Jest/Istanbul Coverage Ignore

Limiting Jest Coverage Noise

I’ve been using Test-Driven Development when writing test code. This is done using jest --watch --changesSince '@{u} to trigger test runs whenever a change is made.

This will run test and coverage report but only for the files touched since upstream tracking branch. The git command equivalent is git diff '@{u}...@' --name-only -- '*.js'

Istanbul Ignores

I’ve also been learning how to correctly istanbul ignore statements. The istanbul docs cover the heuristic that’s used.

What made it click for me is to think of what the “next” thing is.

/* istanbul ignore next */
const foo = {
    /* istanbul ignore next */
    bar: () => {},
    baz: '',
};

In this example, the “next” thing for both ignores is the assignments. It doesn’t work to ignore this and shouldn’t ignore anyways, since it’s trivial to get coverage for assignments or declarations.

There is still lack of function coverage, if we don’t call foo.bar(). To ignore the function execution, we need to get move the ignore closer to the “next” thing:

const foo = {
    bar: /* istanbul ignore next */ () => {},
    baz: '',
};

We’ve now declared that we want to ignore coverage for the entire anonymous function.