> ## Documentation Index
> Fetch the complete documentation index at: https://developers.avacloud.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom HTTP Client

The TypeScript SDK makes API calls using an HTTPClient that wraps the native [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API). This client is a thin wrapper around `fetch` and provides the ability to attach hooks around the request lifecycle that can be used to modify the request or handle errors and response.

The `HTTPClient` constructor takes an optional `fetcher` argument that can be used to integrate a third-party HTTP client or when writing tests to mock out the HTTP client and feed in fixtures.

The following example shows how to use the `beforeRequest` hook to to add a custom header and a timeout to requests and how to use the `requestError` hook to log errors:

```javascript theme={null}
import { Avalanche } from "@avalanche-sdk/chainkit";
import { HTTPClient } from "@avalanche-sdk/chainkit/lib/http";

const httpClient = new HTTPClient({
  // fetcher takes a function that has the same signature as native `fetch`.
  fetcher: (request) => {
    return fetch(request);
  },
});

httpClient.addHook("beforeRequest", (request) => {
  const nextRequest = new Request(request, {
    signal: request.signal || AbortSignal.timeout(5000),
  });

  nextRequest.headers.set("x-custom-header", "custom value");

  return nextRequest;
});

httpClient.addHook("requestError", (error, request) => {
  console.group("Request Error");
  console.log("Reason:", `${error}`);
  console.log("Endpoint:", `${request.method} ${request.url}`);
  console.groupEnd();
});

const sdk = new Avalanche({ httpClient });
```