Why is .forEach returning undefined?

forEach doesn’t return anything. It just loops through elements and while looping you can change element data

And so you can change your function SOtest to

function SOtest () {
  testArray.forEach(test => {
    test.newSegments = test.segments ? testsMap[test.segments] : [];
    test.display = helperFn(); // will add true/false to the test prop

    if (test.display) {
      test.tests.map(t => {
        t.newSegments = t.segments ? testsMap[t.segments] : [];
        t.display = helperFn(); // will add true/false to the test prop
      })
    }
  })
  return testArray;
}