Attempt to fix node.js integration by tweaking imports with babel

Fixes: matrix-org#4287
Signed-off-by: Johannes Marbach <[email protected]>

babel.config.cjs

@@ -1,3 +1,6 @@
+const fs = require("fs");
+const path = require("path");
+
 module.exports = {
     sourceMaps: true,
     presets: [
@@ -22,5 +25,35 @@ module.exports = {
         "@babel/plugin-transform-object-rest-spread",
         "@babel/plugin-syntax-dynamic-import",
         "@babel/plugin-transform-runtime",
+        // Fix up imports & exports so that Node.js doesn't choke on them
+        ...(process.env.NODE_ENV === "test" ? [] : [function appendJsExtensionToImports() {
+           function fixImportOrExport(target, state) {
+                if (!target.node.source) {
+                    return;
+                }
+
+                const source = target.node.source.value;
+
+                if (source && source.startsWith(".") && !source.endsWith(".js")) {
+                    const fullPath = path.join(path.dirname(state.file.opts.filename), source);
+                    if (fs.existsSync(fullPath) && fs.statSync(fullPath).isDirectory()) {
+                        target.node.source.value += "/index.js";
+                    } else {
+                        target.node.source.value += ".js";
+                    }
+                }
+            }
+
+            return {
+              visitor: {
+                ImportDeclaration(target, state) {
+                    fixImportOrExport(target, state);
+                },
+                ExportDeclaration(target, state) {
+                    fixImportOrExport(target, state);
+                },
+              },
+            };
+        }])
     ],
 };