[Compiler Bug]: Eagerly dereferencing a property from an object that may be undefined

[gaearon]

What kind of issue is this?

• React Compiler core (the JS output is incorrect, or your app works incorrectly after optimization)
• babel-plugin-react-compiler (build issue installing or using the Babel plugin)
• eslint-plugin-react-compiler (build issue installing or using the eslint plugin)
• react-compiler-healthcheck (build issue installing or using the healthcheck script)

Link to repro

https://playground.react.dev/#N4Igzg9grgTgxgUxALhAgHgBwjALgAgDMoA7OXASwhPwGEIBbbEhE3ACgEp9gAdG-HGpgCAbTgwImADT4wCXLUmYAuvgC8+AEoIAhuQB0UeQGVcu3Ai798g4QQpgAok1wBPDfgCEEqfxt2JCL41ABiEBCeXBoAfDwBthSE+Oxeji6Y7tx8Ara2hBHsvpgG6JwJ+AC+AdUkATAKsDTsFQA8ACYUAG4hJLQANhRwANbqwGERlfgA9DEB5SS1IJVAA

Repro steps

export function Component() {
  const [crop, setCrop] = React.useState()
  const isEmpty = !crop

  const onFoo = () => {
    if (!isEmpty) {
      foo(crop.x)
    }
  }

  return (
    <div onClick={onFoo} />
  )
}

We end up with

function Component() {
  const $ = _c(5);
  const [crop] = React.useState();
  const isEmpty = !crop;
  let t0;
  if ($[0] !== isEmpty || $[1] !== crop.x) {
    t0 = () => {
      if (!isEmpty) {
        foo(crop.x);
      }
    };
    $[0] = isEmpty;
    $[1] = crop.x;
    $[2] = t0;
  } else {
    t0 = $[2];
  }
  const onFoo = t0;
  let t1;
  if ($[3] !== onFoo) {
    t1 = <div onClick={onFoo} />;
    $[3] = onFoo;
    $[4] = t1;
  } else {
    t1 = $[4];
  }
  return t1;
}

The original code isn't accessing crop.x when isEmpty is true, but the generated code does. So it fails with runtime crash. Turning off the compiler fixes it.

How often does this bug happen?

Every time

What version of React are you using?

18

What version of React Compiler are you using?

19.0.0-beta-6fc168f-20241025

[josephsavona]

Thanks for posting. We’ve seen this previously and @mofeiZ has been working to improve the way we calculate dependencies to get even more precise while also handling cases like this.

You can see a preview of the fix if you add // @enablePropagateDepsInHIR as the first line in playground. We’ll enable this in an updated beta release once we’ve verified and finished the fix.

[gaearon]

Ahh fantastic, thank you

[aalmanasir]

How can I use it

[josephsavona]

The fix is still in development, we’ll enable once we’ve fully vetted the fix.

For now you can opt out of compilation for specific components with https://react.dev/learn/react-compiler#something-is-not-working-after-compilation

[poteto]

This should now be fixed by #31199 (thanks to @mofeiZ!), and included in the latest beta releases of the compiler. Please let us know if you see any more issues!