I have a form with chess Knight position input value (ex. D4). After clicking "Show", the system should show all possible moves (coordinates) from defined position (ex. C2 E2 F3 F5 E6 C6 B3 B5).

```js
let form = document.getElementById("chessKnight")
let btn = document.getElementById("button")
btn.addEventListener("click", chessKnight)

function chessKnight(cell) {
  var possibleCoordinates = [];
  var xCoordinates = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H'];
  var cellX = xCoordinates.indexOf(cell[0]) + 1;
  var cellY = parseInt(cell[1]);

  var cellXpositions = [cellX + 2, cellX - 2, cellX + 1, cellX -
    1
  ].filter(function(cellPosition) {
    return (cellPosition > 0 && cellPosition < 9);
  })

  var cellYpositions = [cellY + 2, cellY - 2, cellY + 1, cellY -
    1
  ].filter(function(cellPosition) {
    return (cellPosition > 0 && cellPosition < 9);
  })

  for (var i = 0; i < cellXpositions.length; i++) {
    for (var j = 0; j < cellYpositions.length; j++) {
      if (!possibleCoordinates.includes([cellXpositions[i],
          cellYpositions[j]
        ])) {
        possibleCoordinates.push([cellXpositions[i],
          cellYpositions[j]
        ]);
      }
    }
  }
  console.log('Possible Coordinates:', possibleCoordinates);
  return possibleCoordinates.length;
}
```

```html
<input id="chessKnight" type="text">
<button id="button">Show</button>
```

The system shows the following: 'Possible Coordinates:' []