Use exports for the default path when main is missing

fixtures/exports/lib/index.js

@@ -0,0 +1 @@
+export const a = 1

fixtures/exports/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "exports",
+  "private": true,
+  "type": "module",
+  "exports": {
+    ".": {
+      "import": "./lib/index.js"
+    }
+  },
+  "devDependencies": {
+    "@size-limit/file": ">= 0.0.0",
+    "size-limit": ">= 0.0.0"
+  },
+  "size-limit": [
+    {
+      "name": "index",
+      "limit": "1 kB"
+    }
+  ]
+}

packages/size-limit/get-config.js

@@ -99,6 +99,20 @@ function toAbsolute(file, cwd) {
   return isAbsolute(file) ? file : join(cwd, file)
 }
 
+// The package root file from `exports`: the field itself when it is a string,
+// otherwise its `.` entry, with conditions resolved in `import`, `default`,
+// `require` order
+function exportsEntry(exports) {
+  if (typeof exports === 'string') return exports
+  if (typeof exports !== 'object' || exports === null) return undefined
+  if ('.' in exports) return exportsEntry(exports['.'])
+  for (let condition of ['import', 'default', 'require']) {
+    let entry = exportsEntry(exports[condition])
+    if (entry) return entry
+  }
+  return undefined
+}
+
 async function findFiles(patterns, cwd) {
   // Node.js `glob` matches a bare directory as a single entry, while
   // `tinyglobby` expanded it to the files inside. Add a `/**` sibling for
@@ -205,7 +219,8 @@ export default async function getConfig(plugins, process, args, pkg) {
               require.resolve(join(dirname(pkg.path), pkg.packageJson.main))
             ]
           } else {
-            processed.files = [join(dirname(pkg.path), 'index.js')]
+            let entry = exportsEntry(pkg.packageJson.exports) || 'index.js'
+            processed.files = [join(dirname(pkg.path), entry)]
           }
         }
         return processed

packages/size-limit/test/get-config.test.js

@@ -111,6 +111,21 @@ it('uses index.js by default', async () => {
   })
 })
 
+it('uses the root entry of exports without main field', async () => {
+  expect(await check('exports')).toEqual({
+    checks: [
+      {
+        files: [fixture('exports', 'lib', 'index.js')],
+        limit: '1 kB',
+        name: 'index',
+        sizeLimit: 1000
+      }
+    ],
+    configPath: 'package.json',
+    cwd: fixture('exports')
+  })
+})
+
 it('overrides limit by CLI arg', async () => {
   expect(await check('simple', ['--limit', '10 kB'])).toEqual({
     checks: [