Strange behavior in Javascript enhanced for...in loop

I am making a Javascript game with the canvas tag, and I am using an enhanced for loop to update the player positions.

In brief:

var actors = new Array();

var player = new Actor(0, 0, img);

actors[0] = player;

function update_positions() {
    //position 1
    for(var a in actors) {
        //position2
        a.xpos += a.xvel;
        a.ypos += a.yvel;
    }
}

Just outside of the for loop at position 1, I can access the correct value of actors[0].xvel. Inside the for loop at position 2, a.xvel is undefined. Can someone explain to me what is happening?

The for...in statement is meant to be used to iterate over object properties, by looking your code seems that actors is an Array (you are setting the initial element with index 0).

This statement will also crawl up the prototype chain, if you have extended the Array.prototype those properties will be iterated, and also the order of iteration is not warranted.

I would recommend you to avoid problems and iterate using a normal for loop:

for (var i = 0; i < actors.length; i++) {
    actors[i].xpos += actor.xvel;
    actors[i].ypos += actor.yvel;
}

If I'm wrong, and actors is not an Array, I would recommend you to use the hasOwnProperty method, to ensure that the property exists in the object itself, not up somewhere in the prototype chain:

for (var name in object) {
  if (object.hasOwnProperty(name)) {
    //
  }
}

it looks like you're trying to access object properties on the name, not the value here. the index, in this case '0', gets assigned to 'a' in the for/in loop.

what you want to do is access the value of the array member, ie: actors[a].

try this:

for(var a in actors) { // a=0 the first time around the loop,  
    actor = actors[a]; // same thing as actors[0];
    actor.xpos += actor.xvel;
    actor.ypos += actor.yvel;
}