JavaScript and TypeScript test coverage

Ask your coding agent to set up Supercov in your project:

Set up Supercov in this JavaScript or TypeScript project using npx supercov.
Read npx supercov docs and run our existing tests. Show the coverage report
and any measurement limits. Don't change application code or tests.

Run it yourself

You’ll need Node.js 22 or newer and your project’s test dependencies. For browser tests, start any services your suite needs.

Run Supercov through npx; no global install is needed:

npx supercov --version

Run your tests

Put Supercov before the command you normally run:

npx supercov -- npm test

If you invoke the runner directly, use the matching command instead:

npx supercov -- npx vitest run
npx supercov -- npx jest
npx supercov -- npx playwright test

This works with JavaScript, JSX, TypeScript and TSX, including React components. Keep your runner and build configuration. Supercov measures an isolated copy and builds it if your tests use compiled output.

React, React Native and Expo

React component tests work with Testing Library and Vitest/jsdom, Vitest Browser Mode, or Babel/Jest. Mixed Vitest Node/browser projects keep their own setup. React Native and Expo use their existing Jest presets and native mocks:

npx supercov -- npm test -- --runInBand

Keep the test command your project normally uses. Supercov 0.0.55 fixes Expo configuration forwarding, Babel runtime imports, duplicate Jest test names and coverage from React effect cleanup during teardown.

RN support here covers JavaScript component tests. It does not cover Hermes, native modules or device E2E. React hydration has been tested in jsdom and Chromium; Next.js Server Components, streaming SSR and server actions are not established by those checks. See React and RN support for scope and the runnable example.

Component tests in a real browser

Vitest Browser Mode runs your component tests in a real browser. Supercov measures it per test like any other Vitest run – lines, branches, MC/DC and assertion coverage. The compatibility suite exercises Chromium through the Playwright provider:

npx supercov -- npx vitest run

Expressions rendered inside JSX are measured in their own right, so an aria-label={label(state)} that never evaluates is not counted as covered because the component rendered once. That is also what lets a UI assertion be explained precisely: toHaveAccessibleName is about the attribute and toHaveTextContent about the child, and each earns credit separately.

expect.element(...), expect.soft(...) and expect.poll(...) count as assertions, so browser assertions can be mapped like any other.

Read the result and list a few gaps:

npx supercov runs latest
npx supercov runs latest gaps --limit 5

With Vitest, Jest, node:test, Playwright, Mocha, AVA and node-tap, Supercov identifies which test ran each piece of code. See runner support for browser and custom-assertion limits.

Try a small example

Download the examples, extract them, and open supercov-examples/javascript. This JavaScript example uses node:test and needs no extra test dependencies.

In src/session.js, checkout requires a signed-in customer and a session that hasn’t expired:

export function checkout(signedIn, expired, price, quantity) {
  if (signedIn && !expired) {
    const total = price * quantity;
    return { status: 'confirmed', total };
  }
  return { status: 'denied', total: 0 };
}

The two tests check a valid order and a signed-out visitor. Run them:

npx supercov -- npm test
npx supercov runs latest

The coverage section recorded with Supercov 2.0.0 is:

Coverage
  Lines      100.00% (5/5)
  Branches   100.00% (2/2)
  MC/DC      50.00% (1/2)

Both branches ran, but no test checks an expired session for a signed-in customer. Inspect that decision:

npx supercov runs latest decision src/session.js:2

The report includes:

signedIn && !expired
C1 covered: signedIn
C2 MISSING: !expired

A test for a signed-in customer with an expired session closes the gap. In the recorded run, MC/DC reaches 100%; line and branch coverage stay the same. See MC/DC for how it works.

Check what the tests assert

These tests check the returned status, but not the total. An assertion such as assert.equal(order.total, 50) would check the calculation for this input.

Supercov can ask Jev whether each test would fail if a statement it ran changed. runs latest assertions assess saves the answers (it needs TYPESAFE_API_KEY), and runs latest assertions then lists the statements no test checks, file by file. A statement that is not asserted runs in some test, but nothing that test checks depends on it. See Assertion coverage.

Already using Vitest coverage? See how the reports differ. Need to know which test covers a line? See per-test coverage in Vitest. npx supercov docs coverage-model prints the guide for your installed version.