Fix JSX entity decoder skipping entities after non-entity ampersand by Copilot · Pull Request #3897 · microsoft/typescript-go

Copilot · 2026-05-16T15:42:26Z

Analysis

The decodeEntities function in internal/transformers/jsxtransforms/jsx.go had a bug where it would find the first & and then search for the next ;, but didn't account for the possibility of another & appearing between them.

For example, with input &&:

Find & at position 0
Find ; at position 5
Extract entity &amp (positions 1-5) — invalid because it contains &
Write the whole thing as-is: &&

The TypeScript reference implementation uses a regex &((#((\d+)|x([\da-fA-F]+)))|(\w+)); where \w+ naturally excludes &, so it correctly matches only & starting from the second &.

Fix

Added an inner loop in the decodeEntities function that skips past all intervening & characters between the current & and the next ;, emitting each as a literal. The loop adjusts the semi index as it advances, so the entity span is only searched once. This correctly handles cases with multiple consecutive non-entity ampersands before a valid entity (e.g. a&b&c& → "a&b&c&").

Test case jsxEntityDecoderAfterNonEntityAmpersand.tsx covers mixed entity/non-entity ampersand sequences including &&, a&b&c&d<e, &&, &&&, and a&b&c&.

Copilot Checklist

I successfully ran these commands at the end of my session, and they completed without error:

npx hereby build
npx hereby test
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When decodeEntities encounters an '&' followed by ';', it now checks if there's another '&' between them. If so, the first '&' is treated as a literal character and processing restarts from the next '&'. This fixes cases like '&&' being incorrectly left as-is instead of decoded to '&&'. Agent-Logs-Url: https://github.com/microsoft/typescript-go/sessions/5ff0998f-b249-4679-a68b-71dd0a3f168c Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot

Pull request overview

Fixes a bug in the JSX text entity decoder (decodeEntities) where a non-entity ampersand could cause the decoder to skip subsequent valid entities, aligning tsgo behavior with the TypeScript reference implementation.

Changes:

Update decodeEntities to detect an intervening & before ; and restart scanning so subsequent entities are still decoded.
Add a new compiler test case covering mixed entity and non-entity ampersand sequences in JSX text.
Add new reference baselines (.js, .types, .symbols, .errors.txt) for the test.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`internal/transformers/jsxtransforms/jsx.go`	Adjusts entity scanning logic to avoid consuming invalid `&...;` spans that contain another `&`.
`testdata/tests/cases/compiler/jsxEntityDecoderAfterNonEntityAmpersand.tsx`	New regression test for entity decoding after non-entity ampersands in JSX text.
`testdata/baselines/reference/compiler/jsxEntityDecoderAfterNonEntityAmpersand.js`	Expected JS emit showing decoded JSX text strings.
`testdata/baselines/reference/compiler/jsxEntityDecoderAfterNonEntityAmpersand.types`	Expected type baseline for the new test.
`testdata/baselines/reference/compiler/jsxEntityDecoderAfterNonEntityAmpersand.symbols`	Expected symbol baseline for the new test.
`testdata/baselines/reference/compiler/jsxEntityDecoderAfterNonEntityAmpersand.errors.txt`	Expected diagnostic baseline for the new test.

jakebailey · 2026-05-16T16:36:49Z

+		// Check if there's another '&' between the current '&' and the ';'.
+		// If so, the current '&' is not part of a valid entity; write it as
+		// a literal and restart from the next '&'.
+		nextAmp := strings.IndexByte(text[1:], '&')


@copilot check and consider similar problems

Good catch. Narrowed the search from text[1:] to text[1:semi] so it only inspects the candidate entity span. I also checked the rest of the function — the other two IndexByte calls (lines 864 and 900) search for & to find the next entity start, so they correctly need to scan the full remaining text. Fixed in 6c7989a.

Narrows the IndexByte scan from text[1:] to text[1:semi] so only the candidate entity span is inspected, avoiding an unnecessary O(n) scan of the remaining text for each entity. Agent-Logs-Url: https://github.com/microsoft/typescript-go/sessions/895e4b37-80ee-4c96-af8b-acf8a085ba78 Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

DanielRosenwasser · 2026-05-18T05:14:16Z

+// @target: esnext
+
+const a = <div>&&amp;</div>;
+const b = <div>a&b&amp;c&d&lt;e</div>;


This is totally missing the case of a&b&c&, isn't it?

Added a&b&c& as test case e. It correctly produces "a&b&c&". Fixed in 0654017.

DanielRosenwasser · 2026-05-18T05:14:32Z

+		// If so, the current '&' is not part of a valid entity; write it as
+		// a literal and restart from the next '&'.
+		nextAmp := strings.IndexByte(text[1:semi], '&')
+		if nextAmp >= 0 {


This probably needs to be a loop.

