[robobun]

Problem

• Most npm packages ship Babel or tsc output. bun build --minify keeps the lowered shapes: var n;return(n=r.x)==null?void 0:n.y, typeof e.b<"u", n.count=n.count+1, o.x=o.x||1, the arguments copy loop. oxc, terser and esbuild print the native syntax, which is smaller.
• Number printing only shortened 1e4 to 1e9. 1e21 printed as 22 digits, 1e100 as 101 digits, 1e300 as 301 digits (Bun always expands numbers in scientific notation even if the scientific form is shorter #29311), 5e-324 as 300 zeros, 0.5 as 0.5, 1000 as 1000, 160000 as 160000 (Bun build converts some numbers to take up more chars instead of less #31074) (src/js_printer/lib.rs, print_non_negative_float).

Fix

• Printer: print esbuild's shortest number form in JS output (exponent or plain, .5 and hex for large integers under --minify-whitespace). Integers below 2^53 come straight from itoa; other values use WTF's double-conversion through the existing WTF__dtoa binding, not core::fmt. JSON output (package.json rewrites) keeps the JSON.stringify form. Also fix a bug where new (a?.b)() and (a?.b)`x` printed without the parentheses the grammar needs.
• Parser, under minify_syntax (new src/js_parser/visit/mangle.rs): { "2": x } to { 2: x } (objects, classes, a["2"]); typeof x.y < "u" and typeof local > "u" to x.y !== void 0 / local === void 0 (an unbound identifier keeps the typeof guard); x = x OP y to x OP= y, and x = x || y to x ||= y (also &&=, ??=) for a declared identifier; a === null || a === void 0 to a == null (a declared identifier, a temp store or this: an unbound global could be an accessor that is read twice); esbuild's a == null ? void 0 : a.b to a?.b and a != null ? a : b to a ?? b; when bundling, 16 or more string literals in an array to "a.b".split(".").
• The lowered forms hold the operand in a temp: (n = r.x) == null ? void 0 : n.y first becomes (n = r.x)?.y. When the function body is fully visited, a var temp whose every use is one of these stores collapses to r.x?.y and its var n is dropped. The same end-of-body check turns Babel's for (var e = arguments.length, t = Array(e), r = 0; r < e; r++) t[r] = arguments[r] into var t = [...arguments] (with .slice(k) for rest parameters) when e and r have no other use. A temp that another symbol links to (a nested var n redeclaration, whose reads count on its own symbol) is never dropped: the parser now marks such symbols with SymbolFlags::IS_LINK_TARGET.
• Verified: test/bundler/transpiler/transpiler.test.js (minify peepholes, 11 cases), test/bundler/bundler_minify.test.ts (3 runtime tests, plus the number cases of Bun always expands numbers in scientific notation even if the scientific form is shorter #29311 and Bun build converts some numbers to take up more chars instead of less #31074 in minify/ShortestNumberForm and minify/IntegerTrailingZeroScientificNotation), and the full test/bundler suite. A corpus bundle of lodash, typescript, yargs, axios and uuid shrank from 4068136 to 4031455 bytes (1145334 to 1136802 gzipped) with identical output.

Fixes #29311

Background

• use_count_estimate counts every visited reference to a symbol. For a var declared in a function body it is final once that body's statements are visited, the same fact the existing let/const single-use inlining in visit_stmts relies on. The temp elimination checks the count then, so it also runs in Bun.Transpiler and bun run, which enable minify_syntax for the bun target.
• A chain root that calls the bare temp keeps this undefined: (n = r.x) == null ? void 0 : n() becomes (0, r.x)?.(). A conditional that is itself called, used as a template tag, or deleted is left alone for the same reason, and so is a branch with a parenthesized inner chain ((a.b?.c).d must still throw when a.b is null).
• x ||= y skips the write when x is truthy, so a property setter, a global accessor or the TypeError of a const could observe the difference, and it names an anonymous function on its right (x ||= function() {} sets name to "x"). The rewrite therefore only applies to a mutable declared identifier and a right side that is not an anonymous function or class, as in oxc. o.x = o.x || 1 stays as written. A computed key must be a literal: a[k] = a[k] + 1 converts k with ToPropertyKey twice and a[k] += 1 once, which an object in k observes through its toString.
• module and exports are declared for every file but only exist at runtime when the file is wrapped as CommonJS, so typeof module > "u" keeps its guard.

