mirror of
https://github.com/facebook/react.git
synced 2025-11-01 09:12:30 +00:00
Mofei Zhang
228e066ab5
```js
function Component() {
useEffect(() => {
let hasCleanedUp = false;
document.addEventListener(..., () => hasCleanedUp ? foo() : bar());
// effect return values shouldn't be typed as frozen
return () => {
hasCleanedUp = true;
}
};
}
```
### Problem
`PruneHoistedContexts` currently strips hoisted declarations and rewrites the first `StoreContext` reassignment to a declaration. For example, in the following example, instruction 0 is removed while a synthetic `DeclareContext let` is inserted before instruction 1.
```js
// source
const cb = () => x; // reference that causes x to be hoisted
let x = 4;
x = 5;
// React Compiler IR
[0] DeclareContext HoistedLet 'x'
...
[1] StoreContext reassign 'x' = 4
[2] StoreContext reassign 'x' = 5
```
Currently, we don't account for `DeclareContext let`. As a result, we're rewriting to insert duplicate declarations.
```js
// source
const cb = () => x; // reference that causes x to be hoisted
let x;
x = 5;
// React Compiler IR
[0] DeclareContext HoistedLet 'x'
...
[1] DeclareContext Let 'x'
[2] StoreContext reassign 'x' = 5
```
### Solution
Instead of always lowering context variables to a DeclareContext followed by a StoreContext reassign, we can keep `kind: 'Const' | 'Let' | 'Reassign' | etc` on StoreContext.
Pros:
- retain more information in HIR, so we can codegen easily `const` and `let` context variable declarations back
- pruning hoisted `DeclareContext` instructions is simple.
Cons:
- passes are more verbose as we need to check for both `DeclareContext` and `StoreContext` declarations
~(note: also see alternative implementation in https://github.com/facebook/react/pull/32745)~
### Testing
Context variables are tricky. I synced and diffed changes in a large meta codebase and feel pretty confident about landing this. About 0.01% of compiled files changed. Among these changes, ~25% were [direct bugfixes](https://www.internalfb.com/phabricator/paste/view/P1800029094). The [other changes](https://www.internalfb.com/phabricator/paste/view/P1800028575) were primarily due to changed (corrected) mutable ranges from https://github.com/facebook/react/pull/33047. I tried to represent most interesting changes in new test fixtures
`
React Compiler
React Compiler is a compiler that optimizes React applications, ensuring that only the minimal parts of components and hooks will re-render when state changes. The compiler also validates that components and hooks follow the Rules of React.
More information about the design and architecture of the compiler are covered in the Design Goals.
More information about developing the compiler itself is covered in the Development Guide.