fix: infer TS module format from package.json (#1580)

* fix: infer TS module format from `package.json`

* chore: typo

* chore: update comments

src/runtimes/node/bundlers/nft/index.ts

@@ -10,9 +10,10 @@ import { cachedReadFile, getPathWithExtension } from '../../../../utils/fs.js'
 import { minimatch } from '../../../../utils/matching.js'
 import { getBasePath } from '../../utils/base_path.js'
 import { filterExcludedPaths, getPathsOfIncludedFiles } from '../../utils/included_files.js'
-import { MODULE_FORMAT, MODULE_FILE_EXTENSION, tsExtensions } from '../../utils/module_format.js'
+import { MODULE_FORMAT, MODULE_FILE_EXTENSION, tsExtensions, ModuleFormat } from '../../utils/module_format.js'
 import { getNodeSupportMatrix } from '../../utils/node_version.js'
-import { getModuleFormat as getTSModuleFormat } from '../../utils/tsconfig.js'
+import { getClosestPackageJson } from '../../utils/package_json.js'
+import { getModuleFormat as getModuleFormatFromTsConfig } from '../../utils/tsconfig.js'
 import type { GetSrcFilesFunction, BundleFunction } from '../types.js'
 
 import { processESM } from './es_modules.js'
@@ -85,6 +86,33 @@ const getIgnoreFunction = (config: FunctionConfig) => {
   }
 }
 
+/**
+ * Returns the module format that should be used when transpiling a TypeScript
+ * file.
+ */
+const getTSModuleFormat = async (mainFile: string, repositoryRoot?: string): Promise<ModuleFormat> => {
+  const fromTsConfig = getModuleFormatFromTsConfig(mainFile, repositoryRoot)
+
+  // If we can infer the module type from a `tsconfig.json` file, use that.
+  if (fromTsConfig !== undefined) {
+    return fromTsConfig
+  }
+
+  // At this point, we need to infer the module type from the `type` field in
+  // the closest `package.json`.
+  try {
+    const packageJSON = await getClosestPackageJson(dirname(mainFile), repositoryRoot)
+
+    if (packageJSON?.contents.type === 'module') {
+      return MODULE_FORMAT.ESM
+    }
+  } catch {
+    // no-op
+  }
+
+  return MODULE_FORMAT.COMMONJS
+}
+
 const traceFilesAndTranspile = async function ({
   basePath,
   cache,
@@ -107,7 +135,7 @@ const traceFilesAndTranspile = async function ({
   runtimeAPIVersion: number
 }) {
   const isTypeScript = tsExtensions.has(extname(mainFile))
-  const tsFormat = isTypeScript ? getTSModuleFormat(mainFile, repositoryRoot) : MODULE_FORMAT.COMMONJS
+  const tsFormat = isTypeScript ? await getTSModuleFormat(mainFile, repositoryRoot) : MODULE_FORMAT.COMMONJS
   const tsAliases = new Map<string, string>()
   const tsRewrites = new Map<string, string>()
 

src/runtimes/node/utils/tsconfig.ts

@@ -6,22 +6,29 @@ import { MODULE_FORMAT } from './module_format.js'
 
 const esmModuleValues = new Set(['es6', 'es2015', 'es2020', 'es2022', 'esnext', 'node16', 'nodenext'])
 
-// Returns the module format that should be used for a TypeScript file at a
-// given path, by reading the associated `tsconfig.json` file if it exists.
+/**
+ * Looks for a `tsconfig.json` file on a given path and, if one exists, returns
+ * the module format inferred from the `module` property. If no file is found
+ * or if no `module` property is defined, the function returns `undefined`.
+ */
 export const getModuleFormat = (path: string, boundary?: string) => {
   const file = getTsconfig(path)
 
   if (!file) {
-    return MODULE_FORMAT.COMMONJS
+    return
   }
 
   // If there is a boundary defined and the file we found is outside of it,
   // discard the file.
   if (boundary !== undefined && relative(boundary, dirname(file.path)).startsWith('..')) {
-    return MODULE_FORMAT.COMMONJS
+    return
   }
 
-  const moduleProp = file.config.compilerOptions?.module?.toLowerCase() ?? ''
+  const moduleProp = file.config.compilerOptions?.module
+
+  if (!moduleProp) {
+    return
+  }
 
   return esmModuleValues.has(moduleProp) ? MODULE_FORMAT.ESM : MODULE_FORMAT.COMMONJS
 }