ignore-if-badidea.js

const test = n => {
  if (n === 1) {
    console.log(n)
  } else if (n === 2) {
    console.log(2)
  } /* istanbul ignore next */ else if (1 === 2) {
    console.log('math is broken')
  } else {
    console.log('n is some other thing')
  }
}

test(1)
test(2)
test(3)
// gets 100% coverage, but STILL gets 100% coverage if test(3) removed

ignore-if-busted.js

const test = n => {
  if (n === 1) {
    console.log(n)
  } else if (n === 2) {
    console.log(2)
  } else /* istanbul ignore if */ if (1 === 2) {
    console.log('math is broken')
  } else {
    console.log('n is some other thing')
  }
}

test(1)
test(2)
test(3)
// does not get 100% coverage

ignore-if-works.js

const test = n => {
  if (n === 1) {
    console.log(n)
  } else if (n === 2) {
    console.log(2)
  } else {
    /* istanbul ignore if */
    if (1 === 2) {
      console.log('math is broken')
    } else {
      console.log('n is some other thing')
    }
  }
}
test(1)
test(2)
test(3)
// gets 100% coverage, fails 100% coverage if test(3) omitted