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    <p>The balls are all colliding; the formula for the collision is correct, i.e.:</p> <pre><code>(dx * dx) + (dy * dy) &lt;= sum of the circles' radii </code></pre> <p>The problem, is that sometimes the screen is not updated to reflect the new color, due to the single-threaded nature of JavaScript, and the <code>else</code> part of the <code>if</code> that resets the color.</p> <p>For example, when <code>ball[0]</code> collides with <code>ball[1]</code>, you set <code>ball[0]</code>'s color to <code>red</code> in the <code>if</code>, which is <strong>OK</strong>. However, if <code>ball[0]</code> <strong><em>does not</em></strong> collide with <code>ball[2]</code> in the next iteration of the inner loop (<code>j=2</code>), for example, it will <strong><em>reset</em></strong> <code>ball[0]</code>'s color back to its original color, and since the screen will only be rendered <strong><em>after</em></strong> the loop is over (again, due to the single-threaded nature of JavaScript), you never see the <code>red</code> color in this situation.</p> <p>So, you have two options. <strong>1)</strong> Either <strong><a href="http://jsbin.com/imofat/829/edit" rel="nofollow"><code>break</code></a></strong> it out of the <em>inner</em> loop when a collision occurs (if you don't need to test every other collision); <strong>2)</strong> Mark which balls have already collided, and only reset the colors of those who have never collided before in the loop, using an <code>Array</code>, for example:</p> <pre><code>var collisions = []; for (var i = 0; i &lt; balls.length; i++) { for (var j = 0; j &lt; balls.length; j++) { if (i!=j) { if (Math.pow(balls[j].center.x - balls[i].center.x, 2) + Math.pow(balls[j].center.y - balls[i].center.y, 2) &lt;= Math.pow(balls[i].radius + balls[j].radius, 2)) { collisions[i] = true; balls[i].setColor('red'); } else { if (!collisions[i]) { balls[i].setColor(balls[i].color); } } } } } </code></pre> <p>Also, the second loop could be simplified to start from <code>i + 1</code> instead of <code>0</code>. This way, you not only reduce the number of collision tests, but also remove the <code>i != j</code> test.</p> <p><strong><a href="http://jsbin.com/imofat/820/edit" rel="nofollow">DEMO</a></strong>.</p>
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    1. COQuite brilliant , thanks a lot!!
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      1. USAditya
 

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