Collision detection accuracy
One thing you can improve on is the getting of the absolute value. In some browsers, bitwise absolute is faster than Math.abs. But it depends on the browser's implementation.

//bitwise absolute
function abs(n){
  return (n^(n&gt;&gt;31))-(n&gt;&gt;31);
}


You could also factor this part out into a variable for readability. The result of this is a boolean. The explanation will be in the next part:

//foo true if less than or equal to 0, false if greater
var foo = dist - (c1.getR() + c2.getR()) &lt;= 0; 


Also, in this bit of code, since done and invinc are boolean, there's no point comparing them to true and false. You can use them directly in the condition. 

if (done === false &amp;&amp; invinc !== true) //when false, and not true (false)

if (!done &amp;&amp; !invinc)                  //when not true (false) and not true (false)


In JS, assignment operations "spill left". You can do the following, assigning true to done and "spill" the same value over to game_over.

game_over = done = true;


Also, due to the structure of the code, the previous only happens when our condition assigned to foo is less than or equal to zero or true. So let's modify the condition to this:

if(foo &amp;&amp; !done &amp;&amp; !invinc) game_over = done = true;


So in the end, your code will look like this:

var dx = Math.abs(c1.getcx() - c2.getcx());
var dy = Math.abs(c1.getcy() - c2.getcy());
var dist = Math.sqrt((dx * dx) + (dy * dy));
var foo = dist - (c1.getR() + c2.getR()) &lt;= 0;

if (foo &amp;&amp; !done &amp;&amp; !invinc) game_over = done = true;