feat: enforce to use function declaration on top-level

packages/eslint-plugin-antfu/src/rules/top-level-function.test.ts

@@ -0,0 +1,51 @@
+import { RuleTester } from '@typescript-eslint/utils/dist/ts-eslint'
+import { it } from 'vitest'
+import rule, { RULE_NAME } from './top-level-function'
+
+const valids = [
+  'function foo() {}',
+  // allow arrow function inside function
+  'function foo() { const bar = () => {} }',
+  // allow arrow function when type is specified
+  'const Foo: Bar = () => {}',
+  // allow let/var
+  'let foo = () => {}',
+  // allow arrow function in as
+  'const foo = (() => {}) as any',
+  // allow iife
+  ';(() => {})()',
+]
+
+const invalids = [
+  [
+    'const foo = (as: string, bar: number) => { return as + bar }',
+    'function foo (as: string, bar: number) { return as + bar }',
+  ],
+  [
+    'const foo = <K, T extends Boolean>(as: string, bar: number): Omit<T, K> => as + bar',
+    'function foo <K, T extends Boolean>(as: string, bar: number): Omit<T, K> { return as + bar }',
+  ],
+  [
+    'export const foo = () => {}',
+    'export function foo () {}',
+  ],
+  [
+    'export const foo = () => ({})',
+    'export function foo () { return {} }',
+  ],
+]
+
+it('runs', () => {
+  const ruleTester: RuleTester = new RuleTester({
+    parser: require.resolve('@typescript-eslint/parser'),
+  })
+
+  ruleTester.run(RULE_NAME, rule, {
+    valid: valids,
+    invalid: invalids.map(i => ({
+      code: i[0],
+      output: i[1],
+      errors: [{ messageId: 'topLevelFunctionDeclaration' }],
+    })),
+  })
+})