feat(#93): adds mockConfig function export for testing

Author: joelgallant

app-config/src/index.test.ts

@@ -1,5 +1,5 @@
 import { inspect } from 'util';
-import { config, loadConfig, resetConfigInternal } from './index';
+import { config, loadConfig, mockConfig, resetConfigInternal } from './index';
 import { withTempFiles } from './test-util';
 
 beforeEach(() => resetConfigInternal());
@@ -142,3 +142,12 @@ describe('loadConfig', () => {
     );
   });
 });
+
+describe('mockConfig', () => {
+  it('mocks out the config expert', async () => {
+    mockConfig({ foo: 'bar' });
+    expect(config).toEqual({ foo: 'bar' });
+    await expect(() => loadConfig()).rejects.toThrow();
+  });
+});
+

app-config/src/index.ts

@@ -10,6 +10,7 @@ export interface ExportedConfig {}
 
 // the export of this module is a proxy in front of this value
 let loadedConfig: ExportedConfig | undefined;
+let isMocked = false;
 
 const assertLoaded = () => {
   if (!loadedConfig) {
@@ -24,6 +25,10 @@ export async function loadConfig(
 ): Promise<ExportedConfig> {
   if (loadedConfig) {
     logger.warn('Called loadConfig, even though config was already loaded elsewhere');
+
+    if (isMocked) {
+      throw new AppConfigError(`Called loadConfig after config was mocked with mockConfig!`);
+    }
   }
 
   const { fullConfig } = await loadValidatedConfig(options, schemaOptions);
@@ -120,3 +125,11 @@ export {
 export function resetConfigInternal() {
   loadedConfig = undefined;
 }
+
+/**
+ * Overrides the configuration internally, setting it to the provided override.
+ */
+export function mockConfig(override: ExportedConfig) {
+  loadedConfig = override;
+  isMocked = true;
+}