Notes

• x = x + 1 is not turned into ++x: ++ coerces a string operand to a number while += 1 concatenates.
• The arguments loop rewrite needs strict mode, as in oxc: a sloppy function can reassign arguments to a non-iterable.
• Module-level temps (var _a at the top of a module) get the (t = x)?.y form only. Parts are visited one at a time, so the final count is not known when the statement list of one part is finished.
• test/bundler/bundler_npm.test.ts (npm/ReactSSR) samples generated columns of the minified react bundle and pins its size; both moved with the smaller output (221989 to 221411 bytes). bundler_bytecode_portable.test.ts pins the bundle hashes, bundler_edgecase.test.ts pinned the 309-digit form of -1.79e308 and bun-build-api.test.ts the quoted numeric keys.
• Related open PRs: Keep required parens around a new expression callee #31813 (parens around a new callee, a wider fix of the same printer class), bundler: inline single-use vars, drop unused locals, and re-merge after each edit #41158 (bundler var inlining with the same IS_LINK_TARGET idea) and minify: port esbuild's statement-level syntax passes #41159 (esbuild's statement-level passes, also adds src/js_parser/visit/mangle.rs). Whichever lands second needs a merge of the shared pieces.
• Minify: pick the shorter of decimal vs scientific for floats #29312 and Print integers in shorter scientific form when possible #31076 were earlier takes on the shortest number form. Both are closed in favor of this PR. Their cases (1e300, 1e-7, 15e19, 16e4, 2048e3, 32e7, the 100 tie) are in bundler_minify.test.ts here, and the output matches esbuild 0.21.5 byte for byte, with and without --minify. js_printer: print integers as plain digits in JSON output #41184 (plain digits in JSON output, the bun pm pkg set side of the same function) stays open.

---

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/bundler/bundler_bytecode_portable.test.ts, test/bundler/bun-build-api.test.ts

[robobun]

^{Updated 11:53 AM PT - Sep 2nd, 2026}

✅ @robobun, your commit 2b10b62c30b0422e6a248782a0d773123dc86672 passed in Build #109427! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 41156

That installs a local version of the PR into your bun-41156 executable, so you can run:

bun-41156 --bun

[robobun]

Status: rebased on main (7c18e0d), review comments addressed, head is 1a73b9e.

CI note: test/cli/run/require-cache.test.ts times out on the x64-asan lane. The same timeout shows on unrelated PR builds on that lane (109373, 109363, 109347, 109316), so it is reported separately. Every other red lane passed on retry.

How it was verified:

• bun bd test test/bundler/transpiler/transpiler.test.js (new minify peepholes block, all fail on the released bun and pass with this branch)
• bun bd test test/bundler/bundler_minify.test.ts (4 new tests that run the rewritten code and compare output) and test/cli/install/bun-pm-pkg.test.ts (JSON numbers stay plain)
• full test/bundler suite with the debug build; the remaining failures are 5 s timeouts on bytecode and compile tests in the debug build, and compile/HelloWorldWithProcessVersionsBun, which compares process.versions.bun to Bun.version with -debug stripped and fails on any debug build.
• a corpus bundle (lodash, typescript, yargs, axios, uuid) with --minify --target=bun runs with identical output: 4068136 to 4031455 bytes, 1145334 to 1136802 gzipped.

