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Browser Object Model: window.onerror

window.onerror can be assigned a (callback-) function that is invoked when an error occurs.
The function receives a message, an URL, line and column number and an error object.
The following HTML document attempts to demonstrate the usage of window.onerror. Whenever the assigned call back function is called, it creates a row in a <table> with the received parameters. 
<!DOCTYPE html>
<!--

  https://github.com/ReneNyffenegger/Browser-Object-Model/tree/master/window/onerror.html

  https://renenyffenegger.ch/development/web/js-html/userAgent.html

-->
<html>
<head>
  <meta content="text/html;charset=utf-8" http-equiv="Content-Type">
  <title>window.onerror</title>
</head>


  <style type='text/css'>

     thead tr
           { font-weight: bold}

     td    {vertical-align: top; padding: 4px}

     td:nth-child(3), /* Line Number */
     td:nth-child(4)  /* Col Number */
           { text-align: right;}

     tr    {border-bottom: 1px solid #444;}

     table {border: 1px solid #000; border-collapse: collapse}

  </style>

  <script type='text/javascript'>

    window.onerror = function(msg, url, lineNr, colNr, err) {

      var tr     = document.createElement('tr');
      var td_msg = document.createElement('td');
      var td_url = document.createElement('td');
      var td_lnr = document.createElement('td');
      var td_cnr = document.createElement('td');
//    var td_err = document.createElement('td');

      td_msg.innerText = msg;
      td_url.innerText = url;
      td_lnr.innerText = lineNr;
      td_cnr.innerText = colNr;
//    td_err.innerText = err;

      tr.appendChild(td_msg);
      tr.appendChild(td_url);
      tr.appendChild(td_lnr);
      tr.appendChild(td_cnr);
//    tr.appendChild(td_err);

      document.getElementById('errors').appendChild(tr);

      return true;

    }

    function main() {
    }

    function call_undefined_function() {

    //
    // Calling an undefined function causes
    // a »Reference Error«:
    //
       undefined_function();
    }

    function cause_eval_error() {
       var v = eval('4 + 5');
       console.log('v = ' + v);

       var w = eval('4 +  ');                          // SyntaxError: expected expression, got end of script
       console.log('w = ' + w);
    }

    function throw_an_error() {
       throw "up";
    }

    function type_error() {
        var ary = new Array('foo', 'bar', 'baz');
        console.log('ary.length   = ' + ary.length  );
        console.log('ary.length() = ' + ary.length()); // TypeError: ary.length is not a function
    }

  </script>
<body onload='main()'>


  <a href='javascript:call_undefined_function();'>Call an undefined function</a><br>
  <a href='javascript:cause_eval_error();'       >Cause an eval error</a>       <br>
  <a href='javascript:throw_an_error();'         >Throw an error</a>            <br>
  <a href='javascript:type_error();'             >Type error</a>


  <table id='errors'>
    <thead><tr>
       <td>Message</td>
       <td>Url</td>
       <td>Line</td>
       <td>Col</td>
<!--   <td>Error</td> -->
    </tr></thead>

  </table>

</body>
</html>
Github repository Browser-Object-Model, path: /window/onerror.html
This HTML document is online here.

Multiple arguments

Because of historical reasons, the window.onerror event is the only event to receive multiple arguments.

TODO

<!DOCTYPE html>
<script>
window.addEventListener("error", e => 
  {
     const { message, filename, lineno, colno, error } = e;
     alert(`Error occurred: ${message}: ${error}\n${filename}\nLine:Col ${lineno}:${colno}\n\n`);

   }
);
</script>
<html>
  …
</html>

See also

The window object and the browser object model.

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