Slot.tsx

import * as React from 'react';
import { composeRefs } from '@radix-ui/react-compose-refs';

/* -------------------------------------------------------------------------------------------------
 * Slot
 * -----------------------------------------------------------------------------------------------*/

interface SlotProps extends React.HTMLAttributes<HTMLElement> {
  children?: React.ReactNode;
}

const Slot = React.forwardRef<HTMLElement, SlotProps>((props, forwardedRef) => {
  const { children, ...slotProps } = props;
  const childrenArray = React.Children.toArray(children);
  const slottable = childrenArray.find(isSlottable);

  if (slottable) {
    // the new element to render is the one passed as a child of `Slottable`
    const newElement = slottable.props.children as React.ReactNode;

    const newChildren = childrenArray.map((child) => {
      if (child === slottable) {
        // because the new element will be the one rendered, we are only interested
        // in grabbing its children (`newElement.props.children`)
        if (React.Children.count(newElement) > 1) return React.Children.only(null);
        return React.isValidElement(newElement)
          ? (newElement.props.children as React.ReactNode)
          : null;
      } else {
        return child;
      }
    });

    return (
      <SlotClone {...slotProps} ref={forwardedRef}>
        {React.isValidElement(newElement)
          ? React.cloneElement(newElement, undefined, newChildren)
          : null}
      </SlotClone>
    );
  }

  return (
    <SlotClone {...slotProps} ref={forwardedRef}>
      {children}
    </SlotClone>
  );
});

Slot.displayName = 'Slot';

/* -------------------------------------------------------------------------------------------------
 * SlotClone
 * -----------------------------------------------------------------------------------------------*/

interface SlotCloneProps {
  children: React.ReactNode;
}

const SlotClone = React.forwardRef<any, SlotCloneProps>((props, forwardedRef) => {
  const { children, ...slotProps } = props;

  if (React.isValidElement(children)) {
    return React.cloneElement(children, {
      ...mergeProps(slotProps, children.props),
      ref: forwardedRef ? composeRefs(forwardedRef, (children as any).ref) : (children as any).ref,
    });
  }

  return React.Children.count(children) > 1 ? React.Children.only(null) : null;
});

SlotClone.displayName = 'SlotClone';

/* -------------------------------------------------------------------------------------------------
 * Slottable
 * -----------------------------------------------------------------------------------------------*/

const Slottable = ({ children }: { children: React.ReactNode }) => {
  return <>{children}</>;
};

/* ---------------------------------------------------------------------------------------------- */

type AnyProps = Record<string, any>;

function isSlottable(child: React.ReactNode): child is React.ReactElement {
  return React.isValidElement(child) && child.type === Slottable;
}

function mergeProps(slotProps: AnyProps, childProps: AnyProps) {
  // all child props should override
  const overrideProps = { ...childProps };

  for (const propName in childProps) {
    const slotPropValue = slotProps[propName];
    const childPropValue = childProps[propName];

    const isHandler = /^on[A-Z]/.test(propName);
    if (isHandler) {
      // if the handler exists on both, we compose them
      if (slotPropValue && childPropValue) {
        overrideProps[propName] = (...args: unknown[]) => {
          childPropValue(...args);
          slotPropValue(...args);
        };
      }
      // but if it exists only on the slot, we use only this one
      else if (slotPropValue) {
        overrideProps[propName] = slotPropValue;
      }
    }
    // if it's `style`, we merge them
    else if (propName === 'style') {
      overrideProps[propName] = { ...slotPropValue, ...childPropValue };
    } else if (propName === 'className') {
      overrideProps[propName] = [slotPropValue, childPropValue].filter(Boolean).join(' ');
    }
  }

  return { ...slotProps, ...overrideProps };
}

const Root = Slot;

export {
  Slot,
  Slottable,
  //
  Root,
};
export type { SlotProps };