## resolveId() function

Returns the ID portion of a reference.

**Signature:**

```typescript
export declare function resolveId(input: Reference | Resource | undefined): string | undefined;
```

## Parameters

| Parameter | Type | Description |
| --- | --- | --- |
| input | Reference \| Resource \| undefined | A FHIR reference or resource. |

**Returns:**

string \| undefined

The ID portion of a reference.