Changes since the first push:

• number printing uses itoa for integers and WTF's double-conversion for the rest, no core::fmt; JSON output keeps the JSON.stringify form (bun pm pkg set port=3000 writes 3000)
• ||=, &&=, ??= are only produced for a declared identifier; o.x = o.x || 1 stays as written
• a[k] = a[k] + 1 stays as written unless k is a literal: JSC converts an object key with toString twice for the plain form and once for +=
• x = x || y is not rewritten for a const or import binding, nor when y is an anonymous function or class (||= would name it)
• a === null || a === void 0 only folds for a declared identifier, a temp store or this
• an inlined enum key keeps its brackets when the value is negative or non-finite
• a conditional used as a template tag is not turned into an optional chain, and a parenthesized inner chain is never merged into the recovered one
• a temp that a nested var redeclaration links to is never dropped (SymbolFlags::IS_LINK_TARGET)
• the .split rewrite only runs when bundling; temp-store candidates are grouped by symbol
• bundler_bytecode_portable.test.ts and bundler_npm.test.ts snapshots follow the new output

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Walkthrough

Summary

Syntax minification now rewrites property keys, checks, assignments, optional chains, argument-copy loops, string arrays, and numeric literals. Parser state defers function-body rewrites until usage counts are finalized. Symbol linking now marks link targets. Tests cover output and runtime behavior.

Changes

Syntax minification

Layer / File(s)	Summary
Deferred mangle engine src/js_parser/p.rs, src/js_parser/visit/mangle.rs	The parser tracks function-body depth, template-tag context, and deferred candidates. The mangle engine applies validated rewrites for temporary stores, argument-copy loops, property keys, compound assignments, nullish checks, optional chains, and long string arrays.
Visitor transformation wiring src/js_parser/visit/mod.rs, src/js_parser/visit/visit_binary.rs, src/js_parser/visit/visit_expr.rs, src/js_parser/visit/visit_stmt.rs	Visitors invoke the new transformations during syntax minification and apply deferred candidates after each function body.
Symbol link-target tracking src/ast/symbol.rs, src/js_parser/p.rs, src/js_parser/lower/lower_esm_exports_hmr.rs	Merged and replaced symbols are marked as link targets. Generated accessors expose the new symbol flag.
Shorter printer output src/js_printer/lib.rs	Number printing now selects shorter decimal, exponent, or hexadecimal forms. Numeric spacing and optional-chain parenthesization use shared handling.
Minification regression coverage test/bundler/bundler_minify.test.ts, test/bundler/transpiler/transpiler.test.js, test/bundler/bun-build-api.test.ts, test/bundler/bundler_edgecase.test.ts, test/bundler/bundler_npm.test.ts, test/bundler/bundler_bytecode_portable.test.ts, test/cli/install/bun-pm-pkg.test.ts	Tests cover transformed syntax, runtime semantics, conservative bailouts, numeric serialization, and updated bundler snapshots.

Suggested reviewers: jarred-sumner

Merge Risk: 🔵 Low · up to d15ff

The minifier introduces shorter rewrites that can change observable behavior for unbound global accessors and constant or imported bindings in narrow cases. The risk is bounded and the PR remains mergeable with explicit owner awareness or follow-up on these edge cases.

🚥 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.
Title check	✅ Passed	The title clearly summarizes the main changes: optional-chain re-sugaring, shorter number output, and minifier peephole optimizations.
Description check	✅ Passed	The description explains the problem, implementation, safeguards, testing, and observed size improvements. It does not use the exact template headings, but it provides the required information and is …

Full details: Description check

Explanation

The description explains the problem, implementation, safeguards, testing, and observed size improvements. It does not use the exact template headings, but it provides the required information and is sufficiently complete.

