fix: needs runtime node-fetch import

`import ... from "node-fetch"` will throw in Edge runtime and browser
because the library attempts to make `node:...` package imports.

The only way to consume this package is to load it when it's needed with
`await import(...)`, and not as an import statement at the top of the
context.

It is the difference between always loading it and only using it when
needed, and only loading it and using it when needed.

closes #50
cc @transmissions11

src/lib/backoff.ts

@@ -1,29 +1,33 @@
 /* eslint-disable no-console */
-import nodeFetch, { RequestInfo as NodeFetchRequestInfo, RequestInit as NodeFetchRequestInit } from "node-fetch";
+import type NodeFetch from "node-fetch";
+import type { RequestInfo as NodeFetchRequestInfo, RequestInit as NodeFetchRequestInit } from "node-fetch";
+
+type Fetch = typeof NodeFetch | typeof fetch;
 
 export interface BackoffOptions {
   maxRetries: number;
   delay: number;
 }
 
-const globalFetch = typeof fetch === "undefined" ? nodeFetch : fetch;
-
 export const fetchWithBackoff = async (
   input: RequestInfo & NodeFetchRequestInfo,
   init?: RequestInit & NodeFetchRequestInit,
-  fetch = globalFetch,
+  fetch?: Fetch,
   { delay, maxRetries }: BackoffOptions = {
     delay: 500,
     maxRetries: 7
   }
 ) => {
+  let _fetch: Fetch | undefined = fetch;
+
+  if (!_fetch) {
+    const { default: nodeFetch } = await import("node-fetch");
+    _fetch = nodeFetch;
+  }
+
   for (let i = 0; i <= maxRetries; i++) {
     try {
-      if (!fetch) {
-        throw new Error("No fetch implementation.");
-      }
-
-      const response = await fetch(input, init);
+      const response = await _fetch(input, init);
 
       if (!response.ok) {
         const errorData = await response.json();

src/lib/types.ts

@@ -59,8 +59,9 @@ export interface OpenAIOptions extends OpenAIStreamOptions {
    */
   controller?: AbortController;
   /**
-   * An optional custom fetch implementation, which will be used to replace the default fetch/node-fetch
-   * call used for making API requests in edge/dom and node environments respectively.
+   * An optional custom fetch implementation, which will be used to replace the
+   * default fetch/node-fetch call used for making API requests in edge/dom and
+   * node environments respectively.
    */
   fetch?: typeof fetch | typeof nodeFetch;
 }