Add support for setting emit flags and synthetic comments by dragomirtitian · Pull Request #64500 · microsoft/TypeScript

Titian Cernicova-Dragomir (dragomirtitian) · 2026-09-28T15:39:45Z

typescript-automation · 2026-09-28T15:39:51Z

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Copilot

Copilot review overview

🟡 Changes recommended

Public API compatibility gaps, omitted enum members, and a nullable protocol entry that can panic must be addressed.

Review effort: Balanced
Findings: 1 · 1 · 1

Open (3)

Null request entries can trigger an emitNode nil dereference · New
Generated enum omits native zero and composed EmitFlags constants · New
Mark emitContext as undefined-compatible for exact optional properties · New

What changed in this PR

Adds emit flags and synthetic-comment support to the TypeScript API printer.

Changes:

Introduces EmitContext and EmitFlags.
Transfers emit metadata alongside encoded AST nodes.
Adds synchronous and asynchronous API tests.

File	Description
`tsc/internal/printer/printer.go`	Enables synthetic comments without a source file.
`tsc/internal/api/session.go`	Reconstructs emit metadata for printing.
`tsc/internal/api/proto.go`	Defines emit metadata protocol types.
`tsc/internal/api/enum_values_generated.go`	Adds generated emit flag values.
`tsc/internal/api/encoder/decoder.go`	Adds per-node decode callbacks.
`tools/scripts/tsc/generate-enums.ts`	Configures `EmitFlags` generation.
`packages/typescript/test/sync/api.test.ts`	Adds generated synchronous tests.
`packages/typescript/test/async/api.test.ts`	Tests comments and emit flags.
`packages/typescript/src/enums/emitFlags.ts`	Adds runtime enum output.
`packages/typescript/src/enums/emitFlags.enum.ts`	Adds typed enum output.
`packages/typescript/src/api/sync/api.ts`	Adds generated synchronous APIs.
`packages/typescript/src/api/proto.generated.ts`	Adds generated protocol declarations.
`packages/typescript/src/api/node/encoder.ts`	Reports encoded node indices.
`packages/typescript/src/api/async/api.ts`	Implements the public emit context API.

Files not reviewed (1)

tsc/internal/api/enum_values_generated.go: Generated file

…nodes # Conflicts: # packages/typescript/src/api/async/api.ts # packages/typescript/src/api/proto.generated.ts # packages/typescript/src/api/sync/api.ts # packages/typescript/test/async/api.test.ts # packages/typescript/test/sync/api.test.ts

- Null and unknown emitNodes entries are rejected as client errors instead of panicking. - EmitFlags exposes None and the composed flags, and None resets a node. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Copilot

🟡 Changes recommended

Synthetic comment handling can change program semantics or produce invalid syntax, and deferred prints retain mutable metadata.

3 open findings

3 resolved since last review

Files not reviewed (1)

tsc/internal/api/enum_values_generated.go: Generated file

🧠 Review effort: Balanced

Copilot

🟡 Changes recommended

Synthetic-comment handling can alter expression semantics, produce invalid syntax, and lose attached metadata.

3 open issues

2 previously missed issues

In code that hasn't changed since last review

Detect line terminators in multiline comments

tsc/internal/printer/printer.go:3061

A multiline comment can contain a line terminator even when both newline flags are false. A leading comment with text a\nb on the operand of return 1 bypasses this check and emits return /*a\nb*/ 1;. Automatic semicolon insertion makes that return undefined; the corresponding throw is invalid syntax. Include line terminators in comment.Text when deciding whether parentheses are needed.

Honor HasLeadingNewLine for trailing comments

tsc/internal/printer/printer.go:5519

The newly exposed hasLeadingNewLine option is ignored for trailing comments. For example, addSyntheticTrailingComment(node, MultiLineCommentTrivia, " note ", false, true) still places the comment on the node's line. Honor comment.HasLeadingNewLine before writing the trailing comment, as the leading-comment emitter already does.

3 resolved since last review

1 file not reviewed

tsc/internal/api/enum_values_generated.go: Generated file

_{Review effort: Balanced}

Andrew Branch (andrewbranch) · 2026-10-19T16:40:23Z

+		if node.Kind != ast.KindPartiallyEmittedExpression && p.willEmitLeadingSyntheticNewLine(node) {
+			return p.emitContext.Factory.NewParenthesizedExpression(node)


Are these new bugs or inherited from Strada?

+	if comment.HasTrailingNewLine || comment.Kind == ast.KindSingleLineCommentTrivia {
 		p.writer.WriteLine()


 func (p *Printer) parenthesizeExpressionForNoAsi(node *ast.Expression) *ast.Expression {
 	if !p.commentsDisabled {
+		if node.Kind != ast.KindPartiallyEmittedExpression && p.willEmitLeadingSyntheticNewLine(node) {
+			return p.emitContext.Factory.NewParenthesizedExpression(node)


Add support for setting emit flags and synthetic comments

9a9cc1e

Copilot AI balanced review requested due to automatic review settings September 28, 2026 15:39

github-project-automation Bot added this to PR Backlog Sep 28, 2026

github-project-automation Bot moved this to Not started in PR Backlog Sep 28, 2026

typescript-automation Bot added the For Uncommitted Bug PR for untriaged, rejected, closed or missing bug label Sep 28, 2026

Copilot started reviewing on behalf of Titian Cernicova-Dragomir (dragomirtitian) September 28, 2026 15:40

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Comment thread tsc/internal/api/session.go

Comment thread tools/scripts/tsc/generate-enums.ts

Comment thread packages/typescript/src/api/async/api.ts Outdated

typescript-automation Bot assigned Andrew Branch (andrewbranch) Sep 28, 2026

Titian Cernicova-Dragomir (dragomirtitian) and others added 3 commits September 28, 2026 17:36

Address code review

aca7a1b

Add regression tests for code review fixes

73a61db

- Null and unknown emitNodes entries are rejected as client errors instead of panicking. - EmitFlags exposes None and the composed flags, and None resets a node. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Andrew Branch (andrewbranch) requested a balanced review from Copilot October 16, 2026 14:26

Copilot started reviewing on behalf of Andrew Branch (andrewbranch) October 16, 2026 14:26

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Comment thread packages/typescript/src/api/async/api.ts Outdated

Comment thread tsc/internal/api/session.go

Comment thread tsc/internal/printer/printer.go

Address code review

8cfaeb9

Andrew Branch (andrewbranch) requested a balanced review from Copilot October 19, 2026 16:15

Copilot started reviewing on behalf of Andrew Branch (andrewbranch) October 19, 2026 16:15

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Add support for setting emit flags and synthetic comments - #64500

Add support for setting emit flags and synthetic comments#64500
Titian Cernicova-Dragomir (dragomirtitian) wants to merge 5 commits into
microsoft:mainfrom
bloomberg:add-support-for-emit-nodes

Titian Cernicova-Dragomir (dragomirtitian) commented Sep 28, 2026 •

edited by jakebailey

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		if node.Kind != ast.KindPartiallyEmittedExpression && p.willEmitLeadingSyntheticNewLine(node) {
		return p.emitContext.Factory.NewParenthesizedExpression(node)

		if comment.HasTrailingNewLine \|\| comment.Kind == ast.KindSingleLineCommentTrivia {
		p.writer.WriteLine()

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Titian Cernicova-Dragomir (dragomirtitian) commented Sep 28, 2026 •

edited by jakebailey

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