feat(transformer): support tagged template expression with `</script` transformation #15664

Dunqing · 2025-11-13T09:00:33Z

Implement a transformation plugin for tagged template expressions containing </script> tags to prevent script tag issues in browser environments.

This plugin transforms tagged template literals containing </script (case-insensitive) to use a helper function call with template caching, matching esbuild's behavior.

Example

Transforms tagged templates to use cached template objects with a helper function:

Input:

foo`</script>`
bar`<script>${content}</script>`

Output:

var _templateObject, _templateObject2;
foo(_templateObject || (_templateObject = babelHelpers.taggedTemplateLiteral(["<\/script>"])));
bar(_templateObject2 || (_templateObject2 = babelHelpers.taggedTemplateLiteral(["<script>", "<\/script>"])), content);

Closes #15306

Dunqing · 2025-11-13T09:00:50Z

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codspeed-hq · 2025-11-13T09:05:26Z

CodSpeed Performance Report

Merging #15664 will not alter performance

_{Comparing 11-13-feat_transformer_support_tagged_template_expression_with__script_transformation (777ea0b) with main (c023ba6)}

Summary

✅ 33 untouched
⏩ 4 skipped¹

4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

Copilot

Pull Request Overview

This PR implements a transformation plugin for tagged template expressions containing </script> tags to prevent script tag parsing issues when JavaScript is embedded in HTML. The transformation converts tagged template literals into function calls using cached template objects via a Babel helper.

Key Changes

Adds a new TaggedTemplateTransform plugin that detects and transforms tagged templates containing </script> (case-insensitive)
Integrates the plugin into the transformer pipeline with proper option handling across napi/TypeScript/Rust boundaries
Includes comprehensive test fixtures covering basic cases, expressions, case variations, and edge cases

Reviewed Changes

Copilot reviewed 33 out of 34 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/oxc_transformer/src/plugins/tagged_template_transform.rs	Core plugin implementation with script tag detection and transformation logic
crates/oxc_transformer/src/plugins/mod.rs	Plugin registration and integration into the traversal pipeline
crates/oxc_transformer/src/plugins/options.rs	Plugin options structure definition
crates/oxc_transformer/src/options/mod.rs	Option mapping from Babel configuration
crates/oxc_transformer/src/options/babel/plugins.rs	Babel plugin entry parsing for "tagged-template-transform"
crates/oxc_transformer/src/common/helper_loader.rs	Added TaggedTemplateLiteral helper enum variant
napi/transform/src/transformer.rs	NAPI bindings for plugin options
napi/transform/index.d.ts	TypeScript definitions for plugin configuration
npm/runtime/src/helpers/esm/taggedTemplateLiteralEscape.js	Runtime helper function (currently unused)
tasks/transform_conformance/src/constants.rs	Added plugin to conformance test suite
tasks/transform_conformance/snapshots/*	Updated test snapshots showing improved pass rates
tasks/transform_conformance/tests/plugin-tagged-template-transform/*	Comprehensive test fixtures for various scenarios

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crates/oxc_transformer/src/plugins/tagged_template_transform.rs

crates/oxc_transformer/src/options/mod.rs

crates/oxc_transformer/src/plugins/tagged_template_transform.rs

npm/runtime/src/helpers/esm/taggedTemplateLiteralEscape.js

overlookmotel · 2025-11-15T19:30:34Z

@Dunqing in case you weren't aware, there's a panic in conformance.

Dunqing · 2025-11-16T12:52:24Z

@Dunqing in case you weren't aware, there's a panic in conformance.

Sorry, I forgot to push the local changes.

sapphi-red

LGTM on my side

overlookmotel · 2025-11-17T10:28:01Z

I'm afraid I don't think this is right. I'll make sure I'm correct about that, and write up what I think the problem is.

