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? mapY array


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Hello dear html5 cracks,

so im following this tutorial about a javascript tile based theme and i came across this line:


tile = (engine.currentMap[mapY] && engine.currentMap[mapY][mapX]) ? engine.currentMap[mapY][mapX] : {ground: 0};

now as i understand it checks wether there is information in the array or not, if there is none it draws a black tile.

now i tryed writing :

tile = (engine.currentMap[mapY] && engine.currentMap[mapY][mapX]) ? engine.currentMap[mapY][mapX] : {ground: 0};

and it wont work anymore. now to my question why cant i just check if there is a tile at mapY,mapX?


thx already,



whole code bit

engine.map.draw = function(){   var i, j, tile;   var mapX = 0;   var mapY = 0;   var iMax = engine.screen.tilesX + engine.viewport.overflowTile;   var jMax = engine.screen.tilesY + engine.viewport.overflowTile;   for(j = -engine.viewport.overflowTile; j<jMax; j++)   {      for(i = -engine.viewport.overflowTile; i<iMax; i++)      {         mapX = i + engine.viewport.x;         mapY = j + engine.viewport.y;         tile = (engine.currentMap[mapY] && engine.currentMap[mapY][mapX]) ? engine.currentMap[mapY][mapX] : {ground: 0};         engine.tile.draw(i, j, tile);      }   }};
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This is because the code is a little hacky. If you take away the first statement the engine will crash, because there is a possibility you tried to check an element of 'undefined' ie:


array[0][0] is OK, but maybe:


array[30] is undefined, so you can't legally write array[30][0], because the interpreter will complain that you tried to access [0] of 'undefined', which is obviously nonsensical. If you have the first statement the interpreter doesn't bother looking at the second one; it's already evaluated as false.


Does that make sense?

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