Merge pull request #26514 from storybookjs/kasper/fix-single-loaders-…

…decorators

Bug: Make sure loaders/decorators can be used as array
(cherry picked from commit b016a4f)

code/lib/preview-api/src/modules/store/csf/composeConfigs.test.ts

@@ -176,6 +176,34 @@ describe('composeConfigs', () => {
     });
   });
 
+  it('allows single array to be written without array', () => {
+    expect(
+      composeConfigs([
+        {
+          argsEnhancers: ['1', '2'],
+          argTypesEnhancers: ['1', '2'],
+          loaders: '1',
+        },
+        {
+          argsEnhancers: '3',
+          argTypesEnhancers: '3',
+          loaders: ['2', '3'],
+        },
+      ])
+    ).toEqual({
+      parameters: {},
+      decorators: [],
+      args: {},
+      argsEnhancers: ['1', '2', '3'],
+      argTypes: {},
+      argTypesEnhancers: ['1', '2', '3'],
+      globals: {},
+      globalTypes: {},
+      loaders: ['1', '2', '3'],
+      runStep: expect.any(Function),
+    });
+  });
+
   it('combines decorators in reverse file order', () => {
     expect(
       composeConfigs([

code/lib/preview-api/src/modules/store/csf/composeConfigs.ts

@@ -3,6 +3,7 @@ import { global } from '@storybook/global';
 
 import { combineParameters } from '../parameters';
 import { composeStepRunners } from './stepRunners';
+import { normalizeArrays } from './normalizeArrays';
 
 export function getField<TFieldType = any>(
   moduleExportList: ModuleExports[],
@@ -16,10 +17,10 @@ export function getArrayField<TFieldType = any>(
   field: string,
   options: { reverseFileOrder?: boolean } = {}
 ): TFieldType[] {
-  return getField(moduleExportList, field).reduce(
-    (a: any, b: any) => (options.reverseFileOrder ? [...b, ...a] : [...a, ...b]),
-    []
-  );
+  return getField(moduleExportList, field).reduce((prev: any, cur: any) => {
+    const normalized = normalizeArrays(cur);
+    return options.reverseFileOrder ? [...normalized, ...prev] : [...prev, ...normalized];
+  }, []);
 }
 
 export function getObjectField<TFieldType = Record<string, any>>(