In meantime, have rebased on latest main + added one formatting nit commit.

overlookmotel · 2025-11-17T11:08:50Z

What we're missing is a 2nd array for raw strings if cooked and raw are not the same:

Input:

foo`</script>`;
foo`</script>\n`;
foo`</script>\u`; // No cooked because invalid escape `\u`

Current output:

// Correct
foo(_temp || (_temp = _taggedTemplateLiteral(["</script>"])));
// `raw` property incorrect in `foo`
foo(_temp2 || (_temp2 = _taggedTemplateLiteral(["</script>\n"])));
// Syntax error!
foo(_temp3 || (_temp3 = _taggedTemplateLiteral(["</script>\u"])));

Output should be:

foo(_temp || (_temp = _taggedTemplateLiteral(["</script>"])));
foo(_temp2 || (_temp2 = _taggedTemplateLiteral(["</script>\n"], ["</script>\\n"])));
//                                                            ^^^^^^^^^^^^^^^^^^
foo(_temp3 || (_temp3 = _taggedTemplateLiteral([void 0], ["</script>\\u"])));
//                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^

Babel REPL

ESBuild REPL

Because this only transforms tagged template literals containing </script>, I think we're OK to merge this, and fix in a follow-up. It'll only be incorrect for tagged template literals which contain both </script> and escapes e.g. \n, and syntax error only occurs if there's an invalid escape sequence in the template.

Additionally, is this transform enabled in benchmarks? I think it may be a perf regression. We should check if expr is an Expression::TaggedTemplateExpression in enter_expression and NOT add #[inline] to transform_tagged_template - because most expressions are not TaggedTemplateExpressions.

overlookmotel · 2025-11-17T11:33:17Z

Actually, this transform is enabled in benchmarks. Effect is only -1%.

That's acceptable for now. We can improve it in a follow-up.

overlookmotel · 2025-11-17T11:25:44Z

crates/oxc_transformer/src/plugins/tagged_template_transform.rs

+
+            let raw_bytes = raw.as_bytes();
+            // Get the bytes up to the last possible starting position of the script tag
+            let max_remain_len = raw_bytes.len().saturating_sub(SCRIPT_TAG.len());


No need for saturating_sub here. You already checked that len < SCRIPT_TAG.len() above, so just raw_bytes.len() - SCRIPT_TAG.len() would do (cheaper).

overlookmotel · 2025-11-17T11:27:55Z

crates/oxc_transformer/src/plugins/tagged_template_transform.rs

+            for (idx, byte) in raw_bytes_iter {
+                if byte == b'<'
+                    && SCRIPT_TAG
+                        .iter()
+                        .zip(raw_bytes[idx..].iter())
+                        .all(|(a, b)| *a == b.to_ascii_lowercase())
+                {
+                    return true;
+                }
+            }


We have a faster way to check for </script in oxc_codegen here:

oxc/crates/oxc_codegen/src/str.rs

Lines 726 to 743 in 3b548dc

/// Check if `slice` is `</script`, regardless of case.

///

/// `slice.len()` must be 8.

//

// `#[inline(always)]` so that compiler can see from caller that `slice.len() == 8`

// and so `slice.try_into().unwrap()` cannot fail. This function is only 4 instructions.

#[expect(clippy::inline_always)]

#[inline(always)]

pub fn is_script_close_tag(slice: &[u8]) -> bool {

// Compiler condenses these operations to an 8-byte read, u64 AND, and u64 compare.

// https://godbolt.org/z/K8q68WGn6

let mut bytes: [u8; 8] = slice.try_into().unwrap();

for byte in bytes.iter_mut().skip(2) {

// `| 32` converts ASCII upper case letters to lower case.

*byte |= 32;

}

bytes == *b"</script"

}

overlookmotel

We've decided to merge, and fix the bug + perf in follow-ups.

overlookmotel · 2025-11-17T11:54:54Z

Merge activity

Nov 17, 11:54 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Nov 17, 11:54 AM UTC: overlookmotel added this pull request to the Graphite merge queue.
Nov 17, 12:01 PM UTC: Merged by the Graphite merge queue.

… transformation (#15664) Implement a transformation plugin for tagged template expressions containing </script> tags to prevent script tag issues in browser environments. This plugin transforms tagged template literals containing `</script` (case-insensitive) to use a helper function call with template caching, matching esbuild's behavior. ### Example Transforms tagged templates to use cached template objects with a helper function: Input: ```js foo`</script>` bar`<script>${content}</script>` ``` Output: ```js var _templateObject, _templateObject2; foo(_templateObject || (_templateObject = babelHelpers.taggedTemplateLiteral(["<\/script>"]))); bar(_templateObject2 || (_templateObject2 = babelHelpers.taggedTemplateLiteral(["<script>", "<\/script>"])), content); ``` Closes #15306

