[compiler] Fix JSX tags prefixed with _ or $ incorrectly treated as host elements#36688
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[compiler] Fix JSX tags prefixed with _ or $ incorrectly treated as host elements#36688sleitor wants to merge 1 commit into
_ or $ incorrectly treated as host elements#36688sleitor wants to merge 1 commit into
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…, `_Bar`, `$Foo`) treated as host elements Previously, `lowerJsxElementName` only checked `if (tag.match(/^[A-Z]/)))` to identify component references. This means tags starting with `_`, `$`, a digit, or any character other than an ASCII letter were incorrectly treated as BuiltinTag (host/intrinsic elements) rather than component references. JSX semantics mirror Babel's JSX transform: a tag is a host element if and only if it starts with a **lowercase** letter (a-z). Everything else (uppercase, `_`, `$`, etc.) refers to an in-scope identifier and should be loaded as a component. Fix: change the guard from `tag.match(/^[A-Z]/)` to `!tag.match(/^[a-z]/)`, which correctly classifies identifiers like `_Bar` and `$Foo` as component references. Fixes facebook#36601
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Summary
Fixes #36601
Problem
In
lowerJsxElementName(BuildHIR.ts), the conditionif (tag.match(/^[A-Z]/))only treats JSX tags starting with an uppercase letter as component references. Tags starting with_,$, or any other non-letter character fell through to theelsebranch and were incorrectly classified asBuiltinTag(host/intrinsic elements).This meant that
<_Bar />or<$Foo />was treated like<div />, causing the compiler to skip memoization of the component and potentially producing incorrect output.Root Cause
JSX semantics (as implemented by Babel's JSX transform) are:
The original code only handled the first half of that rule ("starts with uppercase → component") while ignoring identifiers like
_Barand$Foo.Fix
Change:
To:
This correctly classifies any JSX identifier that does NOT start with a lowercase letter as a component reference, matching JSX spec semantics.
Test
Added fixture
jsx-underscore-prefix-componentthat renders<_Bar />and verifies the compiler correctly memoizes it as a component reference.How did you test this change?
jsx-underscore-prefix-componentyarn workspace babel-plugin-react-compiler lint✅