feature: Implement custom JS helper import

src/domain/helpers.js

@@ -0,0 +1,11 @@
+const HELPER_FILE_PATH = 'helpers.mjs'
+
+export const importCustomHelpers = (withSrcDir, ifPathExists) => (
+  withSrcDir((prefixWithSrcDir) => (
+    ifPathExists(
+      prefixWithSrcDir(HELPER_FILE_PATH),
+      (filePath) => import(filePath),
+      {},
+    )
+  ))
+)

tests/domain/helpers.test.js

@@ -0,0 +1,32 @@
+import { describe, it } from 'node:test'
+import assert from 'node:assert'
+import { withTempDir } from '#tests/helpers'
+import * as fs from 'node:fs/promises'
+import { join } from 'node:path'
+import { withDir, ifPathExists } from '#src/utils/fs'
+import { importCustomHelpers } from '#src/domain/helpers'
+
+describe('#src/domain/helpers', () => {
+  describe('importCustomHelpers()', () => {
+    it('parses exports from a `helpers.js` file', async () => {
+      await withTempDir(async (prefixWithTempDir) => {
+        const srcDirPath = prefixWithTempDir('src')
+        const filePath = join(srcDirPath, 'helpers.mjs')
+
+        await fs.mkdir(srcDirPath)
+        await fs.writeFile(filePath, `
+export const sayHello = () => 'Hello World!'
+
+export const add2 = (num) => num + 2
+`)
+
+        const helpers = await importCustomHelpers(withDir(srcDirPath), ifPathExists)
+
+        assert('sayHello' in helpers)
+        assert('add2' in helpers)
+        assert.strictEqual(helpers.sayHello(), 'Hello World!')
+        assert.strictEqual(helpers.add2(4), 6)
+      })
+    })
+  })
+})