Configure jest to automatically pick changes in typescript modules without need to compile them using tsc

Summary: Right now, if you are changing something in one of the modules, e.g. in "flipper-plugin", you'll need to compile the changed module using `tsc` before running tests, otherwise `jest` won't pick up the changes. `yarn test` has a step to run `tsc` before tests, however when tests are started simply by `jest` command, e.g. by VSCode plugin, then `tsc` is not automatically called before tests. This diff makes things easier by configuring `jest` to automatically transpile imported modules. This means simply running `jest` command will automatically pick up and automatically transpile all the changes.

Reviewed By: passy

Differential Revision: D28310318

fbshipit-source-id: 77e9b91daa59a46ce64b1b70eb9a998d7b72de00

desktop/jest.config.js

@@ -22,4 +22,5 @@ module.exports = {
   coverageReporters: ['json-summary', 'lcov', 'html'],
   testMatch: ['**/**.(node|spec).(js|jsx|ts|tsx)'],
   testEnvironment: 'jest-environment-jsdom-sixteen',
+  resolver: '<rootDir>/jest.resolver.js',
 };

desktop/jest.resolver.js

@@ -0,0 +1,23 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+module.exports = (request, options) => {
+  // Call the defaultResolver, so we leverage its cache, error handling, etc.
+  return options.defaultResolver(request, {
+    ...options,
+    // Use packageFilter to process parsed `package.json` before the resolution (see https://www.npmjs.com/package/resolve#resolveid-opts-cb)
+    packageFilter: (pkg) => {
+      return {
+        ...pkg,
+        // Alter the value of `main` before resolving the package, so jest to try different entry points in the specified order.
+        main: pkg.flipperBundlerEntry || pkg.main,
+      };
+    },
+  });
+};