undefined is not an object (evaluating 'message.message_contacts.filter')

Question

Hey this is a react native code with redux saga.. The code returns a list.map with "filterUnreadMessages = list"... But for some reason the code return this especific error:

undefined is not an object (evaluating 'message.message_contacts.filter')

  const messageIds = list.map(message => message.id);

  const filterUnreadMessages = list
    .map(message =>
      message.message_contacts.filter(
        contact => contact.contact_id === userId && contact.readed_at === null,
      ),

Answer 1

It means that message.message_contacts is undefined, so you can't call .filter on it. Alternatively, message itself is undefined.

You can do a check first:

const filterUnreadMessages = list
  .map(message => {
    if (message !== undefined && Array.isArray(message.message_contacts)) { // <-- check here
      return message.message_contacts.filter(
        contact => contact.contact_id === userId && contact.readed_at === null,
      )
    } else {
      return [];
    }
  }