---
id: fast-foundation.elementdefinitionparams.callback
title: ElementDefinitionParams.callback property
hide_title: true
---
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[@microsoft/fast-foundation](./fast-foundation.md) &gt; [ElementDefinitionParams](./fast-foundation.elementdefinitionparams.md) &gt; [callback](./fast-foundation.elementdefinitionparams.callback.md)

## ElementDefinitionParams.callback property

A callback to invoke if definition will happen.

<b>Signature:</b>

```typescript
callback: ElementDefinitionCallback;
```