next.js/packages/next/src/next-devtools/dev-overlay/utils/merge-refs.ts

import type * as React from 'react'

/**
 * A function that merges React refs into one.
 * Supports both functions and ref objects created using createRef() and useRef().
 *
 * Usage:
 * ```tsx
 * <div ref={mergeRefs(ref1, ref2, ref3)} />
 * ```
 *
 * @param inputRefs Array of refs
 * @returns Merged refs
 */
export default function mergeRefs<T>(
  ...inputRefs: (React.Ref<T> | undefined)[]
): React.Ref<T> | React.RefCallback<T> {
  const filteredInputRefs = inputRefs.filter(Boolean)

  if (filteredInputRefs.length <= 1) {
    const firstRef = filteredInputRefs[0]

    return firstRef || null
  }

  return function mergedRefs(ref) {
    for (const inputRef of filteredInputRefs) {
      if (typeof inputRef === 'function') {
        inputRef(ref)
      } else if (inputRef) {
        ;(inputRef as React.MutableRefObject<T | null>).current = ref
      }
    }
  }
}