[robobun]

Problem

• delete x on a bare identifier is an early SyntaxError in strict mode, and ES modules are strict. Bun accepts it: bun build emits delete x; into ESM output, and node rejects the file with SyntaxError: Delete of an unqualified identifier in strict mode. esbuild errors.
• Cause: the Op::UnDelete arm (src/js_parser/visit/visit_expr.rs:1220) only visits the operand. The port lost esbuild's markStrictModeFeature(deleteBareName) call, and Remove dead code from simdutf_sys, ncrypto, js_parser, bun_install, built-in JS, bindgen, uSockets, and 66 Cargo manifests #39732 then removed the unused variant.

Fix

• Restore StrictModeFeature::DeleteBareName and raise it from the Op::UnDelete arm when the operand is an EIdentifier, before the visit, as esbuild does.
• Correct because mark_strict_mode_feature already implements esbuild's two cases: strict source, and sloppy source bundled to ESM output. cjs and iife output, --no-bundle, and runtime CommonJS are unchanged.
• Only an identifier operand is reported, parenthesized too. delete o.x, delete (0, x) and delete f() stay valid.
• Verified: test/bundler/transpiler/transpiler.test.js, test/bundler/bundler_edgecase.test.ts (new cases fail on main). Self-reviewed: 9 concerns probed (notes), 0 needed a change.

Background

• Scope::strict_mode records why a scope is strict: a "use strict" directive, or implicitly import, export, top-level await, or a class body. It is set after parsing (p.rs:2717), so the check belongs in the visit pass.
• mark_strict_mode_feature (p.rs:4218) reports sloppy-only syntax when the scope is strict, or when the bundler targets ESM output, since the bundle is then one strict module.
• ES2024 13.5.1.1 forbids delete on an identifier reference only. The operand is checked as parsed, so a later rewrite (--define, import binding) cannot change the result.

Notes

• Self-review, compared with node on each case: folded operands (delete (1 ? x : x), handled by the printer's (0, x) wrap, src/js_printer/lib.rs:4377), synthesized deletes (the React compiler only emits property operands), caret placement for delete ((x)), identifier-like operands (undefined, arguments, NaN, async: all errors, as in node), non-identifier operands (this, import.meta, new.target, comma, assignment, call, template: all allowed), TypeScript assertions (delete x!, delete (x as any): errors, TS2703 rejects them too), per-scope strictness (one error per strict scope, sloppy scopes untouched), sloppy CommonJS at runtime and in cjs/iife output (unchanged), and scan-only parses (the visitor is not reached).
• Suites run with the debug build, all green: bundler_edgecase (146), transpiler.test.js (198), bundler_cjs2esm, bundler_cjs, bundler_minify, esbuild/default, esbuild/dce, esbuild/ts, esbuild/extra, transpiler/assign-to-import, transpiler/runtime-transpiler, transpiler/preserve-use-strict-cjs, and test/js/node/test/parallel/test-vm-not-strict.js (sloppy CommonJS delete a at runtime).
• Messages: strict source gets "delete" of a bare identifier cannot be used in strict mode with a note at the directive or keyword. Sloppy source bundled to ESM gets "delete" of a bare identifier cannot be used with the ESM output format due to strict mode.
• The handoff that led to this PR also noted that the UnDelete arm never sets p.delete_target. That half is in js_parser: keep assignment and delete targets as references (string .length, import.meta, module.id, --define) #40804 (and js_parser: set p.delete_target before visiting the delete operand #36734). This PR is independent of it: the strict-mode check reads the operand before the visit, so it does not use delete_target. Both add one statement to the same arm, so whichever lands second has a one-line conflict.
• js_parser: error on delete of bare identifier when bundling to ESM #35962 proposed the same check in July against StrictModeFeature::DeleteBareName. Remove dead code from simdutf_sys, ncrypto, js_parser, bun_install, built-in JS, bindgen, uSockets, and 66 Cargo manifests #39732 removed that variant, so the branch no longer compiles. This PR supersedes it.
• The ESM bundler test deletes a local var, not an import binding. Once js_parser: keep assignment and delete targets as references (string .length, import.meta, module.id, --define) #40804 lands, handle_identifier also reports Cannot assign to import "x" for a delete of an import (esbuild reports both), and bundleErrors matches the exact error set.
• test/bundler/esbuild/lower.test.ts is describe.todo. The LowerForbidStrictModeSyntax entry carried a TODO FIX expectedScanLog comment for this case. It now has the real bundleErrors for delete-1.js and delete-2.js. I verified it passes with the describe enabled. with.js is not listed: bun has no strict-mode check for with.
• The note for an explicit "use strict" directive prints because of the "" keyword here. That is pre-existing ("use strict"; var arguments = 1 on main prints the same) and not changed here.
• Runtime: bun file.js on an ES module with delete x now fails at transpile time with this error instead of JSC's SyntaxError: Cannot delete unqualified property 'x' in strict mode. Both are fatal. A dynamic import() of such a module rejects with a BuildMessage instead of a SyntaxError, like every other parse-time error.
• The runtime transpiler cache version is not bumped. The change adds a diagnostic and does not change emitted code.
• The two expectPrinted("(delete x) ** N") assertions in the exponentiation test now use expectPrinted_. expectPrinted wraps the input in export default (...), which is strict. The parenthesization they assert is unchanged.

