ability at runtime for a context object to access other context objects

.changeset/tiny-chefs-greet.md

@@ -0,0 +1,5 @@
+---
+"counterfact": minor
+---
+
+ability at runtime for a context object to access other context objects

docs/usage.md

@@ -192,6 +192,17 @@ By default, each `_.context.ts` delegates to its parent directory, so you can de
 > [!TIP]
 > You can make the context objects do whatever you want, including things like writing to databases. But remember that Counterfact is meant for testing, so holding on to data between sessions is an anti-pattern. Keeping everything in memory also makes it fast.
 
+> [!TIP]
+> An object with loadContext() function is passed to the constructor of a context class. You can use it load the context from another directory at runtime. This is an advanced use case.
+>
+> ```ts
+> class Context {
+>   constructor({ loadContext }) {
+>     this.rootContext = loadContext("/");
+>   }
+> }
+> ```
+
 ### Security: the `$.auth` object
 
 If a username and password are sent via basic authentication, the username and password can be found via `$.auth.username` and `$.auth.password` respectively.

src/server/module-loader.ts

@@ -175,7 +175,9 @@ export class ModuleLoader extends EventTarget {
 
             // @ts-expect-error TS says Context has no constructable signatures but that's not true?
             // eslint-disable-next-line @typescript-eslint/no-unsafe-argument
-            new endpoint.Context(),
+            new endpoint.Context({
+              loadContext: (path: string) => this.contextRegistry.find(path),
+            }),
           );
         }
       } else {

test/server/module-loader.test.ts

@@ -190,6 +190,36 @@ describe("a module loader", () => {
     });
   });
 
+  it("provides the parent context if the local _.context.ts doesn't export a default", async () => {
+    await usingTemporaryFiles(async ($) => {
+      await $.add(
+        "_.context.js",
+        "export class Context  { constructor({loadContext}) { this.loadContext = loadContext } }",
+      );
+      await $.add("a/_.context.js", "export class Context  { name = 'a' }");
+      await $.add("package.json", '{ "type": "module" }');
+
+      const registry: Registry = new Registry();
+
+      const contextRegistry: ContextRegistry = new ContextRegistry();
+
+      const loader: ModuleLoader = new ModuleLoader(
+        $.path("."),
+
+        registry,
+        contextRegistry,
+      );
+
+      await loader.load();
+
+      // eslint-disable-next-line @typescript-eslint/consistent-type-assertions, @typescript-eslint/no-unsafe-assignment, @typescript-eslint/no-explicit-any
+      const rootContext = contextRegistry.find("/") as any;
+
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-member-access, @typescript-eslint/no-unsafe-call
+      expect(rootContext?.loadContext("/a")?.name).toBe("a");
+    });
+  });
+
   // can't test because I can't get Jest to refresh modules
   it.skip("updates the registry when a dependency is updated", async () => {
     await usingTemporaryFiles(async ($) => {