Load config from window in default config loader (#3238)

Support setting config through the window object. The config
should be set in window.__APP_CONFIG__ which should be an
object.

.changeset/3194.md

@@ -0,0 +1,7 @@
+---
+'@backstage/core': patch
+---
+
+Extend default config loader to read config from the window object.
+
+Config will be read from `window.__APP_CONFIG__` which should be an object.

packages/core/src/api-wrappers/createApp.test.tsx

@@ -18,10 +18,12 @@ import { defaultConfigLoader } from './createApp';
 
 (process as any).env = { NODE_ENV: 'test' };
 const anyEnv = process.env as any;
+const anyWindow = window as any;
 
 describe('defaultConfigLoader', () => {
   afterEach(() => {
     delete anyEnv.APP_CONFIG;
+    delete anyWindow.__APP_CONFIG__;
   });
 
   it('loads static config', async () => {
@@ -73,4 +75,20 @@ describe('defaultConfigLoader', () => {
       'Failed to load runtime configuration, SyntaxError: Unexpected token } in JSON at position 0',
     );
   });
+
+  it('loads config from window.__APP_CONFIG__', async () => {
+    anyEnv.APP_CONFIG = [
+      { data: { my: 'config' }, context: 'a' },
+      { data: { my: 'override-config' }, context: 'b' },
+    ];
+    const windowConfig = { app: { configKey: 'config-value' } };
+    anyWindow.__APP_CONFIG__ = windowConfig;
+
+    const configs = await defaultConfigLoader();
+
+    expect(configs).toEqual([
+      ...anyEnv.APP_CONFIG,
+      { context: 'window', data: windowConfig },
+    ]);
+  });
 });

packages/core/src/api-wrappers/createApp.tsx

@@ -67,6 +67,13 @@ export const defaultConfigLoader: AppConfigLoader = async (
     }
   }
 
+  const windowAppConfig = (window as any).__APP_CONFIG__;
+  if (windowAppConfig) {
+    configs.push({
+      context: 'window',
+      data: windowAppConfig,
+    });
+  }
   return configs;
 };