Code coverage of VSCode extensions with NYC

TL;DR: Adding NYC code coverage reporter to vscode extension unit tests can be tricky. You can check my working test runner here. The article below explains some of the key elements in it.

Standard testing flow

When you generate an extension using Yeoman, the following scaffolding will be created:

  1. An “Extension Tests” target will be created in launch.json
  2. This will start the Extension Host and run a run function, typically exported from ${workspaceFolder}/out/test/suite/index
  3. This run function takes care of everything, namely
    • setting up a Mocha instance
    • adding all relevant files to its list of tests
    • running mocha and reporting results

All the test run within the Extension Host which is creating a couple of challenges.

Challenges of adding NYC to the mix

NYC internals

NYC is primarily a command line tool. Most tutorials on code covering TypeScript will therefore focus on the command line options and the content of the .nycrc file.

Since we will be running it programatically within the Extension Host, this won’t help us and we need to work around that.

Unfortunately, NYC’s documentation of the internal workings is virtually non-existent so it took me a bit of debugging and reverse-engineering to figure out what actually happens and how to translate these command line instructions into a working TypeScript runner

$ cat .nycrc
    "extends": "@istanbuljs/nyc-config-typescript",
    // OPTIONAL if you want coverage reported on every file, including those that aren't tested:
    "all": true

$ cat test/mocha.opts
--require ts-node/register
--require source-map-support/register
<glob for your test files>

$ nyc mocha

Setting up the instance

First of all, we need to create a NYC instance and pass it all the necessary parameters. Also, we need to load some additional modules needed for TypeScript. The following snippet shows the best way I found to simulate the above command line options.

// Simulates the recommended config option
// extends: "@istanbuljs/nyc-config-typescript",
import * as baseConfig from "@istanbuljs/nyc-config-typescript";

// Recommended modules, loading them here to speed up NYC init
// and minimize risk of race condition
import 'ts-node/register';
import 'source-map-support/register';

export async function run(): Promise<void> {
	const testsRoot = path.resolve(__dirname, '..');

  // Setup coverage pre-test, including post-test hook to report
  const nyc = new NYC({
    cwd: path.join(__dirname, '..', '..', '..'),
    reporter: ['text-summary', 'html'],
    all: true,
    silent: false,
    instrument: true,
    hookRequire: true,
    hookRunInContext: true,
    hookRunInThisContext: true,
    include: [ "out/**/*.js" ],
    exclude: [ "out/test/**" ],
  await nyc.wrap();

Notice, that we are actully instrumenting the files in the out directory - they will be translated to your src/**/*.ts files using source maps.

If you want to check your include/exclude configuration, you can run the following:

// Debug which files will be included/excluded
console.log('Glob verification', await nyc.exclude.glob(nyc.cwd));

You should see all the relevant files in the console. If not, tweak the include/exclude parameters until you do.

Instrumenting the JavaScript

First thing that needs to happen in order for the code coverage reporting to work is so called “instrumenting”. NYC does this on the fly by replacing the require function with an instrumented version of it. This is done when the nyc.wrap() function is called (see last line of the snippet above). Any module loaded before this call is not instrumented and won’t appear in your coverage report.

I am using the following safety check to detect race conditions that would load some of the files before the nyc.wrap() call by adding the following right after:

// Check the modules already loaded and warn in case of race condition
// (ideally, at this point the require cache should only contain one file - this module)
const myFilesRegex = /\/out/;
const filterFn = myFilesRegex.test.bind(myFilesRegex);
if (Object.keys(require.cache).filter(filterFn).length > 1) {
  console.warn('NYC initialized after modules were loaded', Object.keys(require.cache).filter(filterFn));

This is also the reason, why I am only (re)creating the .nyc_output folder after the call to nyc.wrap() (see full code below).

Another thing that can help prevent the race condition is to have a look at the activationEvents in package.json. I changed mine from * to onStartupFinished which seems to have helped.

"activationEvents": [

This issue on GitLab has a few pointers in that respect.

Reporting coverage

This is another challenge you will face when running in the Extension host. You can see I am using the ‘text-summary’ and ‘html’ reporters. The HTML files were created as expected, but I couldn’t get the text summary reporter to work. This was due to the fact that NYC reports the results directly into process.stdout stream and they don’t show in the debug console. The following function did the trick:

async function captureStdout(fn) {
  let w = process.stdout.write, buffer = '';
  process.stdout.write = (s) => {
    buffer = buffer + s;
    return true; 
  await fn();
  process.stdout.write = w;
  return buffer;

I then run the reporter like this

// Capture text-summary reporter's output and log it in console
console.log(await captureStdout(;

Putting it all together

You can have a look at the full file in one of my projects here.