***

title: "CreateErrorResultGuard"
generated: true
---------------

<GenerationInfo sourceFile="packages/testing/src/error-result-guard.ts" sourceLine="18" packageName="@vendure/testing" />

Convenience method for creating an [ErrorResultGuard](/current/core/reference/typescript-api/testing/error-result-guard#errorresultguard). Takes a predicate function which
tests whether the input is considered successful (true) or an error result (false).

Note that the resulting variable must *still* be type annotated in order for the TypeScript
type inference to work as expected:

*Example*

```ts
const orderResultGuard: ErrorResultGuard<AddItemToOrderResult>
  = createErrorResultGuard(order => !!order.lines);
```

```ts title="Signature"
function createErrorResultGuard<T>(testFn: (input: T) => boolean): ErrorResultGuard<T>
```

Parameters

### testFn

\<MemberInfo kind="parameter" type={`(input: T) => boolean`} />