [
      "svelte-jester",
      {
        "preprocess": "/some/path/to/svelte.config.js"
      }
    ],
    "^.+\\.ts$": [
      "ts-jest",
      {
        "useESM": true
        // optional: seperate tsconfig for tests
        //"tsconfig": "tsconfig.spec.json",
      }
    ],
  },
  "moduleFileExtensions": [
    "js",
    "ts",
    "svelte"
  ],
  "extensionsToTreatAsEsm": [
    ".svelte",
    ".ts"
  ],

CJS version

1. Create a svelte.config.js at the root of your project:

   const sveltePreprocess = require("svelte-preprocess");
   
   module.exports = {
     preprocess: sveltePreprocess({
       // ...
     }),
   };

   To learn what options you can pass to sveltePreprocess, please refer to the documentation.

2. In your Jest config, enable preprocessing for svelte-jester, and add ts-jest as a transform:

   "transform": {
     "^.+\\.svelte$": [
       "./node_modules/svelte-jester/dist/transformer.cjs",
       {
         "preprocess": true
       }
     ],
     "^.+\\.ts$": "ts-jest"
   },
   "moduleFileExtensions": [
     "js",
     "ts",
     "svelte"
   ]

However if you do not want to create a svelte.config.js at the root of your project or you wish to use a custom config just for tests, you may pass the path to the config file to the preprocess option thus:

  "transform": {
    "^.+\\.svelte$": [
      "./node_modules/svelte-jester/dist/transformer.cjs",
      {
        "preprocess": "/some/path/to/svelte.config.js"
      }
    ],
    "^.+\\.ts$": "ts-jest"
  },
  "moduleFileExtensions": [
    "js",
    "ts",
    "svelte"
  ]

Note that TypeScript supports ES modules, so if you were previously using babel-jest just for ES module transpilation, you can remove babel-jest, babel, and any associated presets and config.

By default, ts-jest will only transpile .ts files though, so if you want to continue using ES modules in .js files, you'll need to configure ts-jest to process .js files as well. To do this, update the file glob above, and follow the instructions in the ts-jest docs.

Preprocess

Preprocessors are loaded from svelte.config.js or svelte.config.cjs.

Add the following to your Jest config:

"transform": {
  "^.+\\.svelte$": ["svelte-jester", { "preprocess": true }]
}

For CJS, replace "svelte-jester" with "./node_modules/svelte-jester/dist/transformer.cjs".

Create a svelte.config.js file and configure it, see svelte-preprocess for more information.

In CJS mode, svelte-jester must start a new a process for each file needing to be preprocessed, which adds a performance overhead.

In ESM mode, this isn't necessary. You can set NODE_OPTIONS=--experimental-vm-modules and "extensionsToTreatAsEsm": [".svelte"] to run in ESM mode. However, be aware that ESM support in Jest is still experimental as according to their docs. Follow the development along at https://github.com/facebook/jest/issues/9430.

Options

preprocess (default: false): Pass in true if you are using Svelte preprocessors. They are loaded from svelte.config.js or svelte.config.cjs.

debug (default: false): If you'd like to see the output of the compiled code then pass in true.

svelteVersion (default: actual Version from Svelte package): If you'd like to override the svelteVersion for some reason.

compilerOptions (default: {}): Use this to pass in Svelte compiler options.

rootMode (default: ""): Pass in upward to walk up from the transforming file's directory and use the first svelte.config.js or svelte.config.cjs found, or throw an error if no config file is discovered. This is particularly useful in a monorepo as it allows you to:

• Run tests from the worktree root using Jest projects where you only want to put svelte.config.js in workspace folders, and not in the root.
• Run tests from the worktree root using Jest projects, but have different svelte.config.js configurations for individual workspaces.
• Have one common svelte.config.js in your worktree root (or any directory above the file being transformed) without needing individual svelte.config.js files in each workspace. Note - this root config file can be overriden if necessary by putting another config file into a workspace folder

The default mode is to load svelte.config.js or svelte.config.cjs from the current project root to avoid the risk of accidentally loading a config file from outside the current project folder.

When upward is set it will stop at the first config file it finds above the file being transformed, but will walk up the directory structure all the way to the filesystem root if it cannot find any config file. This means that if there is no svelte.config.js or svelte.config.cjs file in the project above the file being transformed, it is always possible that someone will have a forgotten config file in their home directory which could cause unexpected errors in your builds.

CJS mode options

showConsoleLog (default: false): If you'd like to see console.logs of the preprocessors then pass in true. Otherwise these will be surpressed, because the compiler could complain about unexpected tokens.

maxBuffer (default: 10485760): Sets limit for buffer when preprocess is true. It defines the largest amount of data in bytes allowed on stdout or stderr for child_process.spawnSync. If exceeded, the child process is terminated and any output is truncated. The default value of 10Mb overrides Node's default value of 1Mb.

"transform": {
  "^.+\\.js$": "babel-jest",
  "^.+\\.svelte$": ["./node_modules/svelte-jester/dist/transformer.cjs", {
    "preprocess": false,
    "debug": false,
    "compilerOptions": {},
    "rootMode": "",
    "maxBuffer": 15000000
  }]
}

Testing Library

This package is required when using Svelte with the Testing Library. If you'd like to avoid including implementation details in your tests, and making them more maintainble in the long run, then consider checking out @testing-library/svelte.

Credits

Thanks to all contributors, inspired by:

• https://github.com/pngwn/svelte-test
• https://github.com/WeAreGenki/minna-ui
• https://github.com/rspieker/jest-transform-svelte

LICENSE

MIT