old_contents: class Sudoku {
      constructor() {
          this.arrangement = [...Array(9)].map(_ => Array(9).fill(null))
          this.solids = []
      }
  }
  
  exports.module = Sudoku
  
new_contents: class Sudoku {
      constructor() {
          this.arrangement = [...Array(9)].map(_ => [...Array(9)].map(__ => []))
          this.excluded = [...Array(9)].map(_ => [...Array(9)].map(__ => []))
          this.solids = []
      }
  
      getBoxes() {
          let tuples = arr => arr.map(x => arr.map(y => [x, y])).reduce((sum, x) => [...sum, ...x], [])
  
          return [
              // Rows
              ...[...Array(9)].map((_, i) => [...Array(9)].map((__, j) => [i, j])),
              // Columns
              ...[...Array(9)].map((_, i) => [...Array(9)].map((__, j) => [j, i])),
              // Squares
              ...tuples([0, 3, 6]).map(([tx, ty]) =>
                  [...Array(3)].map((_, i) => [...Array(3)].map((__, j) => [i + ty, j + tx]))
              )
          ]
      }
  }
  
  exports.module = Sudoku
  
subject: Add Sudoku.getBoxes()