> For the complete documentation index, see [llms.txt](https://docs.relevant-digital.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.relevant-digital.com/technical/integration/javascript-api/relevantdigital.addauctioncallbacks.md).

# relevantDigital.addAuctionCallbacks()

Use `relevantDigital.addAuctionCallbacks()` when you cannot change the [relevantDigital.loadPrebid()](/technical/integration/javascript-api/relevantdigital.loadprebid.md) call. For example, register callbacks from custom JavaScript in the Yield UI.

### Register callbacks

#### Example

```
window.relevantDigital = window.relevantDigital || {};
relevantDigital.cmd = relevantDigital.cmd || [];
relevantDigital.cmd.push(function() {
   relevantDigital.addAuctionCallbacks({
       onSlotAndUnit: function({ slot, unit }) {
           console.info('Relevant Yield auction object', this);
           console.info('Slot and unit', slot, unit);
       },
       onBeforeAdRequest: function(params) {
           console.info('Relevant Yield auction object', this);
           console.info('Parameters', params);
       },        
   }); 
});
```

### Callback behavior

* `this` is the auction object.
* Callbacks registered for the same event run in registration order.
* You can intercept any auction-object function.
* This includes callbacks normally passed to `loadPrebid()`.

### Run a callback after the default handler

Listeners run before the default callback or function. Pass `{ when: 'after' }` as the second argument to run a listener afterward.

Use this option to inspect or modify the default return value.

#### Example

```
relevantDigital.addAuctionCallbacks({
    createAdUnitCode: function(result, params) {
        console.info("Parameters", params);
        return (result || 'unknown') + '-extra'
    },
}, { when: 'after' });  
```

### Return values

* The default return value is the first callback argument.
* The normal callback arguments follow it.
* Return the first argument to preserve the default result.


---

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