feat: detect esm/cjs syntax

src/index.ts

@@ -2,4 +2,5 @@ export * from './analyze'
 export * from './cjs'
 export * from './eval'
 export * from './resolve'
+export * from './syntax'
 export * from './utils'

src/syntax.ts

@@ -0,0 +1,21 @@
+const ESM_RE = /([\s;]|^)(import[\s'"*{]|export\b|import\.meta\b)/m
+
+export function hasESMSyntax (code: string): boolean {
+  return ESM_RE.test(code)
+}
+
+const CJS_RE = /([\s;]|^)(module.exports\b|exports\.|require\s*\(|global\b)/m
+export function hasCJSSyntax (code: string): boolean {
+  return CJS_RE.test(code)
+}
+
+export function detectSyntax (code: string) {
+  const hasESM = hasESMSyntax(code)
+  const hasCJS = hasCJSSyntax(code)
+
+  return {
+    hasESM,
+    hasCJS,
+    isMixed: hasESM && hasCJS
+  }
+}

test/syntax.test.mjs

@@ -0,0 +1,37 @@
+import { expect } from 'chai'
+import { detectSyntax } from 'mlly'
+
+const staticTests = {
+  // ESM
+  // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#syntax
+  'import defaultExport from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import * as name from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import { export1 } from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import { export1 as alias1 } from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import { export1, export2 } from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import { export1, export2 as alias2, export3 } from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import defaultExport, { export1, export2 } from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import defaultExport, * as name from"module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import"module-name"': { hasESM: true, hasCJS: false, isMixed: false },
+  //
+  'import defaultMember from "module-name";': { hasESM: true, hasCJS: false, isMixed: false },
+  'import "./file.mjs"': { hasESM: true, hasCJS: false, isMixed: false },
+  'export default b=""': { hasESM: true, hasCJS: false, isMixed: false },
+  // CJS
+  'exports.c={}': { hasESM: false, hasCJS: true, isMixed: false },
+  'const b=true;module.exports={b};': { hasESM: false, hasCJS: true, isMixed: false },
+  // Mixed
+  'import"module-name";module.exports={};': { hasESM: true, hasCJS: true, isMixed: true },
+  // No clues
+  'import("./file.mjs")': { hasESM: false, hasCJS: false, isMixed: false },
+  'console.log(process.version)': { hasESM: false, hasCJS: false, isMixed: false },
+  'const a={};': { hasESM: false, hasCJS: false, isMixed: false }
+}
+
+describe('detectSyntax', () => {
+  for (const [input, result] of Object.entries(staticTests)) {
+    it(input, () => {
+      expect(detectSyntax(input)).to.deep.equal(result)
+    })
+  }
+})