Add fetch examples

Co-authored-by: Fredrik Adelöw <freben@users.noreply.github.com>
Signed-off-by: Johan Haals <johan.haals@gmail.com>

docs/architecture-decisions/adr013-use-node-fetch.md

@@ -1,7 +1,8 @@
 ---
 id: adrs-adr013
-title: 'ADR013: Use node-fetch for data fetching'
-description: Architecture Decision Record (ADR) for HTTP data fetching packages
+title: 'ADR013: Proper use of HTTP fetching libraries'
+# prettier-ignore
+description: Architecture Decision Record (ADR) for the proper use of fetchApiRef, node-fetch, and cross-fetch for data fetching.
 ---
 
 ## Context
@@ -11,9 +12,57 @@ support burden of keeping said package up to date.
 
 ## Decision
 
-Node packages should use the `node-fetch` package for HTTP data fetching.
-Isomorphic packages should use the `cross-fetch` as a development dependency and
-rely on the built-in `fetch` API.
+Backend (node) packages should use the `node-fetch` package for HTTP data
+fetching. Example:
+
+```ts
+import fetch from 'node-fetch';
+import { ResponseError } from '@backstage/errors';
+
+const response = await fetch('https://example.com/api/v1/users.json');
+if (!response.ok) {
+  throw await ResponseError.fromResponse(response);
+}
+const users = await response.json();
+```
+
+Frontend plugins and packages should prefer to use the
+[`fetchApiRef`](https://backstage.io/docs/reference/core-plugin-api.fetchapiref).
+It uses `cross-fetch` internally. Example:
+
+```ts
+import { useApi } from '@backstage/core-plugin-api';
+const { fetch } = useApi(fetchApiRef);
+
+const response = await fetch('https://example.com/api/v1/users.json');
+if (!response.ok) {
+  throw await ResponseError.fromResponse(response);
+}
+const users = await response.json();
+```
+
+Isomorphic packages should have a dependency on the `cross-fetch` package for
+mocking and type definitions. Preferably, classes and functions in isomorphic
+packages should accept an argument of type `typeof fetch` to let callers supply
+their preferred implementation of `fetch`. This lets them adorn the calls with
+auth or other information, and track metrics etc, in a cross-platform way.
+Example:
+
+```ts
+import crossFetch from 'cross-fetch';
+
+export class MyClient {
+  private readonly fetch: typeof crossFetch;
+
+  constructor(options: { fetch?: typeof crossFetch }) {
+    this.fetch = options.fetch || crossFetch;
+  }
+
+  async users() {
+    return await this.fetch('https://example.com/api/v1/users.json');
+  }
+}
+```
 
 ## Consequences