If within a map with arrow syntax

When you are doing xxx(YY) => (ZZ), it's the same as xxx(YY) => (return ZZ).

The best way to do what you want is to wrap the ZZ with {} to transform it as a function with possible instructions :

{
  aiData.map((row, index) => {
      if(row[3]==='trace') {
        return (
          <tr key={index}>
            <td>{Moment(row[0]).format("DD-MM-YYYY hh:mm:ss")}</td>
            <td>{row[3]}</td>
            <td>{row[2]}</td>
            <td>{row[1]}</td>
          </tr>
        )
      } else if(row[3]==='trace') {
        return (null)
      }        
  })
}