example

JavaScript

var table = [
  ['x', 'x', 'x', 'x'],
  ['x', 'x', 'x', 'x'],
  ['x', 'x', 'x', 'x'],
  ['x', 'x', 'x', 'x'],
  ['x', 'x', 'x', 'x']
]

console.log(JSON.stringify(table));

var appendRow = () => {
  table.push(table[0].slice());
  console.log(JSON.stringify(table));
}

var appendCol = () => {
  table.forEach((element, index, array) => {
    element.push("y");
  });
  console.log(JSON.stringify(table));
}

appendRow();
appendCol();