no conditionals

cond.js

const population = [ [], [], [], [], [], [] ];
population[3][4] = false;
//-------------------------------------------
const population = new Population();
const cell = new AliveCell();
const position = new Position(3, 4);
population.set(position, cell);

class Cell {}

class DeadCell extends Cell {}
class AliveCell extends Cell {}

Cell.DEAD = new DeadCell();
Cell.ALIVE = new AliveCell();

class Cell {
  constructor(isAlive) {
    this.isAlive = isAlive;
  }
  
  nextState(neighbours) {
    if (this.isAlive && (neighbours === 2 || neighbours === 3)) {
      return new Cell(true);
    } else if (!this.isAlive && neighbours === 3) {
      return new Cell(true);
    } else {
      return new Cell(false);
    }
  }
}

class AliveCell extends Cell {
  nextState(neighbours) {
    return [
      Cell.DEAD, Cell.ALIVE, Cell.ALIVE,
      Cell.DEAD, Cell.DEAD, Cell.DEAD,
      Cell.DEAD, Cell.DEAD, Cell.DEAD      
    ][neighbours - 1]
  }
}

class DeadCell extends Cell {
  nextState(neighbours) {
    return [
      Cell.DEAD, Cell.DEAD, Cell.ALIVE,
      Cell.DEAD, Cell.DEAD, Cell.DEAD,
      Cell.DEAD, Cell.DEAD, Cell.DEAD      
    ][neighbours - 1]
  }
}

const cell = population.cellAt(3, 4);
const neighbours = population.neighbours(3, 4);
nextPopulation.put(3, 4, cell.nextState(neighbours));