Question: I'm using [MatterJS](http://brm.io/matter-js) to animate some objects on a canvas. At one point in the animation there is a collision between two of my objects. I'm successfully detecting the event with the below code:

```
Events.on(engine, 'collisionStart', function(event) {

    console.log(event)

});

```

What i'm trying to identify is which two objects are colliding. When I console.log event.pairs I get an empty array. I would have thought it would have returned an array of the two objects that are colliding. Where have I gone wrong?

Answer: 
You Really have no errors. you must see your pair on position zero, the library matterjs have a problem printing this value from event main. you must deploy your response and you will able to see your pairs, so:

```
Events.on(engine, 'collisionStart', function(event) {
     console.log("Evento: ", event)
     var pairs = event.pairs;
     console.log("Pair no visible: ", pairs)
     console.log("Pair visible: ", pairs[0]);
     console.log("colision between " + pairs[0].bodyA.label + " - " + pairs[0].bodyB.label);
});

```

[Here](https://codepen.io/cvalencia/pen/OjYdvE), i am showing a simple example.
Complementing answer of Cristian Agudelo, you need to copy state of pairs and then do what you want:

```
Matter.Events.on(engine, "collisionStart", function(e)
{
    var pairs = e.pairs.slice(); // copy
    ...
}

```