Adding custom code | Fern Documentation

Fern-generated SDKs are designed to be extended with custom code. Your custom code can add additional functionality to the SDK and live in harmony with the generated code.

This page explains how to configure custom logic using a .fernignore file, create custom SDK methods, and add additional dependencies to your SDK.

Adding custom logic

If you want your SDK to do more than just make basic API calls (like combining multiple calls, processing data, adding utilities), you can use .fernignore to protect your custom code from being overwritten during regeneration.

Simply add your custom files to the SDK repository and list them out in .fernignore. Fern won’t override any files that you add in .fernignore.

To get started adding custom code:

Create a new file and add your custom logic

src/helper.ts

1 export function myHelper(): void {
2     return console.log("Hello world!");
3 }

Add your file to `.fernignore`

A .fernignore file is automatically created in your SDK repository when you use GitHub publishing.

.fernignore

1 # Specify files that shouldn't be modified by Fern
2 
3 src/helper.ts

Consume the helper

Now your users can consume the helper function by importing it from the SDK.

1 import { myHelper } from "sdk/helper";
2 
3 myHelper();

Custom SDK methods

Fern also allows you to add custom methods to the SDK itself (e.g. client.my_method() ) by inheriting the Fern generated client and then extending it.

See an example from Flatfile using this process in their TypeScript SDK

Import and extend the generated client

First, import the Fern generated client from ../client and alias it to FernClient. Next, extend FernClient and add whatever methods you want.

src/wrapper/MyClient.ts

1 import { MyClient as FernClient } from "../client"; // alias the Fern generated client
2 
3 export class MyClient extends FernClient { // extend the Fern generated client
4 
5   public myHelper(): void {
6     console.log("Hello world!");
7   }
8 
9 }

See an example from Flatfile doing this in their FlatfileClient

Export the extended client

Update your index.ts file to export the extended client instead of the generated client.

src/index.ts

1 export { MyClient } from "src/wrapper/MyClient"; // instead of "src/client"

See an example index.ts from Flatfile

Update `.fernignore`

Add both the wrapper directory and index.ts to .fernignore.

.fernignore

1 + src/wrapper
2 + src/index.ts

See an example .fernignore from Flatfile

Consume the method

Now your users can consume the helper function by importing it from the SDK.

1 client.myHelper()

Custom dependencies

Pro Feature

This feature is only available on paid plans. Please schedule a demo or email us to get started.

To add packages that your custom code requires, update your generators.yml.

generators.yml

1 - name: fernapi/fern-typescript-sdk
2   version: 2.6.2
3   config:
4     extraDependencies:
5       lodash-es: '1.0.0'
6     extraDevDependencies:
7       "@types/lodash-es": '1.0.0'