7.2 Exclude file/statement/folder from coverage

6 ways to exclude a line, file or statement from coverage reports

Why would I want to exclude/ignore files or lines from coverage?

As stated by the maintainers and authors of the istanbul coverage libary:

Some branches in JS code are typically hard, if not impossible to test. Examples are a hasOwnProperty check, UMD wrappers and so on. Istanbul now has a facility by which coverage can be excluded for certain sections of code.

Istanbul - Ignore code for coverage purposes

What’s more, 100% coverage isn’t necessary or even reasonable in most cases. Some files don’t contain any (business) logic. Or they contain logic that would fail in a very obvious way (eg. a starter crashing on start).

For example the script that would bootstrap an application might bind to a port, which makes it unwieldy to test. The file that imports all the different dependencies and app.use()-es them in an Express setting would be another candidate for maybe avoiding the unit testing/dependency mocking hell.

Another class of files/functions you might want to ignore for coverage purposes are test-specific helpers. It doesn’t matter that some of them aren’t run as part of tests, as they’re not the code under test.

As with a lot of things in software, it’s about tradeoffs.

Exclude/ignore file(s) from Jest coverage by not running relevant tests using configuration

There’s a Jest configuration option testPathIgnorePatterns (see the docs for testPathIgnorePatterns)

The simplest way to configure this is through the package.json:

{
  "jest": {
    "testPathIgnorePatterns": ["<rootDir>/ignore/this/path/"]
  }
}

Exclude/ignore file(s) from coverage by not including it in the coverage collection configuration

As an alternative or augmentation to not running tests (as seen in “Exclude file from Jest coverage by not running relevant tests using configuration”) from Jest coverage by not including it in coverage reports, that’s controlled by the collectCoverageFrom Jest configuration option (see the docs for Jest collectCoverageFrom).

Use something like the following:

{
  "jest": {
    "collectCoverageFrom": ["src/**/{!(ignore-me),}.js"]
  }
}

Important: make sure to wrap the ignored file’s name with ().

Exclude/ignore file from Jest coverage at the file level

We can use istanbul pragmas to ignore files using the following comment at the top of any file:

/* istanbul ignore file */

Exclude/ignore function from Jest coverage

/* istanbul ignore next */
function myFunc() {
  console.log("Not covered but won't appear on coverage reports as such");
}

Exclude/ignore line(s) from Jest coverage

Avoid this if you can, if you’re testing some code you should probably test all of that code.

istanbul ignore next also works to ignore JS statements, definitions and expressions which equates to ignoring/excluding the line from coverage:

// this function will appear as uncovered
function ignoreLine() {
  /* istanbul ignore next */
  console.log("This line won't appear as uncovered");
}

Exclude/ignore statement or clause from Jest coverage

Avoid this if you can, if you’re testing some code you should probably test all of that code.

function myFunc(a) {
  /* istanbul ignore else */
  if (a) {
    // do some work
  } else {
    // do some other work
  }
}