Refactored into a proper inner for loop that skips all intervening & characters in one pass, adjusting semi as it goes. Fixed in 0654017.

jakebailey · 2026-05-18T05:25:03Z

@copilot address comments

…case Agent-Logs-Url: https://github.com/microsoft/typescript-go/sessions/2a9af1f9-b711-45d4-9abb-dccb033ef954 Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

DanielRosenwasser · 2026-05-18T18:27:23Z

+		for nextAmp := strings.IndexByte(text[1:semi], '&'); nextAmp >= 0; nextAmp = strings.IndexByte(text[1:semi], '&') {
+			result.WriteString(text[:nextAmp+1])


I think I'd just prefer

Suggested change

for nextAmp := strings.IndexByte(text[1:semi], '&'); nextAmp >= 0; nextAmp = strings.IndexByte(text[1:semi], '&') {

result.WriteString(text[:nextAmp+1])

for {

nextAmp := strings.IndexByte(text[1:semi], '&')

if nextAmp >= 0 {

break

}

result.WriteString(text[:nextAmp+1])

@copilot do it

Restructured to for { ... break } style in 94e06d5.

DanielRosenwasser

Looks okay, but @jakebailey what do you think of the nit I left?

Agent-Logs-Url: https://github.com/microsoft/typescript-go/sessions/31b2e5d6-fd33-413f-8a6e-04cc6aae425b Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Initial plan

626d71a

Copilot AI assigned Copilot and jakebailey May 16, 2026

Copilot started work on behalf of jakebailey May 16, 2026 15:42 View session

Copilot AI linked an issue May 16, 2026 that may be closed by this pull request

tsgo JSX entity decoder skips entities that follow a non-entity ampersand #3894

Closed

Copilot AI changed the title ~~[WIP] Fix tsgo JSX entity decoder for non-entity ampersands~~ Fix JSX entity decoder skipping entities after non-entity ampersand May 16, 2026

Copilot AI requested a review from jakebailey May 16, 2026 15:57

Copilot finished work on behalf of jakebailey May 16, 2026 15:57

jakebailey approved these changes May 16, 2026

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jakebailey marked this pull request as ready for review May 16, 2026 16:31

Copilot AI review requested due to automatic review settings May 16, 2026 16:31

Copilot AI reviewed May 16, 2026

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Copilot started work on behalf of jakebailey May 16, 2026 16:36 View session

Copilot finished work on behalf of jakebailey May 16, 2026 16:50

Copilot AI requested a review from jakebailey May 16, 2026 16:50

DanielRosenwasser requested changes May 18, 2026

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Copilot started work on behalf of jakebailey May 18, 2026 05:25 View session

Refactor intervening & check into inner loop and add a&b&c& test …

0654017

…case Agent-Logs-Url: https://github.com/microsoft/typescript-go/sessions/2a9af1f9-b711-45d4-9abb-dccb033ef954 Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot finished work on behalf of jakebailey May 18, 2026 05:40

jakebailey requested a review from DanielRosenwasser May 18, 2026 16:45

DanielRosenwasser reviewed May 18, 2026

View reviewed changes

DanielRosenwasser approved these changes May 18, 2026

View reviewed changes

Copilot started work on behalf of jakebailey May 18, 2026 18:28 View session

Restructure inner loop to for { break } style per review feedback

94e06d5

Agent-Logs-Url: https://github.com/microsoft/typescript-go/sessions/31b2e5d6-fd33-413f-8a6e-04cc6aae425b Co-authored-by: jakebailey <5341706+jakebailey@users.noreply.github.com>

Copilot finished work on behalf of jakebailey May 18, 2026 18:43

jakebailey requested a review from DanielRosenwasser May 18, 2026 18:43

DanielRosenwasser approved these changes May 18, 2026

View reviewed changes

DanielRosenwasser enabled auto-merge May 18, 2026 20:57

DanielRosenwasser added this pull request to the merge queue May 18, 2026

Merged via the queue into main with commit 1bf0f86 May 18, 2026
21 checks passed

DanielRosenwasser deleted the copilot/fix-jsx-entity-decoder branch May 18, 2026 21:32

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Fix JSX entity decoder skipping entities after non-entity ampersand#3897

Fix JSX entity decoder skipping entities after non-entity ampersand#3897
DanielRosenwasser merged 5 commits into
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copilot/fix-jsx-entity-decoder

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		for nextAmp := strings.IndexByte(text[1:semi], '&'); nextAmp >= 0; nextAmp = strings.IndexByte(text[1:semi], '&') {
		result.WriteString(text[:nextAmp+1])

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Analysis

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Copilot AI commented May 16, 2026 •

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