refactor to allow custom formatter plugins

features/custom_formatter.feature

@@ -120,6 +120,27 @@ Feature: custom formatter
       <duration-stat>
       """
 
+  Scenario: formatter plugins
+    Given a file named "simple_formatter.js" with:
+    """
+    module.exports = {
+      type: 'formatter',
+      formatter({ on, write }) {
+        on('message', (message) => {
+          if (message.testRunFinished) {
+            write('Test run finished!')
+          }
+        })
+      }
+    }
+    """
+    When I run cucumber-js with `--format ./simple_formatter.js`
+    Then it fails
+    And it outputs the text:
+    """
+    Test run finished!
+    """
+
   Scenario Outline: supported module formats
     Given a file named "features/step_definitions/cucumber_steps.js" with:
       """

src/formatter/builder.ts

@@ -1,7 +1,5 @@
-import path from 'node:path'
 import { EventEmitter } from 'node:events'
 import { Writable as WritableStream } from 'node:stream'
-import { pathToFileURL } from 'node:url'
 import { doesHaveValue, doesNotHaveValue } from '../value_checker'
 import { SupportCodeLibrary } from '../support_code_library_builder/types'
 import { SnippetInterface } from './step_definition_snippet_builder/snippet_syntax'
@@ -10,6 +8,7 @@ import StepDefinitionSnippetBuilder from './step_definition_snippet_builder'
 import JavascriptSnippetSyntax from './step_definition_snippet_builder/javascript_snippet_syntax'
 import getColorFns from './get_color_fns'
 import Formatters from './helpers/formatters'
+import { importCode } from './import_code'
 import Formatter, {
   FormatOptions,
   IFormatterCleanupFn,
@@ -105,14 +104,9 @@ const FormatterBuilder = {
     descriptor: string,
     cwd: string
   ) {
-    let normalized: URL | string = descriptor
-    if (descriptor.startsWith('.')) {
-      normalized = pathToFileURL(path.resolve(cwd, descriptor))
-    } else if (descriptor.startsWith('file://')) {
-      normalized = new URL(descriptor)
-    }
-    let CustomClass = await FormatterBuilder.loadFile(normalized)
-    CustomClass = FormatterBuilder.resolveConstructor(CustomClass)
+    const CustomClass = FormatterBuilder.resolveConstructor(
+      await importCode(descriptor, cwd)
+    )
     if (doesHaveValue(CustomClass)) {
       return CustomClass
     } else {

src/formatter/import_code.ts

@@ -0,0 +1,12 @@
+import { pathToFileURL } from 'node:url'
+import path from 'node:path'
+
+export async function importCode(specifier: string, cwd: string): Promise<any> {
+  let normalized: URL | string = specifier
+  if (specifier.startsWith('.')) {
+    normalized = pathToFileURL(path.resolve(cwd, specifier))
+  } else if (specifier.startsWith('file://')) {
+    normalized = new URL(specifier)
+  }
+  return await import(normalized.toString())
+}

src/formatter/resolve_class_or_plugin.ts

@@ -0,0 +1,28 @@
+import { doesNotHaveValue } from '../value_checker'
+
+export function resolveClassOrPlugin(imported: any) {
+  if (doesNotHaveValue(imported)) {
+    return null
+  }
+  if (typeof imported === 'function') {
+    return imported
+  } else if (typeof imported === 'object' && imported.type === 'formatter') {
+    return imported
+  } else if (
+    typeof imported === 'object' &&
+    typeof imported.default === 'function'
+  ) {
+    return imported.default
+  } else if (
+    typeof imported.default === 'object' &&
+    imported.default.type === 'formatter'
+  ) {
+    return imported.default
+  } else if (
+    typeof imported.default === 'object' &&
+    typeof imported.default.default === 'function'
+  ) {
+    return imported.default.default
+  }
+  return null
+}

src/formatter/resolve_implementation.ts

@@ -1,5 +1,6 @@
-import FormatterBuilder from './builder'
 import builtin from './builtin'
+import { importCode } from './import_code'
+import { resolveClassOrPlugin } from './resolve_class_or_plugin'
 import { FormatterImplementation } from './index'
 
 export async function resolveImplementation(
@@ -9,6 +10,13 @@ export async function resolveImplementation(
   if (builtin[specifier]) {
     return builtin[specifier]
   } else {
-    return await FormatterBuilder.loadCustomClass('formatter', specifier, cwd)
+    const imported = await importCode(specifier, cwd)
+    const resolved = resolveClassOrPlugin(imported)
+    if (!resolved) {
+      throw new Error(
+        `Custom formatter (${specifier}) does not export a function/class`
+      )
+    }
+    return resolved
   }
 }