JSX removing leading whitespace in front of charCode

[MarosPistej]

Describe the bug

About

PR #11474 introduced changes which are causing leading/trailing whitespaces to be trimmed. I believe that issue is in implementation of fn extract_line_content and exact issue i am encountering caused by invocation on line https://github.com/swc-project/swc/pull/11474/changes#diff-8abacd70370fd7f85bb3452458bd542918f7b0a9dc6074dce36d726593c4efe0R1867

Input code

just `white space` in front of `charCode` in JSX:

<span> &#8226; </span>

Config

Link to the code that reproduces this issue

https://play.swc.rs/?version=1.15.11&code=H4sIAAAAAAAAA7MpLkjMs1NQU7YwMjKzVrDRB%2FMBMzIGHBYAAAA%3D&config=H4sIAAAAAAAAA1WPSQ7CMAxF9z1F5DVbWHCHHsIKbpUqk%2BJUIqp6d9JMwM7%2Bg598TELAxhKe4shjXjwGpjD2rHCyEd9ZAZIGWQblI9y6u%2FFlxbBTUc5qQMSwUiwlvrc0aOeYsragZmqaUVYt6ZcnnfGBmP%2BDVxTtqke%2F4qaGBONeezHbHzF5qvgHfEMdNg6D4rk3yxPnB%2BvJL0oTAQAA

SWC Info output

No response

Expected behavior

I expect behavior like in previous version or in babel, preserved leading and trailing space:

Input: <span> &#8226; </span>
Output: React.createElement("span", null, " • ");

Actual behavior

Current behaviour is that leading space is trimmed:

Input: <span> &#8226; </span>
Output: React.createElement("span", null, "• ");

Version

v1.15.11

Additional context

No response

[kdy1]

@claude fix it to match JSX specifications

[claude]

Claude finished @kdy1's task in 14m 39s —— View job • claude/issue-11541-20260210-1324 • Create PR ➔

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Fixing JSX whitespace trimming for character codes

• Gather context and understand the bug
• Understand JSX whitespace specification and reproduce the bug
• Identify the root cause
• Implement the fix
• Write unit tests and create snapshots
• Run full test suite to verify (276 tests pass)
• Commit and push changes

Root cause

In jsx_text_to_str_with_entity_mask (crates/swc_ecma_transforms_react/src/jsx/mod.rs:1850), when processing the first line at a line terminator, trim_leading=true was passed to extract_line_content for all lines, including the first. Per the TypeScript JSX specification, leading whitespace on the first line should be preserved (firstNonWhitespace starts at 0), while only subsequent lines have their leading whitespace trimmed.

The non-entity code path (jsx_text_to_str_impl) already correctly preserved first-line leading whitespace via first_non_whitespace: Option<usize> = Some(0), but the entity-aware path introduced in PR #11474 did not replicate this behavior.

Fix

Added an is_first_line flag to jsx_text_to_str_with_entity_mask that tracks whether we're processing the first line. For the first line, trim_leading=false is passed, preserving leading whitespace. After the first line terminator, the flag is set to false and subsequent lines trim leading whitespace as expected.

Tests added

• react_should_preserve_leading_whitespace_before_charcode — single-line <span> </span> → " • "
• react_should_preserve_leading_whitespace_before_charcode_2 — single-line <span> </span> → " •"
• react_should_preserve_whitespace_before_charcode_multiline — multiline <span>\n \n</span> → "•"
• react_should_preserve_whitespace_around_charcode_multiline — multiline with text → "• text"
• react_should_preserve_first_line_leading_ws_with_entity — first-line leading whitespace preserved: " hello • world"

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