Use alternative implementation for __dirname

packages/snaps-utils/coverage.json

@@ -1,6 +1,6 @@
 {
-  "branches": 96.46,
-  "functions": 98.63,
-  "lines": 98.55,
-  "statements": 94.33
+  "branches": 96.44,
+  "functions": 98.62,
+  "lines": 98.74,
+  "statements": 94.48
 }

packages/snaps-utils/src/eval.ts

@@ -1,7 +1,6 @@
 import { assert } from '@metamask/utils';
 import { fork } from 'child_process';
-import { join, dirname } from 'path';
-import { fileURLToPath } from 'url';
+import { join } from 'path';
 
 import { validateFilePath } from './fs';
 
@@ -21,22 +20,6 @@ export class SnapEvalError extends Error {
   }
 }
 
-/**
- * Get the dirname of the file at the provided `import.meta.url`. This is
- * similar to `__dirname` in CommonJS modules.
- *
- * @param importMetaUrl - The `import.meta.url` of the file to get the dirname
- * of.
- * @returns The dirname of the file at the provided `import.meta.url`.
- */
-export function getDirname(importMetaUrl: string): string {
-  if (importMetaUrl.startsWith('file://')) {
-    return dirname(fileURLToPath(importMetaUrl));
-  }
-
-  return dirname(importMetaUrl);
-}
-
 /**
  * Spawn a new process to run the provided bundle in.
  *
@@ -48,17 +31,11 @@ export async function evalBundle(bundlePath: string): Promise<EvalOutput> {
   await validateFilePath(bundlePath);
 
   return new Promise((resolve, reject) => {
-    const worker = fork(
-      // eslint-disable-next-line @typescript-eslint/ban-ts-comment
-      // @ts-ignore - `import.meta` is not supported in the current environment.
-      join(getDirname(import.meta.url), 'eval-worker.js'),
-      [bundlePath],
-      {
-        // To avoid printing the output of the worker to the console, we set
-        // `stdio` to `pipe` and handle the output ourselves.
-        stdio: 'pipe',
-      },
-    );
+    const worker = fork(join(__dirname, 'eval-worker.js'), [bundlePath], {
+      // To avoid printing the output of the worker to the console, we set
+      // `stdio` to `pipe` and handle the output ourselves.
+      stdio: 'pipe',
+    });
 
     let stdout = '';
     let stderr = '';

tsup.config.ts

@@ -1,6 +1,42 @@
 import { builtinModules } from 'module';
 import type { Options } from 'tsup';
 
+// `Plugin` is not exported from `tsup`, so we have to define it ourselves.
+type Plugin = Options['plugins'][number];
+
+const DIRNAME_SHIM = `import { fileURLToPath } from 'url'
+import path from 'path'
+
+const getFilename = () => fileURLToPath(import.meta.url)
+const getDirname = () => path.dirname(getFilename())
+
+export const __dirname = /* @__PURE__ */ getDirname()`;
+
+/**
+ * A `tsup` plugin that adds a `__dirname` shim to the beginning of each chunk
+ * that uses `__dirname`. This is necessary because `__dirname` is not available
+ * in ESM, so we have to use `import.meta.url` and `fileURLToPath` to get the
+ * dirname of the current file.
+ *
+ * Note: This breaks source maps in the files that use `__dirname`.
+ */
+const dirnameShimPlugin: Plugin = {
+  name: 'dirname-shim-plugin',
+
+  renderChunk(code, info) {
+    if (
+      info.type !== 'chunk' ||
+      this.format === 'cjs' ||
+      !code.includes('__dirname')
+    ) {
+      return undefined;
+    }
+
+    // Not sure how this impacts source maps
+    return { code: `${DIRNAME_SHIM}\n${code}`, map: info.map };
+  },
+};
+
 const config: Options = {
   // Clean the dist folder before bundling.
   clean: true,
@@ -32,6 +68,10 @@ const config: Options = {
   // the bundles will work in both Node.js and browsers.
   platform: 'neutral',
 
+  // The plugins to use when bundling. We add a plugin that adds a `__dirname`
+  // shim to the beginning of each chunk that uses `__dirname`.
+  plugins: [dirnameShimPlugin],
+
   // Hide unnecessary logs from the console. Warnings and errors will still be
   // shown.
   silent: true,
@@ -43,18 +83,6 @@ const config: Options = {
   // Split the output into chunks. This is useful for tree-shaking.
   // https://tsup.egoist.dev/#code-splitting
   splitting: true,
-
-  esbuildOptions: (options, { format }) => {
-    if (format === 'cjs') {
-      options.define = {
-        // Using `import.meta` in Node.js without ESM is a syntax error, so
-        // we replace `import.meta.url` with `__filename` in CommonJS bundles.
-        // This only works with the `getDirname` function in `snaps-utils`.
-        // eslint-disable-next-line @typescript-eslint/naming-convention
-        'import.meta.url': '__filename',
-      };
-    }
-  },
 };
 
 export default config;