Coverage Maps

If you specify a --coverage-map=<file> option, then you can be very precise about which files under test should be covered by a specific test.

A coverage map is a Node.js module that exports a single function. This function takes a test file as an argument, and returns a single filename or an array of program files that should be covered by that test.

This is useful in cases where you want to have a part of your program at foo.js which should be 100% covered by foo.test.js, for example. Coverage generated with a coverage map will help guarantee that you are actually testing a piece of code directly, rather than "accidentally" adding lines of coverage via integration.

When used with --changed or --watch, this also means that you limit which file's changes will trigger which test to re-run. Also, because less of your program is instrumented for each test, the overall suite can run significantly faster.

Module Structure

The coverage map must export a single function. This function takes a test filename as an argument, and returns one of the following values:

  • A filename of a single program file that should be covered by this test.
  • An array of filenames of program files that should be covered by this test.
  • An empty array, indicating that all program files should be covered, following NYC's default coverage logic.
  • Any falsey value, indicating that this test should not provide coverage. This is useful for regression tests that don't add coverage, but do verify some edge-case bug.


For example, you might have a file at index.js which is the main entry point of your library or application, and exports a bunch of other functionality from inside a lib folder. If the file at lib/foo.js loads the index (perhaps just for one or two things), then this effectively means that any test that loads lib/foo.js also loads everything else.

Now any change anywhere in the program triggers a full test suite run, so why bother using --changed at all?

With a --coverage-map argument, you can specify that the test at foo.test.js should only cover foo.js.

Here's an example coverage-map file, which maps *.test.js to the corresponding *.js.

// map.js
module.exports = testFile => testFile.replace(/\.test\.js$/, '.js')

To run tap with this coverage map, run this command:

tap --coverage-map=map.js

Or, you can add this config to your package.json file:

  "tap": {
    "coverage-map": "map.js"