add isESM helper

Author: lobsterkatie

packages/nextjs/src/utils/isESM.ts

@@ -0,0 +1,17 @@
+/**
+ * Determine if the given source code represents a file written using ES6 modules.
+ *
+ * The regexes used are from https://github.com/component/is-module, which got them from
+ * https://github.com/formatjs/js-module-formats, which says it got them from
+ * https://github.com/ModuleLoader/es-module-loader, though the originals are now nowhere to be found.
+ *
+ * @param moduleSource The source code of the module
+ * @returns True if the module contains ESM-patterned code, false otherwise.
+ */
+export function isESM(moduleSource: string): boolean {
+  const importExportRegex =
+    /(?:^\s*|[}{();,\n]\s*)(import\s+['"]|(import|module)\s+[^"'()\n;]+\s+from\s+['"]|export\s+(\*|\{|default|function|var|const|let|[_$a-zA-Z\xA0-\uFFFF][_$a-zA-Z0-9\xA0-\uFFFF]*))/;
+  const exportStarRegex = /(?:^\s*|[}{();,\n]\s*)(export\s*\*\s*from\s*(?:'([^']+)'|"([^"]+)"))/;
+
+  return importExportRegex.test(moduleSource) || exportStarRegex.test(moduleSource);
+}

packages/nextjs/test/utils/isESM.test.ts

@@ -0,0 +1,69 @@
+import { isESM } from '../../src/utils/isESM';
+
+// Based on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
+describe('import syntax', function () {
+  it('recognizes import syntax', function () {
+    expect(isESM("import dogs from 'dogs';")).toBe(true);
+    expect(isESM("import * as dogs from 'dogs';")).toBe(true);
+    expect(isESM("import { maisey } from 'dogs';")).toBe(true);
+    expect(isESM("import { charlie as goofball } from 'dogs';")).toBe(true);
+    expect(isESM("import { default as maisey } from 'dogs';")).toBe(true);
+    expect(isESM("import { charlie, masiey } from 'dogs';")).toBe(true);
+    expect(isESM("import { masiey, charlie as pickle } from 'dogs';")).toBe(true);
+    expect(isESM("import charlie, { maisey } from 'dogs';")).toBe(true);
+    expect(isESM("import maisey, * as dogs from 'dogs';")).toBe(true);
+    expect(isESM("import 'dogs';")).toBe(true);
+  });
+});
+
+// Based on https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Statements/export
+describe('export syntax', function () {
+  it('recognizes exported declarations', () => {
+    expect(isESM('export var maisey, charlie;')).toBe(true);
+    expect(isESM('export let charlie, maisey;')).toBe(true);
+    expect(isESM("export var maisey = 'silly', charlie = 'goofy';")).toBe(true);
+    expect(isESM("export let charlie = 'goofy', maisey = 'silly';")).toBe(true);
+    expect(isESM("export const maisey = 'silly', charlie = 'goofy';")).toBe(true);
+    expect(isESM('export function doDogStuff() { /* ... */ }')).toBe(true);
+    expect(isESM('export class Dog { /* ... */ }')).toBe(true);
+    expect(isESM('export function* generateWayTooManyPhotosOnMyPhone() { /* ... */ }')).toBe(true);
+    expect(isESM('export const { maisey, charlie } = dogObject;')).toBe(true);
+    expect(isESM('export const { charlie, masiey: maiseyTheDog } = dogObject;')).toBe(true);
+    expect(isESM('export const [ maisey, charlie ] = dogArray;')).toBe(true);
+  });
+
+  it('recognizes lists of exports', () => {
+    expect(isESM('export { maisey, charlie };')).toBe(true);
+    expect(isESM('export { charlie as charlieMcCharlerson, masiey as theMaiseyMaiseyDog };')).toBe(true);
+    expect(isESM('export { charlie as default  };')).toBe(true);
+  });
+
+  it('recognizes default exports', () => {
+    expect(isESM("export default 'dogs are great';")).toBe(true);
+    expect(isESM('export default function doDogStuff() { /* ... */ }')).toBe(true);
+    expect(isESM('export default class Dog { /* ... */ }')).toBe(true);
+    expect(isESM('export default function* generateWayTooManyPhotosOnMyPhone() { /* ... */ }')).toBe(true);
+    expect(isESM('export default function () { /* ... */ }')).toBe(true);
+    expect(isESM('export default class { /* ... */ }')).toBe(true);
+    expect(isESM('export default function* () { /* ... */ }')).toBe(true);
+  });
+
+  it('recognizes exports directly from another module', () => {
+    expect(isESM("export * from 'dogs';")).toBe(true);
+    expect(isESM("export * as dogs from 'dogs';")).toBe(true);
+    expect(isESM("export { maisey, charlie } from 'dogs';")).toBe(true);
+    expect(
+      isESM("export { maisey as goodGirl, charlie as omgWouldYouJustPeeAlreadyIWantToGoToBed } from 'dogs';"),
+    ).toBe(true);
+    expect(isESM("export { default } from 'dogs';")).toBe(true);
+    expect(isESM("export { default, maisey } from 'dogs';")).toBe(true);
+  });
+});
+
+describe('potential false positives', () => {
+  it("doesn't get fooled by look-alikes", () => {
+    expect(isESM("'this is an import statement'")).toBe(false);
+    expect(isESM("'this is an export statement'")).toBe(false);
+    expect(isESM('import(dogs)')).toBe(false);
+  });
+});