It means you're trying to access a property of an undefined value. Make sure you are addressing the right value or that you are fetching it correctly. Could also be an error where you trying to read value which has not been initialized yet.

I think this could be a case where you're probably accessing a value declared, but not defined yet. Had this been not even declared, then I believe you would have seen 'not defined' instead of undefined. Seeing your code would be really helpful here - but for now what I can suggest is that where you're accessing this value, you could add a check there like this.

if (typeof forEachFeature == 'undefined') {
  //Here you can access the value as required
}

This would ensure that if the property is not available, you wouldn't be accessing it - hence not triggering an error.