Repro before the fix:

$ printf 'export const x = 1;\n' > x.js
$ printf 'import { x } from "./x.js";\nvar y = 1;\ndelete y;\n' > del.js
$ bun build del.js
// x.js
var x = 1;

// del.js
var y = 1;
delete y;

$ printf 'var x = 1; delete x; console.log("del", typeof x);\n' > d.cjs
$ printf "import './d.cjs';\n" > entry.mjs
$ bun build entry.mjs --format=esm --target=node --outfile=out.mjs && node out.mjs
SyntaxError: Delete of an unqualified identifier in strict mode.

After:

$ bun build del.js
3 | delete y;
           ^
error: "delete" of a bare identifier cannot be used in strict mode
    at del.js:3:8

1 | import { x } from "./x.js";
    ^
note: This file is implicitly in strict mode because of the "import" keyword here

$ bun build entry.mjs --format=esm --target=node --outfile=out.mjs
1 | var x = 1; delete x; console.log("del", typeof x);
                      ^
error: "delete" of a bare identifier cannot be used with the ESM output format due to strict mode
    at d.cjs:1:19

$ bun build entry.mjs --format=cjs --target=node --outfile=out.cjs && node out.cjs
del number

[robobun]

Status: reproduced on main with bun build of an ES module that contains delete x (the output is emitted, node rejects it with SyntaxError: Delete of an unqualified identifier in strict mode) and with a sloppy CommonJS file bundled to --format=esm. Fix and tests are in this PR. Supersedes #35962.

CI: the new tests pass on every lane. The one red lane (darwin x64, test/js/web/url/url.test.ts, an ICU IDNA table check) fails on main as well and is unrelated to this change.

[coderabbitai]

Walkthrough

The parser now detects bare-identifier delete expressions and reports strict-mode diagnostics. Bundler and transpiler tests cover strict rejection, sloppy-mode preservation, and valid non-bare operands.

Changes

Strict bare-name delete handling

Layer / File(s)	Summary
Parser feature and diagnostic src/js_parser/parser.rs, src/js_parser/visit/visit_expr.rs, src/js_parser/p.rs	Adds DeleteBareName, records it for bare-identifier operands, and emits the "delete" of a bare identifier diagnostic.
Bundler strictness coverage test/bundler/bundler_edgecase.test.ts, test/bundler/esbuild/lower.test.ts	Tests strict output errors, sloppy CommonJS and IIFE preservation, and valid non-bare operands.
Transpiler parsing and printing coverage test/bundler/transpiler/transpiler.test.js	Tests strict-mode rejection, valid operands, sloppy-mode printing, and plain printing for exponentiation cases.

Suggested reviewers: jarred-sumner

Merge Risk: 🟡 Moderate · up to 583cc

The change can mishandle conditional expressions that fold to a bare identifier, potentially turning valid code into a strict-mode build error or changing delete behavior. This bounded correctness issue should be fixed before merge.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description clearly explains the problem, fix, scope, compatibility behavior, and verification. It does not use the template headings exactly, but it provides the required information in equivalen…
Title check	✅ Passed	The title is concise, specific, and accurately summarizes the main parser change: reporting bare-identifier delete expressions in strict mode.

Full details: Description check

Explanation

The description clearly explains the problem, fix, scope, compatibility behavior, and verification. It does not use the template headings exactly, but it provides the required information in equivalent sections.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/js_parser/visit/visit_expr.rs`:
- Around line 1220-1230: Update the Op::UnDelete handling and its e_if folding
path so a statically folded conditional cannot expose a bare identifier as the
delete operand; after visiting, re-check for Data::EIdentifier and preserve the
non-reference shape by wrapping newly exposed identifiers as (0, x), maintaining
strict-mode validation and non-strict delete behavior.

