feat: treat pseudo ESM as ESM with custom loader, disable custom Node Loader by default

packages/vite-node/src/server.ts

@@ -1,4 +1,4 @@
-import { join } from 'pathe'
+import { resolve } from 'pathe'
 import type { TransformResult, ViteDevServer } from 'vite'
 import createDebug from 'debug'
 import type { DebuggerOptions, FetchResult, RawSourceMap, ViteNodeResolveId, ViteNodeServerOptions } from './types'
@@ -66,7 +66,7 @@ export class ViteNodeServer {
 
   async resolveId(id: string, importer?: string): Promise<ViteNodeResolveId | null> {
     if (importer && !importer.startsWith(this.server.config.root))
-      importer = join(this.server.config.root, importer)
+      importer = resolve(this.server.config.root, importer)
     const mode = (importer && this.getTransformMode(importer)) || 'ssr'
     return this.server.pluginContainer.resolveId(id, importer, { ssr: mode === 'ssr' })
   }

packages/vitest/src/runtime/loader.ts

@@ -1,32 +1,94 @@
 import { pathToFileURL } from 'url'
-import { isNodeBuiltin } from 'mlly'
+import { readFile } from 'fs/promises'
+import { hasCJSSyntax, isNodeBuiltin } from 'mlly'
 import { normalizeModuleId } from 'vite-node/utils'
 import { getWorkerState } from '../utils'
-import type { Loader, Resolver } from '../types/loader'
+import type { Loader, ResolveResult, Resolver } from '../types/loader'
+import { ModuleFormat } from '../types/loader'
+
+// TODO fix in mlly (add "}" as a possible first character: "}export default")
+const ESM_RE = /([\s;}]|^)(import[\w,{}\s*]*from|import\s*['"*{]|export\b\s*(?:[*{]|default|class|type|function|const|var|let|async function)|import\.meta\b)/m
+function hasESMSyntax(code: string) {
+  return ESM_RE.test(code)
+}
+
+interface ContextCache {
+  isPseudoESM: boolean
+  source: string
+}
+
+const cache = new Map<string, ContextCache>()
+
+const getPotentialSource = async (filepath: string, result: ResolveResult) => {
+  if (!result.url.startsWith('file://') || result.format === 'module')
+    return null
+  let source = cache.get(result.url)?.source
+  if (source == null)
+    source = await readFile(filepath, 'utf8')
+  return source
+}
+
+const detectESM = (url: string, source: string | null) => {
+  const cached = cache.get(url)
+  if (cached)
+    return cached.isPseudoESM
+  if (!source)
+    return false
+  return (hasESMSyntax(source) && !hasCJSSyntax(source))
+}
 
 // apply transformations only to libraries
 // inline code preccessed by vite-node
+// make Node pseudo ESM
 export const resolve: Resolver = async (url, context, next) => {
   const { parentURL } = context
-  if (!parentURL || !parentURL.includes('node_modules') || isNodeBuiltin(url))
+  const state = getWorkerState()
+  const resolver = state?.rpc.resolveId
+
+  if (!parentURL || isNodeBuiltin(url) || !resolver)
     return next(url, context, next)
 
   const id = normalizeModuleId(url)
   const importer = normalizeModuleId(parentURL)
-  const state = getWorkerState()
-  const resolver = state?.rpc.resolveId
-  if (resolver) {
-    const resolved = await resolver(id, importer)
-    if (resolved) {
-      return {
-        url: pathToFileURL(resolved.id).toString(),
-        shortCircuit: true,
-      }
+  const resolved = await resolver(id, importer)
+
+  let result: ResolveResult
+  let filepath: string
+  if (resolved) {
+    const resolvedUrl = pathToFileURL(resolved.id).toString()
+    filepath = resolved.id
+    result = {
+      url: resolvedUrl,
+      shortCircuit: true,
     }
   }
-  return next(url, context, next)
+  else {
+    const { url: resolvedUrl, format } = await next(url, context, next)
+    filepath = new URL(resolvedUrl).pathname
+    result = {
+      url: resolvedUrl,
+      format,
+      shortCircuit: true,
+    }
+  }
+
+  const source = await getPotentialSource(filepath, result)
+  const isPseudoESM = detectESM(result.url, source)
+  if (typeof source === 'string')
+    cache.set(result.url, { isPseudoESM: isPseudoESM || false, source })
+  if (isPseudoESM)
+    result.format = ModuleFormat.Module
+  return result
 }
 
-export const load: Loader = (url, context, next) => {
-  return next(url, context, next)
+export const load: Loader = async (url, context, next) => {
+  const result = await next(url, context, next)
+  const cached = cache.get(url)
+  if (cached?.isPseudoESM && result.format !== 'module') {
+    return {
+      source: cached.source,
+      format: ModuleFormat.Module,
+    }
+  }
+  return result
 }

packages/vitest/src/types/loader.ts

@@ -1,34 +1,36 @@
 import type { Awaitable } from './general'
 
-interface ModuleContext {
+interface ModuleContext extends Record<string, unknown> {
   conditions: string[]
   parentURL?: string
 }
 
-enum ModuleFormat {
+export enum ModuleFormat {
   Builtin = 'builtin',
   Commonjs = 'commonjs',
   Json = 'json',
   Module = 'module',
   Wasm = 'wasm',
 }
 
-interface ResolveResult {
+export interface ResolveResult {
   url: string
+  shortCircuit?: boolean
   format?: ModuleFormat
 }
 
 export interface Resolver {
   (url: string, context: ModuleContext, next: Resolver): Awaitable<ResolveResult>
 }
 
-interface LoaderContext {
+interface LoaderContext extends Record<string, any> {
   format: ModuleFormat
   importAssertions: Record<string, string>
 }
 
 interface LoaderResult {
   format: ModuleFormat
+  shortCircuit?: boolean
   source: string | ArrayBuffer | SharedArrayBuffer | Uint8Array
 }
 

test/esm/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "@vitest/test-esm",
+  "private": true,
+  "scripts": {
+    "test": "vitest",
+    "coverage": "vitest run --coverage"
+  },
+  "dependencies": {
+    "css-what": "6.1.0",
+    "tslib": "2.4.0"
+  },
+  "devDependencies": {
+    "vitest": "workspace:*"
+  }
+}

test/esm/test/executes.spec.ts

@@ -0,0 +1,8 @@
+import { __assign } from 'tslib'
+import { parse } from 'css-what'
+import { expect, test } from 'vitest'
+
+test('imported libs have incorrect ESM, but still work', () => {
+  expect(__assign({}, { a: 1 })).toEqual({ a: 1 })
+  expect(parse('a')).toBeDefined()
+})

test/esm/vite.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    deps: {
+      external: [/tslib/, /css-what/],
+    },
+  },
+})