Question

Now we can start on the World object type. The constructor takes a plan (the array of strings representing the world’s grid, described earlier ) and a legend as arguments. A legend is an object that tells us what each character in the map means. It contains a constructor for every character—except for the space character, which always refers to null , the value we’ll use to represent empty space.

function elementFromChar(legend, ch) {
  if (ch == " ")
    return null;
  var element = new legend[ch]();
  element.originChar = ch;
  return element;
}

function World(map, legend) {
  var grid = new Grid(map[0].length, map.length);
  this.grid = grid;
  this.legend = legend;

  map.forEach(function(line, y) {
    for (var x = 0; x < line.length; x++)
      grid.set(new Vector(x, y),
               elementFromChar(legend, line[x]));
  });
}

In elementFromChar , first we create an instance of the right type by looking up the character’s constructor and applying new to it. Then we add an originChar property to it to make it easy to find out what character the element was originally created from.

We need this originChar property when implementing the world’s toString method. This method builds up a maplike string from the world’s current state by performing a two-dimensional loop over the squares on the grid.

function charFromElement(element) {
  if (element == null)
    return " ";
  else
    return element.originChar;
}

World.prototype.toString = function() {
  var output = "";
  for (var y = 0; y < this.grid.height; y++) {
    for (var x = 0; x < this.grid.width; x++) {
      var element = this.grid.get(new Vector(x, y));
      output += charFromElement(element);
    }
    output += "\n";
  }
  return output;
};

A wall is a simple object—it is used only for taking up space and has no act method.

function Wall() {}

When we try the World object by creating an instance based on the plan from earlier in the chapter and then calling toString on it, we get a string very similar to the plan we put in.

var world = new World(plan, {"#": Wall,
                             "o": BouncingCritter});
console.log(world.toString());
// → ############################
//   #      #    #      o      ##
//   #                          #
//   #          #####           #
//   ##         #   #    ##     #
//   ###           ##     #     #
//   #           ###      #     #
//   #   ####                   #
//   #   ##       o             #
//   # o  #         o       ### #
//   #    #                     #
//   ############################

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.