[flag] warn for setState in render on initial mount

[rickhanlonii]

Overview

Add a feature flag disableSetStateInRenderOnMount to start experimenting with warning for setState in initial render.

Motivation

The main reason to start warning about this now is that supporting this prevents us from landing SSR optimizations such as inlining hooks and removing code to support updates during SSR. But this also indicates a performance issue in the app that can usually be easily fixed.

Pattern isn't useful

Usually, calling setState in initial render of a component is an accident, by forgetting to pass the initial value into useState():

functionComponent({value}){const[prev,setPrev]=useState();// 🚩 oops!if(prev!==value){setPrev(value);}}

For those use cases, the warning will fire and applying an easy fix to pass the inital state will give performance benefits since we won't need to immediately re-render a component on mount.

functionComponent({value}){const[prev,setPrev]=useState(value);// 😎if(prev!==value){setPrev(value);}}

Now this will setState in render on updates.

Future SSR improvements

If this pattern is fixed, we could compile the SSR bundle code from:

functionComponent({value}){const[prev,setPrev]=useState(value);if(prev!==value){setPrev(value);}constderivedValue=useMemo(()=>{returnvalue.filter(Boolean);},value);//...}

To:

functionComponent({value}){const[prev,setPrev]=[value,()=>{}];constderivedValue=value.filter(Boolean);//...}

This could be a huge speedup in SSR performance.

Experiment

To confirm how many cases fall into the easy fix, I'm rolling out a feature flag to test it in a large app to see what use cases are depending on this, and how difficult the fixes are.

Review

It's a pain to update the tests for Fiber and Fizz in different PRs because of the server integration tests.

Here's how the different warnings are implemented in separate commits:

• Fiber warning commit4d1f6b1
• Fizz warning commit8b35055

[react-sizebot]

Comparing: 4842fbe...7820ad0

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	611.08 kB	611.08 kB	=	108.02 kB	108.02 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	677.02 kB	677.02 kB	=	118.99 kB	118.99 kB
facebook-www/ReactDOM-prod.classic.js	=	697.74 kB	697.74 kB	=	122.68 kB	122.68 kB
facebook-www/ReactDOM-prod.modern.js	=	688.05 kB	688.05 kB	=	121.07 kB	121.07 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
test_utils/ReactAllWarnings.js	+0.35%	67.70 kB	67.94 kB	+0.21%	17.05 kB	17.09 kB

Generated by 🚫 dangerJS against 7820ad0

[gaearon]

Maybe “during the initial render”? Set state “on mount” usually refers to doing it in an effect which isn’t what this seems to be about.

Also, “this behavior is deprecated” maybe could be “this is deprecated, pass the initial value to useState instead” or something like that so it’s clear how to fix it.

[rickhanlonii]

Thanks @gaearon, message updated.