@loopback/docs/site/apidocs/openapi-spec-builder.builderbase.withextension.md

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---
lang: en
title: 'API docs: openapi-spec-builder.builderbase.withextension'
keywords: LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI
sidebar: lb4_sidebar
editurl: https://github.com/loopbackio/loopback-next/tree/master/packages/openapi-spec-builder
permalink: /doc/en/lb4/apidocs.openapi-spec-builder.builderbase.withextension.html
---

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[Home](./index.md) &gt; [@loopback/openapi-spec-builder](./openapi-spec-builder.md) &gt; [BuilderBase](./openapi-spec-builder.builderbase.md) &gt; [withExtension](./openapi-spec-builder.builderbase.withextension.md)

## BuilderBase.withExtension() method

Add a custom (extension) property to the spec object.

**Signature:**

```typescript
withExtension(key: string, value: any): this;
```

## Parameters

<table><thead><tr><th>

Parameter


</th><th>

Type


</th><th>

Description


</th></tr></thead>
<tbody><tr><td markdown="1">

key


</td><td markdown="1">

string


</td><td markdown="1">

The property name starting with "x-".


</td></tr>
<tr><td markdown="1">

value


</td><td markdown="1">

any


</td><td markdown="1">

The property value.


</td></tr>
</tbody></table>

**Returns:**

this