# ConverterFunction

```ts
type ConverterFunction<TReturn> = (raw) => TReturn & object;
```

Custom converter for an environment variable or an array element. It receives a present value with templates
already expanded. envapt applies the fallback itself. A return of `undefined` or `null` means the value could
not be converted. From `getAs` and an optional `@Envapt`, an error the converter throws propagates to the
caller unchanged. A required read reports it as an `InvalidEnvValue` issue carrying its message.

## Type Parameters

| Type Parameter | Default type |
| ------ | ------ |
|  `TReturn` | `unknown` |

## Type Declaration

| Name | Type |
| ------ | ------ |
| `~standard?` | `never` |
| `prototype?` | `undefined` |

## See

[https://envapt.materwelon.dev/docs/converters#custom-converters](https://envapt.materwelon.dev/docs/converters#custom-converters)