A* is an algorithm for finding the shortest path between two points. It is very useful in game development. Any tile-based game that requires this kind of movement will probably utilize some form of A*. Tower Defense games, City Building games, Rogue-Likes, the list goes on.

Thus EasyStar.js is a simple A* API written in Javascript.

• The ability to spread out your calculations over multiple calls. EasyStar.js lets you specify how many calculations should be performed each call.
• The ability to add separate points to avoid, outside of those that are avoided based on tile type.
• The ability to specify which tile types are walkable, and which are unwalkable.
• EasyStar calls a callback if it finds your path, or if there is no possible path.
• Easy!

var easyStar = new EasyStar.js(acceptableTiles,callback);

easyStar.setGrid(twoDimensionalArray);

easyStar.setPath(startX,startY,endX,endY);

easyStar.calculate();

To use EasyStar.js, first create a grid that represents the map in your game. Maybe you have made this map with a level editor, or by procedural generation. Here we create a hard-coded map.

//Create our TileMap
var grid = [];
for (var y = 0; y < MAP_HEIGHT; y++) grid.push([]);
grid[0].push(0,0,1,0,0,0,0,0,0,1,0,0,0,0,1,0,0,0,0,0);
grid[1].push(0,2,2,2,2,2,0,2,2,2,2,2,2,2,2,2,2,2,2,0);
grid[2].push(0,2,2,2,2,2,0,2,2,2,2,0,2,2,2,2,2,2,2,0);
grid[3].push(0,2,2,2,2,2,0,2,2,2,2,0,2,2,2,3,3,3,2,0);
grid[4].push(0,2,2,2,2,2,0,2,2,2,2,0,2,2,2,3,3,3,2,2);
grid[5].push(0,2,2,2,2,2,0,2,2,2,2,0,2,2,2,3,3,3,2,0);
grid[6].push(0,2,2,2,2,2,0,2,2,2,2,0,2,2,2,2,2,2,2,0);
grid[7].push(0,2,0,0,1,0,0,0,2,2,2,0,0,0,2,2,2,2,2,0);
grid[8].push(0,2,2,2,2,2,2,0,2,2,2,2,2,0,2,2,2,2,2,0);
grid[9].push(0,2,2,2,2,2,2,0,0,1,0,0,0,0,0,1,0,0,2,0);
grid[10].push(0,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,0);
grid[11].push(0,2,2,2,2,2,2,2,2,2,2,0,0,1,0,0,2,0,0,0);
grid[12].push(0,2,2,2,2,2,2,2,2,2,2,0,2,2,2,2,2,2,2,0);
grid[13].push(0,2,2,2,2,2,2,2,2,2,2,0,2,2,2,2,2,2,2,0);
grid[14].push(0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0);

Next you should create a list of tiles "walkable". In our case, I want the tiles 2 and 3 to be "walkable", but all other values should not be.

var acceptableTiles = [2,3];

Next I want to setup my callback for when a path is found.

function onPathFound(path) {
	if (path==null) {
		alert("Path was not found.");
	} else {
		alert("Path was found. The first Point is " + path[0].x + " " + path[0].y);
	}
}

Now I have everything I need to create my EasyStar instance.

var easyStar = new EasyStar.js(acceptableTiles,onPathFound);

I also need to let EasyStar.js know what my grid looks like, so I need to call the setGrid method.

easyStar.setGrid(grid);

We are getting close now!

All I need to do is give it a path that I want. In the case of my example, I give EasyStar a path every time the user clicks. Let us say that I want to find a path from the upper left hand corner of the map, to a position a few tiles to the right.

easyStar.setPath(0,0,3,0); //startX, startY, endX, endY

EasyStar.js will not yet start calculating my path. In order for EasyStar to actually start calculating, I must call the calculate() method. It is good practice to call the calculate() method on a setInterval method.

If you have a large collision map, then it is possible that these calculations could slow down your page. For this reason, it might be a good idea to give EasyStar.js a smaller iterationsPerCalculations value via

easyStar.setIterationsPerCalculation(value); 

This way it may take longer for you to find a path, but you won't completely halt your game trying to find a path.

In this example, lets just assume that we don't have this problem, and that our collisionGrid is small, and that our path is easy to find -- which it is!

The only thing left to do is to calculate the path.

easyStar.calculate();

..and we're done!

EasyStar.js is licensed under the MIT license, which means you can use it for almost anything, even commercial use.

If you have any questions, comments, or suggestions you can email me at [email protected]