fix(react): fallback to default export when variant named export not …

[rikaweb]

PR: fix(react): fallback to default export when variant named export not found

Checkboxes to select

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added
• Manual Test/Other (Please elaborate)
• Bug fix (non-breaking change which fixes an issue)

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Description / Motivation

When using SXA rendering variants in Next.js App Router with XM Cloud, components with params.FieldNames set (e.g., HeroST (Right)) fail to render with error:

Placeholder headless-main contains unknown component HeroST (Right). 
Ensure that a React component exists for it, and that it is registered in your component-map file.

Root Cause

In packages/react/src/components/Placeholder/placeholder-utils.tsx, the component resolution logic looks for a named export matching params.FieldNames (e.g., component.Right). If not found, it throws an error instead of falling back to the default export.

Before:

const renderedComponent =
  exportName && exportName !== DEFAULT_EXPORT_NAME
    ? ((component as ReactModule)[exportName] as ComponentType)  // ❌ No fallback
    : (component as ReactModule).default || ...

Solution

Gracefully fallback to the default export when a named variant export is not found:

const variantComponent =
  exportName && exportName !== DEFAULT_EXPORT_NAME
    ? ((component as ReactModule)[exportName] as ComponentType)
    : undefined;

const defaultComponent =
  (component as ReactModule).default ||
  (component as ReactModule).Default ||
  (component as ComponentType);

// Use variant if found, otherwise fallback to default export
const renderedComponent = variantComponent || defaultComponent;

This allows components to:

1. Optionally provide named exports for specific variants (e.g., export const Right = ...)
2. Handle all variants via params.FieldNames prop in the default export

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Testing Details

• Tested with HeroST component and HeroST (Right) variant in XM Cloud
• Component now renders correctly using default export
• Component receives params.FieldNames prop to handle variant logic
• Backwards compatible: existing named exports still work as before