This doesn't appear to be related to arrays at all.
Based on your code snippet, it appears that conversationMember.Name is expected to be a string (due to the use of .toLowerCase()), indicating that in this context, includes refers to String.prototype.includes, rather than Array.prototype.includes. This assumption is supported by the fact that self.conversationSearchTerm also seems to be a string (as .toLowerCase() is called on it).
The issue arises from using includes on something meant to be a string but isn't. A simple solution is to default it to an empty string if it's falsy:
return (conversationMember.Name || '').toLowerCase().includes(
(self.conversationSearchTerm || '').toLowerCase()
);
Additionally, you can eliminate the need for var self = this; as this is accessible within your filter due to it being an arrow function. Your function, whether a computed or a method, could be written like this:
filteredConversations() {
return this.conversations.filter(c =>
c.MembershipData.some(md =>
(md.Name || '').toLowerCase().includes(
(this.conversationSearchTerm || '').toLowerCase()
)
)
);
}
Lastly, to handle cases where any of your conversations lack a MembershipData array, you can default it to an empty array dynamically:
...
(c.MembershipData || []).some(md =>
...
If a conversation lacks an array in its MembershipData, the function will then exclude it from the result - since calling .some(condition) on an empty array will return false.