…#15664

…ces (#15830) Fixes the issue identified in #15664 where the tagged template transform plugin incorrectly handled template literals containing escape sequences. ### Examples ```js // Input foo`</script>\n` // Before (incorrect): foo(_t || (_t = taggedTemplateLiteral(["</script>\\n"]))); // After (correct): foo(_t || (_t = taggedTemplateLiteral(["</script>\n"], ["</script>\\n"]))); // Input with invalid escape foo`</script>\u` // Before (syntax error): foo(_t || (_t = taggedTemplateLiteral(["</script>\\u"]))); // After (correct): foo(_t || (_t = taggedTemplateLiteral([void 0], ["</script>\\u"])));

github-actions bot added the A-transformer Area - Transformer / Transpiler label Nov 13, 2025

github-actions bot added the C-enhancement Category - New feature or request label Nov 13, 2025

Dunqing force-pushed the 11-13-feat_transformer_support_tagged_template_expression_with__script_transformation branch 4 times, most recently from 97af8ab to 7745501 Compare November 13, 2025 13:47

Dunqing marked this pull request as ready for review November 13, 2025 13:50

Dunqing requested a review from overlookmotel as a code owner November 13, 2025 13:50

Copilot AI review requested due to automatic review settings November 13, 2025 13:50

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sapphi-red mentioned this pull request Nov 14, 2025

codegen: add a way to keep </script> as-is for tagged template #15306

Open

Dunqing changed the title ~~feat(transformer): support tagged template expression with </script transformation~~ feat(transformer): support tagged template expression with </script transformation Nov 14, 2025

Boshen added the P-high Priority - High label Nov 14, 2025

Dunqing force-pushed the 11-13-feat_transformer_support_tagged_template_expression_with__script_transformation branch 3 times, most recently from 5a07258 to 1d1dc5a Compare November 17, 2025 03:32

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overlookmotel force-pushed the 11-13-feat_transformer_support_tagged_template_expression_with__script_transformation branch from 1d1dc5a to 0f876fe Compare November 17, 2025 10:26

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graphite-app bot force-pushed the 11-13-feat_transformer_support_tagged_template_expression_with__script_transformation branch from 777ea0b to 1b18457 Compare November 17, 2025 11:55

graphite-app bot merged commit 1b18457 into main Nov 17, 2025
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graphite-app bot deleted the 11-13-feat_transformer_support_tagged_template_expression_with__script_transformation branch November 17, 2025 12:01

graphite-app bot removed the 0-merge Merge with Graphite Merge Queue label Nov 17, 2025

camc314 added a commit to camc314/rolldown that referenced this pull request Nov 17, 2025

chore: expose new <script transform option based on oxc-project/oxc…

9b305a7

…#15664

camc314 added a commit to camc314/rolldown that referenced this pull request Nov 17, 2025

chore: expose new <script transform option based on oxc-project/oxc…

b7366f6

…#15664

camc314 added a commit to camc314/rolldown that referenced this pull request Nov 17, 2025

chore: expose new <script transform option based on oxc-project/oxc…

e66dae5

…#15664

camc314 added a commit to camc314/rolldown that referenced this pull request Nov 17, 2025

chore: expose new <script transform option based on oxc-project/oxc…

f9b3a6a

…#15664

Boshen pushed a commit to camc314/rolldown that referenced this pull request Nov 17, 2025

chore: expose new <script transform option based on oxc-project/oxc…

7827168

…#15664

Boshen mentioned this pull request Nov 17, 2025

Keep </script> as-is for tagged templates rolldown/rolldown#6829

Open

This was referenced Nov 18, 2025

fix(transformer/tagged-template-transform): handle \n escape sequences #15830

Merged

perf(transformer/tagged-template-transform): improve performance #15834

Open

	/// Check if `slice` is `</script`, regardless of case.
	///
	/// `slice.len()` must be 8.
	//
	// `#[inline(always)]` so that compiler can see from caller that `slice.len() == 8`
	// and so `slice.try_into().unwrap()` cannot fail. This function is only 4 instructions.
	#[expect(clippy::inline_always)]
	#[inline(always)]
	pub fn is_script_close_tag(slice: &[u8]) -> bool {
	// Compiler condenses these operations to an 8-byte read, u64 AND, and u64 compare.
	// https://godbolt.org/z/K8q68WGn6
	let mut bytes: [u8; 8] = slice.try_into().unwrap();
	for byte in bytes.iter_mut().skip(2) {
	// `\| 32` converts ASCII upper case letters to lower case.
	*byte \|= 32;
	}
	bytes == *b"</script"
	}

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feat(transformer): support tagged template expression with `</script` transformation #15664

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