// creates a DOM Element
var table_row = document.createElement("tr");

for (var i = 0; i < columns.length; i++) {
  // main row
  table_row.innerHTML += "<th>" + columns[i] + "</th>";
}

table_layout.appendChild(table_row); //add in table element

// still points to the DOM Element, so you are clearing its content
// table_row.innerHTML = "";

for (var j = 0; i < columns.length; j++) {
  // create a new DOM Element to use
  table_row = document.createElement("tr");
  // then update its contents
  table_main_row.innerHTML += '<td></td>';
}