fix(exports): use correct indentation for output (#599)

Co-authored-by: Kevin Deng <sxzz@sxzz.moe>

Author: msigwart

src/features/exports.test.ts

@@ -1,7 +1,7 @@
 import path from 'node:path'
 import process from 'node:process'
 import { describe, test } from 'vitest'
-import { generateExports } from './exports.ts'
+import { detectIndent, generateExports } from './exports.ts'
 import type { OutputChunk } from 'rolldown'
 
 const cwd = process.cwd()
@@ -402,6 +402,29 @@ describe.concurrent('generateExports', () => {
   })
 })
 
+describe('detectIndent', () => {
+  test('two spaces', ({ expect }) => {
+    expect(detectIndent(stringifyJson(2))).toBe(2)
+  })
+  test('four spaces', ({ expect }) => {
+    expect(detectIndent(stringifyJson(4))).toBe(4)
+  })
+  test('tab', ({ expect }) => {
+    expect(detectIndent(stringifyJson('\t'))).toBe('\t')
+  })
+  test('empty', ({ expect }) => {
+    expect(detectIndent('')).toBe(2)
+  })
+  test('empty line', ({ expect }) => {
+    expect(detectIndent('{\n\n  "foo": 42 }')).toBe(2)
+  })
+})
+
+function stringifyJson(indentation: string | number): string {
+  const contents = JSON.stringify({ foo: 42 }, null, indentation)
+  return contents
+}
+
 function genChunk(fileName: string) {
   // eslint-disable-next-line @typescript-eslint/consistent-type-assertions
   return {

src/features/exports.ts

@@ -65,17 +65,29 @@ export async function writeExports(
   }
 
   const original = await readFile(pkg.packageJsonPath, 'utf8')
-  let contents = JSON.stringify(
-    updatedPkg,
-    null,
-    original.includes('\t') ? '\t' : 2,
-  )
+  let contents = JSON.stringify(updatedPkg, null, detectIndent(original))
   if (original.endsWith('\n')) contents += '\n'
   if (contents !== original) {
     await writeFile(pkg.packageJsonPath, contents, 'utf8')
   }
 }
 
+export function detectIndent(jsonText: string): string | number {
+  const lines = jsonText.split(/\r?\n/)
+
+  for (const line of lines) {
+    const match = line.match(/^(\s+)\S/)
+    if (!match) continue
+
+    if (match[1].includes('\t')) {
+      return '\t'
+    }
+    return match[1].length
+  }
+
+  return 2
+}
+
 type SubExport = Partial<Record<'cjs' | 'es' | 'src', string>>
 
 export async function generateExports(