title: React native props declaration confusion
body: <p>I have came across this SendBird With React-Native chatting app and i am studying the code structure of it. The following line of codes is causing me confusion:</p>
  <p>Function.js</p>
  <pre><code>export const sbAdjustMessageList = (list) =&gt; {
  return list.map((message, i) =&gt; {
      message['time'] = sbUnixTimestampToDate(message.createdAt);
      message['readCount'] = 0;
      if (message.isUserMessage() || message.isFileMessage()) {
          message['isUser'] = (message.sender.userId === SendBird.getInstance().getCurrentUserId());
      } else {
          message['isUser'] = false;
      }
      if (message.sender) {
          message.sender['isShow'] = true;
          if(!message.sender.profileUrl) {
              message.sender.profileUrl = 'default-image';
          }
      }
  
      if (i &lt; list.length - 1) {
          let prevMessage = list[i + 1];
          if (message.isUserMessage() || message.isFileMessage()) {
              if (prevMessage.isUserMessage() || prevMessage.isFileMessage()) {
                  if (prevMessage.sender.userId === message.sender.userId) {
                      message.sender.isShow = false;  
                  }
              }
          }
      }
      return message
  });
  </code></pre>
  <p>}</p>
  <p>Main.js</p>
  <pre><code>_renderFileMessageItem = rowData =&gt; {
      const message = rowData.item;
      if (message.isUserMessage()) {
        return &lt;TextItem isUser={message.isUser} message={message.message} /&gt;;
      } else if (sbIsImageMessage(message)) {
        return &lt;ImageItem isUser={message.isUser} message={message.url.replace(&quot;http://&quot;, &quot;https://&quot;)} /&gt;;
      } else {
        return &lt;FileItem isUser={message.isUser} message={message.name} /&gt;;
      }
    };
  </code></pre>
  <p>In Function.js, the is this declaration of <code>message['isUser']</code> then it is exported to Main.js to be used. However in Main.js, <code>isUser</code> becomes a props to the imported component. Also, there is no initiation of <code>isUser</code> in constructor class.</p>
  <p>My question is what does <code>message['isUser']</code> here means? Is it a javascript thing or a redux features (The sample app uses redux and react-redux)?</p>