score:0

let newConversationList = conversationsList.map((conversationElement) =>
     (conversationElement.thread_id === message.thread_id) ? {
         ...conversationElement, 
         date_created: message.date_created,
         new_for: [...conversationElement.new_for, user.id]
    } : { 
        ...conversationElement, 
        new_for: [...conversationElement.new_for]
    }

score:0

 let newConversationList = conversationsList.map((conversationElement) =>
     (conversationElement.thread_id === message.thread_id) ? {
         ...conversationElement, //spread/copy like this before mutating
         date_created: message.date_created,
         new_for: [...conversationElement.new_for, user.id]
    } : { 
        ...conversationElement, //spread/copy like this before mutating
        new_for: [...conversationElement.new_for]
    }

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