Untitled

let array = [[5, 3, '-'],
             [6, '-', '-'],
             ['-', 9, 8]];


const arrNumbers = [1,2,3,4,5,6,7,8,9]


const foo = arr => {
  let arrCopy = JSON.parse(JSON.stringify(array))
  let hollowCellsIndex = [];
  let usedNums = [];
  for (let i = 0; i < arr.length; i += 1) {
    for (let j = 0; j < arr[i].length; j += 1) {
      if (arr[i][j] === '-') hollowCellsIndex.push([i, j]);
      if (arr[i][j] !== '-') usedNums.push(arr[i][j]);
    }
  }
  // console.log(array[1][0]);
  return [hollowCellsIndex, usedNums];
};

console.log(foo(array));