Fix JSX indentation in JavaScript output

[Copilot]

• Explore the codebase and understand the issue
• Examine existing tsx test (jsxUnicodeEscapeSequence.tsx)
• Create test case based on the issue (jsxNestedIndentation.tsx)
• Reproduce the bug and confirm TypeScript's expected output
• Accept initial test baselines (shows bug: all on one line)
• Fix the bug in printer.go by uncommenting EFStartOnNewLine handling
• Re-run tests and accept new baselines (4488 → 37 .diff files!)
• Run full test suite to ensure no regressions
• Verify linting and formatting pass
• Fix regression in commentsOnJSXExpressionsArePreserved test
  • Fixed inJsxChild flag state management for nested transforms
  • Fixed children newline logic to check count after filtering
  • All tests now pass with 3 more .diff files removed

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Indentation removed in JavaScript output for JSX (minor)</issue_title>
<issue_description>## Steps to reproduce

import * as React from 'react'
declare function Child(props: { children?: React.ReactNode }): React.ReactNode;
function Test() {
    return <Child>
        <Child>
            <Child></Child>
        </Child>
    </Child>
}

Behavior with typescript@5.8

The output preserves the newlines and indentation of the tags:

function Test() {
    return React.createElement(Child, null,
        React.createElement(Child, null,
            React.createElement(Child, null)));
}

Behavior with tsgo

The output collapses everything on a single line:

function Test() {
    return React.createElement(Child, null, React.createElement(Child, null, React.createElement(Child, null)));
}

</issue_description>

<agent_instructions>Examine an existing tsx test first. Then create a test based on the issue and accept its baselines. Then fix the bug.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #1791

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[jakebailey]

I think this is good?

[Copilot]

Pull Request Overview

This PR fixes JSX indentation in JavaScript output to match TypeScript's behavior. The issue was that nested JSX elements were being collapsed to a single line instead of preserving proper indentation and newlines.

• Fixed JSX indentation by enabling the EFStartOnNewLine flag handling in the printer
• Improved state management for JSX child transformations to prevent context corruption
• Updated newline logic to check children count after filtering out null children

Reviewed Changes

Copilot reviewed 91 out of 91 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/jsxNestedIndentation.tsx	New test case to verify JSX nested indentation behavior
testdata/tests/cases/compiler/jsxNestedIndentation.js	Expected JavaScript output with proper indentation
internal/transformers/jsxtransforms/jsx.go	Fixed JSX child state management and newline logic for proper indentation
internal/printer/printer.go	Enabled EFStartOnNewLine flag handling to support multiline JSX output
Various baseline files	Updated test baselines showing the fix applied across many JSX tests

[weswigham]

This is probably better, since it makes sense to count the result nodes instead of the input nodes. However, in strada we did just always set the startOnNewLine flag on jsx elements which were children of other jsx elements, irrespective of sibling node count, so we're probably still different (we did, however, not set it on jsx text children - only tag-like children). In any case, we might need a followup change that swaps len(newChildren) > 1 to something like len(core.Filter(newChildren,isNotWhitespaceOnlyJsxText)) > 1 so whitespace only jsx text doesn't count as a "real" child element for the purpose of indentation - this is all different from strada, though, so it's kinda... opinion-y?

I dunno - maybe more diffs go away if we align with strada more instead and only set the flag on tag-like children irrespective of sibling count.

[jakebailey]

Yeah, this all seems like style. I tried doing:

if tx.inJsxChild {
	tx.EmitContext().AddEmitFlags(res, printer.EFStartOnNewLine)
}

And it seems to create more differences, but maybe that's not what you mean.

I did try out redoing the tag-like-children thing and only got more diffs, so I must be doing something wrong here. I'll just merge this as a slight improvement for now.

But I agree, this is all sort of style opiniony.

[jakebailey]

Er, well, need another approval to merge but oh well.

[weswigham]

Yeah, I was thinking moreso replacing

	// Add StartOnNewLine flag only if there are multiple actual children (after filtering)
	if len(newChildren) > 1 {
		for _, child := range newChildren {
			tx.EmitContext().AddEmitFlags(child, printer.EFStartOnNewLine)
		}
	}

with

for _, child := range newChildren {
    if !(ast.IsJsxElement(child) || ast.IsJsxSelfClosingElement(child) || ast.IsJsxFragment(child)) continue;
	tx.EmitContext().AddEmitFlags(child, printer.EFStartOnNewLine)
}

which probably more closely matches the strada condition for indentation.

[jakebailey]

Just tried it, and that's seemingly worse than this PR too. Very weird....