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• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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Run ID: 334e5bfc-fbd7-4335-ae5b-e296650580cd

📥 Commits

Reviewing files that changed from the base of the PR and between 85fba53 and 583cce0.

📒 Files selected for processing (6)

• src/js_parser/p.rs
• src/js_parser/parser.rs
• src/js_parser/visit/visit_expr.rs
• test/bundler/bundler_edgecase.test.ts
• test/bundler/esbuild/lower.test.ts
• test/bundler/transpiler/transpiler.test.js

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[claude]

LGTM — small, well-scoped restoration of esbuild's deleteBareName strict-mode check with thorough coverage.

What was reviewed:

• mark_strict_mode_feature body always returns Ok(()), so .expect("unreachable") is provably infallible and matches the four other visit-pass call sites.
• Check runs on the operand as parsed (before visit_expr_in_out), so --define/import rewrites can't flip the verdict — matches esbuild's ordering; parenthesized delete (x) is covered because the parser unwraps parens to EIdentifier.
• The two expectPrinted → expectPrinted_ swaps preserve the original exponentiation-parenthesization assertion; only the implicit export default wrapper (now strict) is dropped.
• Variant matrix is covered: strict source (directive/import/export/TLA/class/nested function), sloppy→ESM output, sloppy cjs/iife preserved and executed under Node, non-identifier operands (member/index/optional/comma/conditional/call) stay valid.

Extended reasoning...

Overview

This PR restores the strict-mode early error for delete <bare-identifier>, which was lost during the Zig→Rust port and whose enum variant was subsequently deleted as unused. The native change is three hunks: a new StrictModeFeature::DeleteBareName variant in parser.rs, its message text wired into the mark_strict_mode_feature match in p.rs, and an EIdentifier check in the Op::UnDelete arm of visit_expr.rs that calls mark_strict_mode_feature before visiting the operand. Test coverage spans bundler_edgecase.test.ts (six new itBundled cases), esbuild/lower.test.ts (TODO → real bundleErrors), and transpiler.test.js (a new dedicated test block plus un-commenting a previously disabled assertion).

Security risks

None. This adds a diagnostic to the JS parser; no new input parsing, no allocation on user-controlled sizes, no I/O, no privilege boundaries. The only runtime-visible effect is that invalid strict-mode input now fails at Bun's transpile time with a BuildMessage instead of at JSC parse time with a SyntaxError — both are fatal, so no behavior is loosened.

Level of scrutiny

Low-to-medium. The native diff is ~10 lines and mirrors esbuild's control flow exactly (check the operand's shape as parsed, before the visit rewrites it). I read the full body of mark_strict_mode_feature (p.rs:4218-4291): it unconditionally returns Ok(()), so .expect("unreachable") is a genuine invariant, and it is the exact pattern used at every other visit-pass call site (visit_expr.rs:186, visit/mod.rs:100, visit/mod.rs:621). The message text "delete" of a bare identifier composes with the existing ... cannot be used in strict mode / ... cannot be used with the ESM output format due to strict mode suffixes to match the error strings the tests assert. The check being placed before visit_expr_in_out is deliberate and correct per ES2024 §13.5.1.1 — the early error applies to the operand's syntactic form, not what it resolves to.

Other factors

The two expectPrinted → expectPrinted_ changes in the exponentiation test do not weaken what was being tested: the input and expected output are byte-identical, and the property under test (unary-before-** requires parens) is still asserted; only the helper's implicit export default wrapper — which now makes the input a strict-mode error — is removed. The lower.test.ts change sits inside describe.todo so it is not live in CI, but it replaces a stale TODO comment with real expectations and honestly notes the remaining with gap rather than papering over it. Test coverage hits both strictness triggers, all three output formats, parenthesized identifiers, and the full set of non-identifier operands that must remain valid. No CODEOWNERS apply to the changed paths, there are no outstanding third-party reviews, and the bug hunt exited on dry_streak.

[robobun]

^{Updated 5:41 PM PT - Aug 28th, 2026}

❌ @robobun, your commit 583cce0 has 1 failures in Build #108049 (All Failures):

• test/js/web/url/url.test.ts - code 1: TypeError: Invalid URL on 🍎 any x64

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🧪   To try this PR locally:

bunx bun-pr 40861

That installs a local version of the PR into your bun-40861 executable, so you can run:

bun-40861 --bun