examples of minimal ES2015 projects, with testing and test coverage.
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ES2015 Code Coverage Examples

In this tutorial I'll walk you through setting up test coverage for your fancy pants ES2015 JavaScript projects. We'll cover two topics: instrumenting native JavaScript code, and writing backwards compatible ES2015 code using babel.

For this tutorial, feel free to follow along in the repository bcoe/es2015-coverage

ES2015 With The Babel ES2015 preset

ES6/ES2015 is composed of a daunting array of JavaScript language features. Babel comes to the rescue, providing collections of plugins that track the standardization process.

For this tutorial we will be using the ES2015 preset, a collection of Babel plugins that encompass many of the exciting features currently in the pipeline for JavaScript.


By relying on the preset, there are only a few dependencies that we need to add test coverage to our ES2015 project:

npm i babel-cli babel-register babel-plugin-istanbul babel-preset-es2015 cross-env mocha chai nyc --save-dev
  • babel-cli: is the command-line interface for babel; we use it during the build step.
  • babel-register: automatically compiles ES2015 JavaScript as it's required in your tests.
  • babel-plugin-istanbul: this plugin adds coverage instrumentation to your ES2015 code as it's compiled.
  • nyc: outputs the coverage information to disk, and handles running reports.
  • cross-env: used to set NODE_ENV=test in a cross-platform compatible way.
  • mocha/chai: the test framework that I happen to be using for this tutorial.



We place a .babelrc in the root of our project which is used by babel-cli to apply the compilation process:

  "presets": [
  "env": {
    "test": {
      "plugins": [
  • presets.es2015: indicates that we should load the babel-preset-es2015 set of plugins.
  • env.test.plugins.istanbul: indicates that we should run the babel-plugin-istanbul plugin only when NODE_ENV=test.

package.json: configuring nyc

{"nyc": {
  "require": [
  "sourceMap": false,
  "instrument": false
  • nyc.require.babel-register: indicates that we should automatically run require('babel-register') as nyc loads our tests. This allows us to write ES2015 code in our tests without running a build step (code is automatically compiled by babel as it is loaded).
  • nyc.sourceMap=false/nyc.instrument=false: indicates that we should not use nyc for instrumenting tests with coverage or handling source-maps; this logic is instead handled by babel and babel-plugin-istanbul.

package.json: script stanza

{"scripts": {
  "build": "babel index.js -d src",
  "test": "cross-env NODE_ENV=test nyc --reporter=lcov --reporter=text mocha test.js",
  "prepublish": "npm run build"
  • scripts.test:
    • cross-env NODE_ENV=test: sets the NODE_ENV environment variable resulting in the babel-plugin-istanbul plugin being loaded.
    • nyc --reporter=lcov --reporter=text: indicates that nyc should be used to run mocha, and that it should output both an lcov and a text report.
    • mocha test.js: it doesn't get much simpler than this; use mocha to run test.js.
  • scripts.build: this script uses babel-cli to compile your ES2015 code.
  • scripts.prepublish: we automatically run the build step before publishing our package to npm.

Writing Tests

Because nyc automatically loads babel-register there is no build step necessary for your tests. Just write your tests using ES2015 syntax and require() the pre-compiled ES2015 JavaScript files:

import CoverageBabel from './index'


describe('CoverageBabel', () => {
  it('returns hello world message', () => {
    const cls = new CoverageBabel('Ben')
    cls.helloMessage().should.equal('hello Ben')

Shipping Your Code

When it's time to publish your code to npm, simply use babel-cli to compile your ES2015 JavaScript into ES5 compatible code:

babel index.js -d src

This command will read in your ES2015 ./index.js file and output the ES5 ./src/index.js file. Only ./src/index.js should be published to npm, which can be achieved by adding a files stanza to your package.json:

{"files": [

Instrumenting Native ES2015 Code

If you're using newer versions's of Node.js various ES2015 features are already supported. node.green provides a useful chart for viewing compatibility information.

nyc uses the same parser underneath the hood as babel and understands native ES2015 constructs; with zero configuration you can start instrumenting your ES2015 code!

{"scripts": {
  "test": "nyc --reporter=text --reporter=lcov mocha test.js"