// Board indexes:
//  0 | 1 | 2
// ---+---+---
//  3 | 4 | 5
// ---+---+---
//  6 | 7 | 8

const getRowNumGivenWidth = w => i => Math.floor(i/w);

// I want to be able to declare nextIndexInRowGivenWidth() here, outside of main()
// but getRowNum() needs to be defined beforehand


const main = () => {

  // User input:
  const width = 3;

  // ...


  const getRowNum = getRowNumGivenWidth(width);

  const nextIndexInRowGivenWidth = width => currentIndex => {
    const rowNum = getRowNum(currentIndex);
    const nextIndex = currentIndex + 1;

    if (getRowNum(nextIndex) != rowNum)
      result = nextIndex - width;
    else
      result = nextIndex;

    return result;
  };


  const nextIndexInRow = nextIndexInRowGivenWidth(width);

  const board = [0, 1, 2, 3, 4, 5, 6, 7, 8];

  board.map(x => console.log(x, " -> ", nextIndexInRow(x)));

  // ...

}

main();