https://www.careercup.com/question?id=5749882624147456 Isles question in ES6 javascript

isles.js

#!/usr/bin/env node
"use strict"

// Write a function that receives a position in 2 dimensional (x,y) array, which was initially initialized with 'o' (signals "water"), the function changes the value/state of that position to 'x' (signals "land") and returns the number of isles in the board.
// For example, for 3x3 board, it will initially look like the following:
// o o o
// o o o
// o o o
// After calling the function with the position (1,2), the board will look like the following:
// o o x
// o o o
// o o o
// and the functions returns 1
// An isle is defined as 'x' surrounded horizontally and vertically with 'o'
// In the following board there is only one isle
// o o o
// o x x
// o x o

let matrix = [
  ["o", "o", "o"],
  ["o", "o", "o"],
  ["o", "o", "o"]
]

class Grid {
  constructor(matrix) {
    this.loadFromMatrix(matrix)
  }

  loadFromMatrix(matrix) {
    this.points = matrix.reduce((yAcc, yCurr, yIdx) => {
      yAcc[yIdx] = yCurr.reduce((xAcc, xCurr, xIdx) => {
        xAcc[xIdx] = new Point(xCurr, yIdx, xIdx, matrix)
        return xAcc
      }, {})
      return yAcc
    }, {})

    this.islandCount = this.countIsles()
  }

  markIsle(x, y) {
    var currentNode = this.points[y][x]
    if (currentNode.value == 'x') {
      return this.islandCount
    }

    if (currentNode) {
      currentNode.value = 'x'
      this.islandCount = this.countIsles()
      return this.islandCount
    }
    return 'Error outside of graph'
  }

  // hasCycle
  countIsles() {
    let visited = {}, count = 0
    Object.keys(this.points).forEach((yKey) => {
      Object.keys(this.points[yKey]).forEach((xKey) => {
        var current = this.points[yKey][xKey]
        if (current.value == 'x' && !visited[current.key]) {
          this.dfs(current, visited)
          count++
        }
      })
    })
    return count
  }

  // dfs
  dfs(current, visited) {
    visited[current.key] = true
    var neighbors = current.neighbors(this.points)
    neighbors.forEach((neighbor, idx) => {
      if (!neighbor) return
      if (neighbor.value == 'x' && !visited[neighbor.key]) {
        this.dfs(neighbor, visited)
      }
    })
  }
}

class Point {
  constructor(value, yIdx, xIdx, matrix) {
    this.y = yIdx
    this.x = xIdx
    this.key = yIdx + "," + xIdx
    this.value = value
  }

  neighbors(matrix) {
    let left, right, top, bot
    if (matrix[this.y]) {
      left = matrix[this.y][this.x - 1]
      right = matrix[this.y][this.x + 1]
    }
    if (matrix[this.y - 1]) {
      top = matrix[this.y - 1][this.x]
    }
    if (matrix[this.y + 1]) {
      bot = matrix[this.y + 1][this.x]
    }

    return [left, right, top, bot]
  }
}
console.log("matrix: ", JSON.stringify(matrix))
// matrix:  [["o","o","o"],["o","o","o"],["o","x","o"]]
var g = new Grid(matrix)
console.log(JSON.stringify(g))
// {"points":{"0":{"0":{"y":0,"x":0,"key":"0,0","value":"o"},"1":{"y":0,"x":1,"key":"0,1","value":"o"},"2":{"y":0,"x":2,"key":"0,2","value":"o"}},"1":{"0":{"y":1,"x":0,"key":"1,0","value":"o"},"1":{"y":1,"x":1,"key":"1,1","value":"o"},"2":{"y":1,"x":2,"key":"1,2","value":"o"}},"2":{"0":{"y":2,"x":0,"key":"2,0","value":"o"},"1":{"y":2,"x":1,"key":"2,1","value":"x"},"2":{"y":2,"x":2,"key":"2,2","value":"o"}}},"islandCount":1}

console.log("g.markIsle(1, 2): ", g.markIsle(1, 2))
// g.markIsle(1, 2):  1
console.log("g.markIsle(2, 1): ", g.markIsle(2, 1))
// g.markIsle(2, 1):  2
console.log("g.markIsle(1, 1): ", g.markIsle(1, 1))
// g.markIsle(1, 1):  1
console.log("g.markIsle(0, 0): ", g.markIsle(0, 0))
// g.markIsle(0, 0):  2
console.log("g.points: ", g.points)
// g.points:  { '0':
//    { '0': Point { y: 0, x: 0, key: '0,0', value: 'x' },
//      '1': Point { y: 0, x: 1, key: '0,1', value: 'o' },
//      '2': Point { y: 0, x: 2, key: '0,2', value: 'o' } },
//   '1':
//    { '0': Point { y: 1, x: 0, key: '1,0', value: 'o' },
//      '1': Point { y: 1, x: 1, key: '1,1', value: 'x' },
//      '2': Point { y: 1, x: 2, key: '1,2', value: 'x' } },
//   '2':
//    { '0': Point { y: 2, x: 0, key: '2,0', value: 'o' },
//      '1': Point { y: 2, x: 1, key: '2,1', value: 'x' },
//      '2': Point { y: 2, x: 2, key: '2,2', value: 'o' } } }