Fix: Import Babel config from package.json and export in babel.config.js to fix Jest tests in custom plugins that have a package.json

babel.config.js

@@ -0,0 +1,24 @@
+const fs = require("fs");
+
+/**
+ * @summary Babel 7 doesn't transpile files in sub-directories that have a package.json when Babel is configured
+ * through package.json or .babelrc. This causes Jest test failures in custom plugins that have a package.json. It
+ * isn't an issue with non-test files because those are imported through the main Reaction app (see /server/plugins.js
+ * and client/plugins.js). Babel does transpile these files when it is configured through the new babel.config.js.
+ * Meteor currently only loads Babel config through .babelrc or package.json. So, in order to support Babel transpiling
+ * of Jest tests, we load the babel config defined in package.json and export it here.
+ * See this Github comment: https://github.com/facebook/jest/issues/6053#issuecomment-383632515
+ */
+module.exports = function (api) {
+  api.cache(false);
+
+  /**
+   * Meteor only reads the babel config from .babelrc or package.json. So with just babel.config.js,
+   * the Meteor app fails but the Jest tests pass. With just package.json, the Jest tests in custom plugins
+   * fail but the app runs.
+   */
+  const file = fs.readFileSync("./package.json");
+  const packageJSON = JSON.parse(file);
+
+  return packageJSON.babel;
+};