fix: improve null/undefined warning messages for useEffect, useInsertionEffect, and useLayoutEffect

[BalajiRKB]

Summary

The existing dev-mode console.warn messages in useEffect, useInsertionEffect, and useLayoutEffect (in packages/react/src/ReactHooks.js) are vague when the create callback is null or undefined.

Before (all three hooks had this):

React Hook useEffect requires an effect callback. Did you forget to pass a callback to the hook?

This message doesn't distinguish between null and undefined, and doesn't explain what the callback should look like, which is a common source of confusion for beginners.

After:

React Hook useEffect received a null effect callback. useEffect must be passed a function that optionally returns a cleanup function. Did you accidentally pass nothing or the result of a function call instead of the function itself?

Changes

• useEffect — improved warning to mention null or undefined explicitly and explain the expected signature
• useInsertionEffect — same improvement, consistent phrasing
• useLayoutEffect — same improvement, consistent phrasing

All three hooks now use template literals to dynamically include whether the value was null or undefined, making it easier for developers to quickly understand and fix the issue.

Test Plan

No new tests required — this is a dev-mode warning message improvement only. Existing tests for null callback behaviour remain unchanged.

Related

Addresses common confusion reported by beginners around vague hook error messages.

[meta-cla]

Hi @BalajiRKB!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!

[BalajiRKB]

This PR fixes the issue reported in #36362.

The vague warning messages for useEffect, useInsertionEffect, and useLayoutEffect when a null or undefined callback is passed have been improved to:

• Explicitly mention whether null or undefined was received
• Explain the expected function signature
• Stay consistent across all three hooks

[meta-cla]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!

[sohammeta]

"Did you accidentally pass nothing" phrasing is confusing when create === null, because the user did pass something — they passed null explicitly