mine-sweeter.js

// Javascript one-liner to calculate total minesweeper neighbours

// Polyfill for ECMA 2019
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat
if (!Array.prototype.flat) {
  Array.prototype.flat = function () {
    return this.reduce((acc, val) => acc.concat(val), []);
  }
}

var input1 = [
  [' ',' ',' ',' ',' '],
  [' ',' ',' ',' ',' '],
  ['X',' ',' ',' ',' '],
  [' ','X',' ',' ',' '],
  [' ',' ','X',' ',' '],
]

var input2 = [
  [' ',' ','X'],
  [' ',' ','X'],
  [' ','X',' '],
  ['X',' ',' '],
];

function findNeighbors(input) {
  return input.map(
    (r,i) => r.map(
      (c, j) => {
        if (c == 'X') return c;

        return [, ...input]
          .splice(i, 3)
          .map(r => [,...r]
            .splice(j, 3))
            .flat()
            .filter(cell => cell == 'X').length
      }
    )
  );
}

function mountConsoleBoard(output) {
  output.forEach(r => console.log('| ' + r.join(' | ') + ' |'))
}


mountConsoleBoard(findNeighbors(input2));

// Output
/**
| 0 | 2 | X |
| 1 | 3 | X |
| 2 | X | 2 |
| X | 2 | 1 |
*/