coverage-istanbul-loader

3.0.5 verified Sat Apr 25 auth: no javascript

A Webpack loader that uses Istanbul to add code coverage instrumentation to JavaScript/TypeScript files. Current stable version is 3.0.5. Release cadence is low; last major update was in 2020. Key differentiators include support for modern Istanbul API (istanbul-lib-instrument) and better source map handling compared to the older istanbul-instrumenter-loader. Ships TypeScript definitions. Requires Node >=10 and Webpack 4+. Works with Karma, TypeScript, and custom options.

Common errors

error Error: Cannot find module '@jsdevtools/coverage-istanbul-loader' ↓

cause Loader not installed or typo in package name.

fix

npm install @jsdevtools/coverage-istanbul-loader --save-dev

error TypeError: loader.run is not a function ↓

cause Using CommonJS require() on ESM-only v3.

fix

Use dynamic import: const loader = (await import('@jsdevtools/coverage-istanbul-loader')).default;

error Module build failed: Error: Cannot find module 'istanbul-lib-instrument' ↓

cause Missing peer dependency.

fix

npm install istanbul-lib-instrument --save-dev

Warnings

breaking Version 3.0 is ESM-only. Using require() will fail with 'require() of ES modules not supported'. ↓

fix Use import or dynamic import(), or downgrade to v2.x.

gotcha Loader must be placed before other loaders (like babel-loader) in webpack config to instrument original source. ↓

fix Ensure this loader is listed last in the 'use' array (i.e., executed first).

deprecated Option 'instrumenter' is deprecated in favor of 'istanbul-lib-instrument' options. ↓

fix Replace 'instrumenter' with 'istanbul-lib-instrument' options (e.g., 'esModules').

gotcha Source maps may not work correctly if options.produceSourceMap is not set to true. ↓

fix Set produceSourceMap: true in loader options.

Install

npm install coverage-istanbul-loader

yarn add coverage-istanbul-loader

pnpm add coverage-istanbul-loader

Imports

default

wrong

const CoverageIstanbulLoader = require('@jsdevtools/coverage-istanbul-loader')

correct

import CoverageIstanbulLoader from '@jsdevtools/coverage-istanbul-loader'

ESM-only since v3. CommonJS require will fail.

loader function

wrong

const loader = require('@jsdevtools/coverage-istanbul-loader')

correct

const { default: loader } = require('@jsdevtools/coverage-istanbul-loader')

In CommonJS, must grab .default property.

Webpack rule usage

wrong

use: 'coverage-istanbul-loader' (without @jsdevtools scope)

correct

module.exports = { module: { rules: [ { test: /\.js$/, use: { loader: '@jsdevtools/coverage-istanbul-loader', options: { esModules: true } } } ] } }

Full scoped package name required.

Quickstart

Configures Webpack to instrument JavaScript files for code coverage using Istanbul, with modern options.

// webpack.config.js
module.exports = {
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: '@jsdevtools/coverage-istanbul-loader',
          options: {
            esModules: true,
            produceSourceMap: true
          }
        }
      }
    ]
  }
};
// Run webpack, output bundle will have coverage instrumentation.
// Then run tests with Karma + karma-coverage-istanbul-reporter.