Using a 2 dimensional array to populate a Javascript table

Your code basically had two issues, when I ran it:

1. In your method populateTable() the variables _r and _c were not defined, so I passed them in as arguments.

2. The method document.getElementByTagName does not exists, it's plural document.getElementsByTagName which in turn returns an array of all <table> elements so you have to select which one you want - I opted for the first one since I assume you only have one table on your page.

Here are the changes:

function newPuzzle(r, c)
{
   var table = document.createElement("table");

   for (var i = 0; i < r; i++)
   {
      var row = document.createElement('tr');
      for (var j = 0; j < c; j++)
      {
         var column = document.createElement('td');

         if (i%2 == j%2)
         {
               column.className = "even";
         }
         else
         {
               column.className = "odd";
         }     
         row.appendChild(column);    
      }
      table.appendChild(row);
   }
   document.body.appendChild(table);
   // here we pass _r and _c as arguments
   populateTable(r,c);
}

function populateTable(_r,_c)
{

   var cell = new Array(_r);
   for (var i = 0; i < _r; i++)
   {
      cell[i] = new Array(_c);
   }

   for (var i = 0; i < cell.length; ++i)
   {
        var entry = cell[i];
        for (var j = 0; j < entry.length; ++j)
        {
            // getElementsByTagName returns an array of all table elements
            var gridTable = document.getElementsByTagName("table");

            // we select the first table element with the array index [0]
            var _cells = gridTable[0].rows[i].cells[j].innerHTML = "2"; 
            //the 2 is just for testing
        }
   }
}

An interactive JS fiddle: https://jsfiddle.net/Ls76kk2a/2/

An optimization tip: Give your table a unique ID like this:

var table = document.createElement("table");
table.id = "mySuperCoolTable";

Then you can be sure you get the right one with:

var gridTable = document.getElementById("mySuperCoolTable");

This only returns one (or none) table because the ID must be unique.

Response to your comment:

Here's an example how to populate all elements but the last one:

function populateTable(_r,_c)
{

   var cell = new Array(_r);
   for (var i = 0; i < _r; i++)
   {
      cell[i] = new Array(_c);
      for (var j = 0; j < _c; j++) {
        cell[i][j] = i*_c + j;
      }
   }

   // fix the last one
   cell[_r-1][_c-1] = "X";

   for (var i = 0; i < cell.length; ++i)
   {
        var entry = cell[i];
        for (var j = 0; j < entry.length; ++j)
        {
            var gridTable = document.getElementsByTagName("table");
            var _cells = gridTable[0].rows[i].cells[j].innerHTML = cell[i][j]; 
        }
   }
}

And here's the JS fiddle: https://jsfiddle.net/Ls76kk2a/3/