Using javascript variable Value form outside the method

There are a few ways you can to this.

What you have currently would work with a little modification:

var rownum;
$('#myTable').find('tr').click( function(){
  rownum=$(this).index()
  alert('You clicked row '+ (rownum) );
});

Hovever, the preferred method would be to pass the variable where it's needed:

function alertRowNumber(rowNumber) {
  alert('You clicked row '+ (rowNumber) );
}

$('#myTable').find('tr').click( function(){
  alertRowNumber($(this).index)
});

There may be an even better solution for your specific scenario, if you want to post an in-context snippet.

  <script langguage='text/javascript'>  
    var rownum = "";
    $('#myTable').find('tr').click( function(){
      rownum=$(this).index();
      alert('You clicked row '+ (rownum) );
    });
 </script>

Do not usevar, just define rownum outside so it will be global and you can access anywhere. Also you can not call find() directly.

You can set rownum global and access from outside of the function:

var rownum;
$('#myTable').find('tr').click( function(){
  rownum = $(this).index();
  alert('You clicked row '+ (rownum) );
});

Or pass it rownum as parameter to other function

$('#myTable').find('tr').click( function(){
  var rownum = $(this).index();      
  myOtherFunction(rownum);
});

function myOtherFunction(num)(){
  alert(num)
}

Declare rownum directly below script tag

<script type=text/javascript>
var rownum;

function A(){
    rownum=5;
}
function B(){
    alert(rownum);//will throw 5
}

You can use namespacing technique.

SomeNameSpace={} // Creating name spacing object
   SameNameSpace.rownum='';
   $('#myTable').find('tr').click( function(){
      SameNameSpace.rownum=$(this).index()
      alert('You clicked row '+ (SameNameSpace.rownum) );
    });

So anywhere you can use this variable by simply calling SameNameSpace.rownum

Using global variable may create issues in later, as other developer may create a variable by same name & in global space that will be a conflict

Refer Here for more information on namespace

Make function any and you will get value for sure ...

 $('#myTable').find('tr').click( function(){
    rownum = $(this).index();
    //  alert('You clicked row '+ (rownum) );
    get(rownum);
 });

function get(rownum)
{
    alert(rownum);
}

This was how I did it using html5 session storage

//function to set line index on clicking a table row - note there were several tables so i selected the first using .eq();  

$("table").eq(0).on('click','tr',function () 
    {
        var lineNum = $(this).closest("tr").index();
        sessionStorage.setItem('lineNumber',lineNum);
    });

//the following retrieves the line index from session storage and converts it to a number from a string (all session storage values are stored as strings)

var line_Number=parseFloat(sessionStorage.getItem('lineNumber'));