# Jest coverage: V8 vs Babel

> Keep Babel, Jest's default, unless coverage is too slow or your code skips Babel. The two providers count differently: the same tests gave 100% and 82% lines.

Web page: https://supercov.com/compare/jest-coverage · Published 2026-10-12
All Supercov pages for agents: https://supercov.com/llms.txt · Full text: https://supercov.com/llms-full.txt

## Babel or V8: which provider should you use?

Keep Babel, Jest's default, unless coverage makes your runs too slow or your
code does not go through Babel. Choose V8 then. The two count different
things, so expect the percentages to change when you switch.

[`coverageProvider`](https://jestjs.io/docs/configuration#coverageprovider-string)
accepts `babel` or `v8`. Babel rewrites your source with counters before it
runs. V8 runs the code as it is, reads coverage from the engine and maps it
back to your source.

## Switch providers

```sh
npx jest --coverage --coverageProvider=v8
```

Or set it in your Jest configuration:

```js
module.exports = {
  collectCoverageFrom: ['src/**/*.js'],
  coverageProvider: 'v8',
};
```

Set `collectCoverageFrom` with either provider. Without it, a file that no
test imports is missing from the report instead of showing 0%.

## Same tests, different percentages

We ran three passing tests against one 17-line file with each provider, on
Jest 30.5.2 and Node.js 24.21.0:

| Provider | Lines | Branches | Functions |
| --- | --- | --- | --- |
| Babel | 5/5 (100%) | 6/7 (85.71%) | 2/2 (100%) |
| V8 | 14/17 (82.35%) | 4/5 (80%) | 2/3 (66.66%) |

Three things explain the gap:

- **Lines.** Babel counts the lines that hold a statement. V8 counts every
  line of the file, blank lines and closing braces included.
- **Ignore comments.** The file has `/* istanbul ignore next */` above an
  untested function. Babel leaves that function out. V8 does not read the
  comment and reports the function as uncovered. V8 reads
  `/* c8 ignore next 3 */` instead; with that comment the results swap to
  17/17 lines for V8 and 5/6 for Babel.
- **Branches.** Babel counts the `if`, each side of `&&`, the default
  parameter and both sides of the ternary. V8 counts the blocks the engine
  tracked.

A percentage that moves when you change provider says nothing about your
tests. Reset your coverage thresholds when you switch.

## What neither provider shows

Both reports mark `if (signedIn && !expired)` as fully covered, because the
tests take the `if` both ways. No test tries a signed-in session that has
expired. That is what [MC/DC](https://supercov.com/compare/mcdc-vs-branch-coverage.md) measures.
Neither provider says which test ran a line.

## What Supercov adds

```sh
npx supercov -- npx jest
npx supercov runs latest gaps
```

Supercov runs your own Jest command, with your configuration, setup files and
reporters. On the same file it reported 7/9 lines, 4/6 branches and 1/3 MC/DC
conditions, and named `!expired` as the condition no test showed changing the
result. Coverage is recorded per test, parameterized tests included.

Using Vitest? See [Vitest coverage: V8 vs Istanbul](https://supercov.com/compare/vitest-coverage.md).


Example evidence: [fixture source, commands and recorded results (JSON)](https://supercov.com/downloads/tool-comparisons-2026-10-12.json). Measured October 12, 2026 with Supercov 4.3.0. These small examples are not a performance benchmark or a general conformance test.