feat: throw error if API package.json does not exist

Author: geoffrich

README.md

@@ -23,7 +23,7 @@ export default {
 };
 ```
 
-You will need to create an `api/` folder in your project root containing a [`host.json`](https://docs.microsoft.com/en-us/azure/azure-functions/functions-host-json) (see sample below). The adapter will output the `render` Azure function for SSR in that folder. The `api` folder needs to be in your repo so that Azure can recognize the API at build time. However, you can add `api/render` to your .gitignore so that the generated function is not in source control.
+You will need to create an `api/` folder in your project root containing a [`host.json`](https://docs.microsoft.com/en-us/azure/azure-functions/functions-host-json) and a `package.json` (see samples below). The adapter will output the `render` Azure function for SSR in that folder. The `api` folder needs to be in your repo so that Azure can recognize the API at build time. However, you can add `api/render` to your .gitignore so that the generated function is not in source control.
 
 ### Sample `host.json`
 
@@ -37,6 +37,14 @@ You will need to create an `api/` folder in your project root containing a [`hos
 }
 ```
 
+### Sample `package.json`
+
+It's okay for this to be empty. Not including it causes the Azure Function build to fail.
+
+```json
+{}
+```
+
 ## Azure configuration
 
 When deploying to Azure, you will need to properly [configure your build](https://docs.microsoft.com/en-us/azure/static-web-apps/build-configuration?tabs=github-actions) so that both the static files and API are deployed.

files/api/package.json

@@ -1,4 +1,4 @@
-import { writeFileSync } from 'fs';
+import { writeFileSync, existsSync } from 'fs';
 import { join, posix } from 'path';
 import { fileURLToPath } from 'url';
 import esbuild from 'esbuild';
@@ -61,6 +61,12 @@ export default function ({ debug = false, customStaticWebAppConfig = {} } = {})
 				}
 			};
 
+			if (!existsSync(join('api', 'package.json'))) {
+				throw new Error(
+					'You need to create a package.json in your `api` directory. See the adapter README for details.'
+				);
+			}
+
 			const tmp = builder.getBuildDirectory('azure-tmp');
 			const publish = 'build';
 			const staticDir = join(publish, 'static');