list_messages.sol

pragma solidity >=0.7.0 <0.9.0;

contract Chat {
    
  // the list of old messages in the chat
  mapping(uint => Message) public messagesList;
  function listMessages() public constant returns (uint[]){
    
    // if the chat is empty
    if(lastMessageId == 0) {
      return new uint[](0);
    }
    
    // give me the ids.
    uint[] memory ids = new uint[](lastMessageId);
    
    // loads all the message ids on 'ids' list.
    for (uint i = 1; i <= lastMessageId; i++) {
      
      // if the sender is different than me.
      if(messages[i].sender != msg.sender) {
        ids[numOfMessages] = messagesList[i].id;
      }
    }
    return ids;
  }
}