karma-rollup-preprocessor

7.0.8 verified Mon Apr 27 auth: no javascript maintenance

Karma preprocessor to bundle ES modules using Rollup. Current stable version 7.0.8 (last released 2021). Release cadence is sporadic with long gaps between major versions. Key differentiators: integrates Rollup into Karma test pipeline, supports all Rollup options, allows multiple configured preprocessors for different transpilation setups. Requires Rollup >=1.0.0 as peer dependency.

Common errors

error Error: Invalid configuration: preprocessors.rollup expected a string or an array of strings. ↓

cause Preprocessor name used incorrectly (e.g., 'rollupPreprocessor' instead of 'rollup').

fix

In preprocessors config, use 'rollup' as the preprocessor name, not 'rollupPreprocessor'.

error TypeError: Cannot read property 'plugins' of undefined ↓

cause rollupPreprocessor object is missing or not correctly placed in Karma config.

fix

Ensure rollupPreprocessor is defined inside config.set({...}) as a top-level key.

error Error: ENOENT: no such file or directory, open '...' ↓

cause Input file not found because Rollup is trying to resolve a missing module.

fix

Install missing npm packages and configure resolve plugins (e.g., @rollup/plugin-node-resolve).

Warnings

deprecated rollup-plugin-buble and rollup-plugin-babel are no longer maintained; use @rollup/plugin-buble and @rollup/plugin-babel instead. ↓

fix Replace require('rollup-plugin-buble') with require('@rollup/plugin-buble') and update configuration accordingly.

gotcha Karma's file watcher must be disabled for files processed by rollupPreprocessor (watched: false). ↓

fix Set `watched: false` in the files array for patterns handled by rollup preprocessor.

gotcha The Rollup input option is handled automatically; setting it manually can cause issues. ↓

fix Do not specify `input` in rollupPreprocessor options; the preprocessor sets it to each test file.

gotcha Source map file paths may be incorrect on Windows when using absolute paths. ↓

fix Use relative paths or update to version 7.0.8 which includes a fix.

Install

npm install karma-rollup-preprocessor

yarn add karma-rollup-preprocessor

pnpm add karma-rollup-preprocessor

Imports

rollupPreprocessor
wrong
```
module.exports = { rollupPreprocessor: ... }
```
correct
```
config.set({ rollupPreprocessor: { ... } })
```
Configuration is set inside Karma config function, not exported directly.
rollup
wrong
```
preprocessors: { 'test/**/*.spec.js': ['rollupPreprocessor'] }
```
correct
```
preprocessors: { 'test/**/*.spec.js': ['rollup'] }
```
The preprocessor name is 'rollup', not 'rollupPreprocessor'. The configuration key is 'rollupPreprocessor'.

customPreprocessors

wrong

customPreprocessors: { rollupBabel: { base: 'rollupPreprocessor', options: { ... } } }

correct

customPreprocessors: { rollupBabel: { base: 'rollup', options: { ... } } }

The base preprocessor is referenced as 'rollup', not 'rollupPreprocessor'.

Quickstart

Basic Karma configuration with rollupPreprocessor for bundling test files.

// karma.conf.js
module.exports = function(config) {
  config.set({
    files: [
      { pattern: 'test/**/*.spec.js', watched: false }
    ],
    preprocessors: {
      'test/**/*.spec.js': ['rollup']
    },
    rollupPreprocessor: {
      plugins: [
        require('rollup-plugin-commonjs')(),
        require('rollup-plugin-node-resolve')()
      ],
      output: {
        format: 'iife',
        name: 'MyTest',
        sourcemap: 'inline'
      }
    }
  });
};