Mirror of facebook/react#34542
Original author: pritjasani08

Summary

This PR fixes issue #33689 where the React Compiler wasn't properly handling cases where named functions call other named functions when they're defined in a "weird order" (i.e., when a function is called before it's declared).

Problem

The getFunctionReferencedBeforeDeclarationAtTopLevel function only checked for function references at the top level scope, but didn't account for nested function calls where one named function calls another named function that's defined later in the code.

Solution

Enhanced the function to:

1. Check if a function call is within another function that's being compiled
2. Detect when a named function calls another named function
3. Properly handle the ordering by adding the called function to referencedBeforeDeclaration when appropriate

Changes

• Modified: compiler/packages/babel-plugin-react-compiler/src/Entrypoint/Program.ts

  • Enhanced getFunctionReferencedBeforeDeclarationAtTopLevel function to handle nested function calls
  • Added logic to detect and handle named functions calling other named functions defined later

• Added: Test fixtures to verify the fix

  • compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/named-functions-order.js
  • compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/named-functions-order.expect.md

Test Cases

The fix handles these scenarios:

1. Named function calling another named function defined later in the same component
2. Nested function calls where inner functions call functions defined later
3. Complex scenarios with multiple levels of function nesting

Example

function Component(props) {
  function firstFunction() {
    return secondFunction(); // Calls secondFunction before it's declared
  }
  
  function secondFunction() {
    return props.message || "Hello";
  }
  
  return firstFunction();
}

This now works correctly with the React Compiler.

##Checklist

• I have read the Contributing Guide
• I have added tests that prove my fix is effective or that my feature works
• All new and existing tests pass
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

Fixes #33689