Excluding code blocks from Jest coverage reports

Oct 30, 2016 using tags reactjs, jest, istanbul

I realized recently after poking around in the source code that Jest uses Istanbul under the hood to generate its code coverage report (via the --coverage option). This is useful for situations where you need to legitimately ignore blocks or lines from coverage reports.

Basically, the magic comment that makes this all work is:

/* istanbul ignore next */

Let’s say for example you needed to ignore the following dispatch(actions.initiateRequest(data)) call from your coverage report as it’s not very practical to test this code-path in redux. Here’s what that might look like.

function mapDispatchToProps(dispatch) {
  return {
    onSubmitFunction: function(data) {
      /* istanbul ignore next */
      return dispatch(actions.initiateRequest(data));

Simple as that! The general form of this magic comment looks something like:

/* istanbul ignore <word>[non-word] [optional-docs] */

The Istanbul project site has some very detailed documentation explaining this and other use-cases, read through there for how the ignore if and ignore else options work too!