You could iterate the array and build a new row if the remainder with 5 is zero.

var array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
    tr;

array.forEach((v, i) => {
    var td = document.createElement('td');
    
    if (!(i % 5)) {
        tr = document.createElement('tr');
        document.getElementById('table0').appendChild(tr);
    }
    td.appendChild(document.createTextNode(v));
    tr.appendChild(td);
});

<table id="table0"></table>

Here's a function that will use the dom instead of string concatenation.

function createTable(tableData) {
  var table = document.createElement('table');
  var tableBody = document.createElement('tbody');

  tableData.forEach(function(rowData) {
    var row = document.createElement('tr');

    rowData.forEach(function(cellData) {
      var cell = document.createElement('td');
      cell.appendChild(document.createTextNode(cellData));
      row.appendChild(cell);
    });

    tableBody.appendChild(row);
  });

  table.appendChild(tableBody);
  document.body.appendChild(table);
}

createTable([["row 1, cell 1", "row 1, cell 2"], ["row 2, cell 1", "row 2, cell 2"]]);

var arr = ["1","2","-3","4","-5","-6","7"];

var table = document.createElement("table"),          // create the table element
    tbody = document.createElement("tbody");          // create the table body element
    
table.appendChild(tbody);                             // add the table body to table

// ### POSITIVE ROW ######################

var tr = document.createElement("tr");                // create the table row element
arr.forEach(function(e) {                             // for each element e in arr
  var td = document.createElement("td");              // create a table cell element
  if(e >= 0) {                                        // if e is positive
    td.textContent = e;                               // then change the text content of this cell to e
    td.style.background = "#ccc";                     // and change the background as well
  }
  tr.appendChild(td);                                 // add the cell to the row (if the number is negative the cell will be empty and without background)
});
tbody.appendChild(tr);                                // add the row to table


//### NEGATIVE ROW ######################

tr = document.createElement("tr");                    // same but for negative numbers ...
arr.forEach(function(e) {
  var td = document.createElement("td");
  if(e < 0) {
    td.textContent = e;
    td.style.background = "#ccc";
  }
  tr.appendChild(td);
});
tbody.appendChild(tr);

document.body.appendChild(table);

You can group some of the code in a function and reuse it:

// elt is shortcut for document.createElement
function elt(tag) {
  return document.createElement(tag);
}

// take an array and a condition function conditionFN and return a row of all the element that passed the condition function with a background of #ccc
function makeRow(arr, conditionFN) {
  var tr = elt("tr");
  arr.forEach(function(e) {
    var td = elt("td");
    if (conditionFN(e)) {
      td.textContent = e;
      td.style.background = "#ccc";
    }
    tr.appendChild(td);
  });
  
  return tr;
}

// take an array and return the equivalent table element
function makeTable(arr) {
  var table = elt("table"),
      tbody = elt("tbody");

  table.appendChild(tbody);
  
  tbody.appendChild(makeRow(arr, function(e) { return e >= 0; })); // the condition function only allows positive numbers => positive row
  tbody.appendChild(makeRow(arr, function(e) { return e < 0; }));  // the condition function only allows negative numbers => negative row
  
  return table;
}



document.body.appendChild(makeTable([1, 2, -3, 4, -5, -6, -7, 8, 9, 10]));

Change:

var tr = document.createElement('TR'); // this should be inside the first loop
tableBody.appendChild(tr); // this should be just before the first loop is over

for (i=0; i<stock.length; i++) {
    for (j=0; j<stock[i].length; j++)    {
      var td = document.createElement('TD')
      td.appendChild(document.createTextNode(stock[i][j]));
      td.appendChild(td) // this should be tr.appendChild(td)
    }
}

to this:

for (i = 0; i < stock.length; i++) {
    var tr = document.createElement('TR');
    for (j = 0; j < stock[i].length; j++) {
        var td = document.createElement('TD')
        td.appendChild(document.createTextNode(stock[i][j]));
        tr.appendChild(td)
    }
    tableBody.appendChild(tr);
}

Fiddle

You could get first the headers of the table and while iterating the column names, you could collect the length of every data array and take the max length of it for later iterating the td elements.

var data = [{ Header: "Column1", Values: ["75", "79", "83"] }, { Header: "Column2", Values: ["77", "81", "86", "90"] }, { Header: "Column3", Values: ["98", "117"] }],
    table = document.createElement('table'),
    tr = document.createElement('tr'),
    rows = [],
    max = 0,
    i;

table.appendChild(tr);
data.forEach(function (o) {
    var th = document.createElement('th');
    th.appendChild(document.createTextNode(o.Header));
    tr.appendChild(th);
    max = Math.max(max, o.Values.length);
});
for (i = 0; i < max; i++) {
    tr = document.createElement('tr'),        
    table.appendChild(tr);
    data.forEach(function (o) {
        var td = document.createElement('td');
        tr.appendChild(td);
        td.appendChild(document.createTextNode(i in o.Values ? o.Values[i] : ''));
    });
}
document.body.appendChild(table);