useCombinedRefs - Old and new

use-combined-refs-new.ts

/**
 * A combined ref implementation using the callback ref cleanups feature.
 * This will work in React 19.
 */

import { Ref, useCallback } from 'react';

type OptionalRef<T> = Ref<T> | undefined;

type Cleanup = (() => void) | undefined | void;

function setRef<T>(ref: OptionalRef<T>, value: T): Cleanup {
  if (typeof ref === 'function') {
    const cleanup = ref(value);

    if (typeof cleanup === 'function') {
      return cleanup;
    }
    return () => ref(null);
  } else if (ref) {
    ref.current = value;
    return () => (ref.current = null);
  }
}

export function useCombinedRefs<T>(...refs: OptionalRef<T>[]) {
  // biome-ignore lint/correctness/useExhaustiveDependencies: The hook already lists all dependencies
  return useCallback((value: T | null) => {
    const cleanups: Cleanup[] = [];

    for (const ref of refs) {
      const cleanup = setRef(ref, value);
      cleanups.push(cleanup);
    }

    return () => {
      for (const cleanup of cleanups) {
        cleanup?.();
      }
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, refs);
}

use-combined-refs-old.ts

/**
 * A combined ref implementation that will work in React 18
 */

import { ForwardedRef, useCallback, useRef } from 'react';

type OptionalRef<T> = ForwardedRef<T> | undefined;

function setRef<T>(ref: OptionalRef<T>, value: T) {
  if (typeof ref === 'function') {
    ref(value);
  } else if (ref) {
    ref.current = value;
  }
}

export function useCombinedRefs<T>(...refs: OptionalRef<T>[]) {
  const previousRefs = useRef<OptionalRef<T>[]>([]);

  return useCallback((value: T | null) => {
    let index = 0;
    for (; index < refs.length; index++) {
      const ref = refs[index];
      const prev = previousRefs.current[index];

      // eslint-disable-next-line eqeqeq
      if (prev != ref) setRef(prev, null);
      setRef(ref, value);
    }

    for (; index < previousRefs.current.length; index++) {
      const prev = previousRefs.current[index];
      setRef(prev, null);
    }

    previousRefs.current = refs;

    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, refs);
}

I first assumed the "new" one supported old and new React, but this is not the case. Still useful for app developers but if you're building a library where the React version can be anything a more solid implementation is something like https://github.com/vercel/next.js/blob/canary/packages/next/src/client/use-merged-ref.ts

/*! The MIT License (MIT) Copyright (c) 2025 Vercel, Inc. */
import { useCallback, useRef, type Ref } from 'react'

// This is a compatibility hook to support React 18 and 19 refs.
// In 19, a cleanup function from refs may be returned.
// In 18, returning a cleanup function creates a warning.
// Since we take userspace refs, we don't know ahead of time if a cleanup function will be returned.
// This implements cleanup functions with the old behavior in 18.
// We know refs are always called alternating with `null` and then `T`.
// So a call with `null` means we need to call the previous cleanup functions.
export function useMergedRef<TElement>(
  refA: Ref<TElement>,
  refB: Ref<TElement>
): Ref<TElement> {
  const cleanupA = useRef<(() => void) | null>(null)
  const cleanupB = useRef<(() => void) | null>(null)

  // NOTE: In theory, we could skip the wrapping if only one of the refs is non-null.
  // (this happens often if the user doesn't pass a ref to Link/Form/Image)
  // But this can cause us to leak a cleanup-ref into user code (e.g. via `<Link legacyBehavior>`),
  // and the user might pass that ref into ref-merging library that doesn't support cleanup refs
  // (because it hasn't been updated for React 19)
  // which can then cause things to blow up, because a cleanup-returning ref gets called with `null`.
  // So in practice, it's safer to be defensive and always wrap the ref, even on React 19.
  return useCallback(
    (current: TElement | null): void => {
      if (current === null) {
        const cleanupFnA = cleanupA.current
        if (cleanupFnA) {
          cleanupA.current = null
          cleanupFnA()
        }
        const cleanupFnB = cleanupB.current
        if (cleanupFnB) {
          cleanupB.current = null
          cleanupFnB()
        }
      } else {
        if (refA) {
          cleanupA.current = applyRef(refA, current)
        }
        if (refB) {
          cleanupB.current = applyRef(refB, current)
        }
      }
    },
    [refA, refB]
  )
}

function applyRef<TElement>(
  refA: NonNullable<Ref<TElement>>,
  current: TElement
) {
  if (typeof refA === 'function') {
    const cleanup = refA(current)
    if (typeof cleanup === 'function') {
      return cleanup
    } else {
      return () => refA(null)
    }
  } else {
    refA.current = current
    return () => {
      refA.current = null
    }
  }
}

@luwes thanks for the heads-up. Indeed, my implementation isn't backwards compatible. For a backwards compatible solution which also supports more-than-two refs please check this hook in the Radix UI package.