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In `@src/js_parser/visit/mangle.rs`:
- Around line 838-843: Replace the repeated full-candidates scans in the
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candidate while preserving existing matching behavior.
- Around line 261-268: Update the logical/nullish compound-assignment guard in
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• src/js_parser/p.rs
• src/js_parser/visit/mangle.rs
• src/js_parser/visit/mod.rs
• src/js_parser/visit/visit_binary.rs
• src/js_parser/visit/visit_expr.rs
• src/js_parser/visit/visit_stmt.rs
• src/js_printer/lib.rs
• test/bundler/bun-build-api.test.ts
• test/bundler/bundler_edgecase.test.ts
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[Jarred-Sumner]

@robobun rebase

[robobun]

Rebased onto main (7c18e0d) and force-pushed as d440fa5. The review threads above are addressed in the same push: number printing now goes through itoa and WTF's double-conversion instead of core::fmt, the logical compound assignments only apply to declared identifiers, an inlined enum key keeps its brackets when negative or non-finite, and a conditional used as a template tag is left alone.

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Inline comments:
In `@src/js_parser/visit/visit_expr.rs`:
- Around line 1695-1697: Update the guard around mangle_string_array_to_split in
the expression visitor to require both minify_syntax and p.options.bundle, while
preserving the existing assign_target == AssignTarget::None condition.

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Inline comments:
In `@src/js_parser/visit/mangle.rs`:
- Around line 975-996: Update the doc comment for remove_uninitialized_decl to
document that it only scans top-level statements and may leave uninitialized var
declarations inside nested blocks when their stores are collapsed; do not change
the function’s behavior.

In `@src/js_printer/lib.rs`:
- Line 944: Define a private constant adjacent to FLOAT_BUF_LEN for the required
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• src/js_parser/visit/visit_expr.rs
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In `@src/js_parser/visit/mangle.rs`:
- Around line 223-227: Update the EIndex repeatability check in the
compound-assignment folding logic to accept only literal property keys, rather
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optional-chain checks, but replace the index is_repeatable_operand condition
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In `@src/js_parser/visit/mangle.rs`:
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[robobun]

Heads up: #41159 adds src/js_parser/visit/mangle.rs with mangle_stmts, mangle_if, mangle_if_expr, values_look_the_same and try_to_insert_optional_chain, and moves the tail of visit_stmts into it. This PR adds the same file path with its own mangle_if_expr. Whichever lands second rebases and drops the duplicate helpers.

[claude]

Re-reviewed after 1a73b9e and didn't find any new bugs; the earlier inline findings all look addressed in the current diff. Given the size of the new visit/mangle.rs pass and the number of semantics-preserving guards it depends on, a human pass over the transform rules is still worthwhile.

What was reviewed:

• mangle_object_property_key/mangle_class_property_key now write back the unwrapped key so the printer's negative/non-finite recompute guard still fires.
• mangle_if_expr gates on is_call_target || is_template_tag via the new p.template_tag, and find_chain_root bails when a plain access sits above an inner ?. start.
• Logical compound-assignment folding now requires a bare declared EIdentifier target; member targets keep the plain assignment.
• print_non_negative_float branches on IS_JSON to keep the JSON.stringify form for package.json rewrites, and the now-unused pow10_exp_1e4_to_1e9 is removed.

Extended reasoning...

Overview

Two commits: a rewrite of the JS printer's number formatting to emit shortest round-tripping forms (with a separate IS_JSON branch that preserves JSON.stringify output for package.json rewrites), and a new 1,176-line src/js_parser/visit/mangle.rs implementing minify-syntax peepholes — computed-key simplification, typeof x < "u" folding, compound-assignment recovery, a == null collapsing, optional-chain re-sugaring with per-function-body temp elimination, and the Babel arguments copy-loop rewrite. Supporting changes add SymbolFlags::IS_LINK_TARGET so aliased symbols aren't dropped based on use_count_estimate, plus template_tag/fn_body_depth/mangle_candidates state on P. ~550 lines of new tests across bundler_minify.test.ts and transpiler.test.js, snapshot updates in four other bundler test files, and a bun pm pkg regression test.

Security risks

None identified. This is compiler/minifier output shaping with no auth, network, filesystem, or crypto surface. The failure mode is emitting semantically different JavaScript, which is a correctness concern rather than a security one.

