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im new here. and i new to php. now i want to make a selectable seat where when user click it will change color and update the value "status" to the database. but now i got this error Uncaught TypeError: Cannot read property 'src' of null here is my code <script language="javascript"> function changeImage(pos) { var pos = <?php echo(json_encode($data[pos])); ?>; if (document.getElementById(pos).src == "$base/images/kuning.png") { document.getElementById(pos).src = "$base/images/hijau.png"; alert("ok1"); } else if (document.getElementById(pos).src == "$base/images/hijau.png") { document.getElementById(pos).src = "$base/images/merah.png"; alert("ok2"); } else if (document.getElementById(pos).src == "$base/images/merah.png") { document.getElementById(pos).src = "$base/images/kuning.png"; alert("ok3"); } } </script> <?php $q = "SELECT * FROM schedule WHERE inv_id=1"; $sql = mysql_query($q); echo "<table>"; while($data=mysql_fetch_array($sql)) { if($data["status"]=="done") { $image = "<a href='#'><img src='$base/images/hijau.png' id='$data[pos]' border='0' width='33' height='26' alt='$data[pos]' onclick='changeImage($data[pos])'/></a>"; } elseif($data["status"]=="onschedule") { $image = "<a href='#'><img src='$base/images/kuning.png' id='$data[pos]' border='0' width='33' height='26' alt='$data[pos]' onclick='changeImage()'/></a>"; } elseif($data["status"]=="late") { $image = "<a href='#'><img src='$base/images/merah.png' id='$data[pos]' border='0' width='33' height='26' alt='$data[pos]' onclick='changeImage()'/></a>"; } echo "<td><center>$image</br>$data[date]</center></td>"; } echo "</table>"; ?>