Level of scrutiny

High. Each transform in mangle.rs encodes a non-obvious equivalence claim (short-circuit assignment vs. setter observability, this binding through call/tag/delete targets, optional-chain boundary preservation across parenthesized inner chains, ToPropertyKey call counts, sloppy-mode arguments reassignment, IS_LINK_TARGET soundness for aliased var redeclarations). The prior review rounds surfaced six real defects in exactly these guards, all of which the current revision addresses — but the density of edge cases and the fact that this ships in bun run's default minify_syntax path for the bun target argue for a maintainer read of the transform rules before merge.

Other factors

This is the first automated pass after commit 1a73b9e, which landed fixes for every open inline thread from earlier runs (inlined-enum key unwrapping, template-tag this preservation, logical OP= restricted to bare identifiers, inner-chain boundary bail-out, JSON number formatting, dead helper removal). No candidate issues were investigated and ruled out beyond re-verifying those fixes in the current source. Test coverage is substantial and includes runtime assertions, but the PR description itself lists four overlapping open PRs touching the same files, so coordination on merge order is a human call.

[robobun]

Folded the number tests of #29312 and #31076 into this PR (commit 2b10b62, test only, no source change) and added Fixes #29311 to the body. Both PRs are now closed in favor of this one.

What I checked on this branch:

• bun build --minify of 1e300, 1e-7, 0.5, 1e21, 5e-324, 160000, 50000, 2048000, 320000000, 1000, 1e4 prints 1e300,1e-7,.5,1e21,5e-324,16e4,5e4,2048e3,32e7,1e3,1e4. Without --minify it prints the same forms with 0.5 instead of .5. Both match esbuild 0.21.5 byte for byte.
• The new tests minify/IntegerTrailingZeroScientificNotation, minify/ShortestNumberForm and minify/ShortestNumberFormWithoutMinify in test/bundler/bundler_minify.test.ts fail on bun 1.4.1 and pass here. The full file passes (58 tests).
• The regression test of Minify: pick the shorter of decimal vs scientific for floats #29312 was not taken as is. Two of its expectations differ from esbuild: it wanted 1.5e20 where esbuild and this PR print 15e19, and it wanted unminified output to keep the long decimal form. The values it covers are in minify/ShortestNumberForm.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[oddharsh]

Measured this PR's CI build (2b10b62) on a real site's client scripts, the same way as #41159 (comment) and #41158 (comment).

This PR shares #41158's merge base (7c18e0d), so the control is the same bracket: the canary binary before it (561641b) and bun@1.4.0-canary.20260902.1 (d6af50f) produce byte-identical output on every input below.

Each peephole I tried fires as described (Bun.build({ minify: true, format: "esm" }), function bodies shown):

input	base	this PR
var n;return(n=r.x)==null?void 0:n.y	unchanged	return n.x?.y
return typeof e.b<"u"	unchanged	return t.b!==void 0
n=n+1	unchanged	n+=1
x=x||1	unchanged	t||=1
return a===null||a===void 0	unchanged	return n==null
[0.5,1000,160000,1e21,0.16]	[0.5,1000,160000,1000000000000000000000,0.16]	[.5,1e3,16e4,1e21,.16]
{"2":1}	unchanged	{2:1}

On the site (aadhar.sh, 26 hand-written browser scripts, each minified on its own):

	base	this PR
raw bytes	368,417	368,050 (-0.10%)
brotli q11	128,394	128,211 (-0.14%)

Most of that comes from the number printer. Across the output, numbers with a leading 0. went from 99 to 21 and integers ending in 000 from 31 to 0. The parser peepholes barely fire here: ?. went from 19 to 20, logical assignments from 32 to 37, and ==null comparisons from 217 to 204. That fits the PR's own framing, since these scripts are authored as modern TypeScript and already use the native syntax; the gains it reports on lodash and typescript come from Babel and tsc output, which this site doesn't ship.

For comparison on the same files: #41158 saves 97 B brotli, #41159 saves 255 B, and oxc-minify 0.156.0 is 2,408 B ahead of the base.

These are size numbers only: I didn't check how the minified scripts behave.