fix(es/compat): Fix parameter default value evaluation order with object rest #11352

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Pull request overview

This PR fixes parameter default value evaluation order issues when using object rest patterns in function parameters (addresses #9198). The implementation introduces a new ParamCollector struct that transforms complex parameter patterns into simpler temporary parameters, then destructures them in the function body using array destructuring to preserve proper evaluation order.

Key changes:

Refactored parameter handling to use ParamCollector for collecting and transforming parameters with object rest patterns
Changed temporary variable naming from _param to indexed names like _0, _1, etc.
Changed generated variable declarations from var to let for better scoping semantics

Reviewed changes

Copilot reviewed 48 out of 48 changed files in this pull request and generated 5 comments.

File	Description
crates/swc_ecma_compat_es2018/src/object_rest.rs	Core refactoring: added `ParamCollector` struct and rewrote `visit_mut_function`, `visit_mut_arrow_expr`, and `visit_mut_constructor` to use the new parameter collection approach
Multiple test output files	Updated expected outputs reflecting the new transformation strategy with indexed parameter names and `let` declarations
crates/swc/tests/fixture/issues-9xxx/9198/*	New test cases specifically for issue #9198 demonstrating the fix for parameter evaluation order
crates/swc/tests/tsc-references/*.js	Updated TypeScript compiler reference outputs to match the new transformation patterns

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crates/swc/tests/tsc-references/objectRestParameterES5.2.minified.js

crates/swc/tests/tsc-references/objectRestParameterES5.1.normal.js

crates/swc/tests/tsc-references/objectRestParameter.2.minified.js

crates/swc/tests/tsc-references/objectRestParameter.1.normal.js

crates/swc/tests/fixture/issues-9xxx/9198/2/output/index.js

magic-akari · 2025-12-04T13:36:24Z

@kdy1, I've identified a destructuring evaluation order bug introduced by #11337. Please hold off on publishing a new swc release until this PR is merged.

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crates/swc_ecma_compat_es2018/src/object_rest.rs

…atterns

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Copilot · 2025-12-05T05:51:56Z

crates/swc_ecma_compat_es2018/src/object_rest.rs

+        let original_pat = match pat {
+            Pat::Rest(rest_pat) => mem::replace(&mut *rest_pat.arg, temp.clone().into()),
+            Pat::Assign(pat) => {
+                let init = AssignPat {
+                    span: DUMMY_SP,
+                    left: Box::new(temp.clone().into()),
+                    right: Expr::undefined(DUMMY_SP),
+                };
+                mem::replace(pat, init).into()
+            }
+            pat => mem::replace(pat, temp.clone().into()),
+        };
+
+        self.collected_pats.push(original_pat);
+        self.temp_exprs.push(temp.as_arg());
+    }


When collecting rest parameters (e.g., ...args), the temp variable is added as a regular array element instead of being spread. For example, in function x1(a, b = a, { c, ...d }, ...{ e }), the generated code uses [_1, _2, _3] but it should be [_1, _2, ..._3] to properly spread the rest parameter.

The issue is in line 1005 where temp.as_arg() is used. For rest parameters, we need to use a spread element instead of a regular argument.

Copilot · 2025-12-05T05:51:57Z

crates/swc_ecma_compat_es2018/src/object_rest.rs

+            // 2. Visit pattern
+            declarator.name.visit_mut_with(self);


[nitpick] The comment says "Visit pattern" but this happens after the init has been visited and before the pattern is processed by the lowering logic. This could lead to confusion about when pattern transformations happen. Consider clarifying that this visits any nested patterns within the declarator name, or move this comment to be more specific about what it's doing.

kdy1 · 2025-12-05T10:11:03Z

Thanks!

magic-akari added 2 commits December 4, 2025 18:38

fix(es/compat): Fix parameter default value evaluation order with obj…

e810f84

…ect rest

chore: update test cases

2df37b3

chore: update test cases

118e61e

fix(es/compat): Preserve function length

b2d5ac7

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magic-akari added 3 commits December 5, 2025 12:39

fix(es/compat): Fix variable definition order in nested object rest p…

13b848b

…atterns

chore: update test cases

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perf

fb9aa12

magic-akari force-pushed the fix/issue-9198 branch from a0c61aa to fb9aa12 Compare December 5, 2025 04:46

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fix: swc_ast_unknown

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Create little-grapes-attend.md

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kdy1 requested a review from a team as a code owner December 5, 2025 10:10

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kdy1 merged commit 2ebb261 into swc-project:main Dec 5, 2025
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kdy1 added this to the Planned milestone Dec 5, 2025

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fix(es/compat): Fix parameter default value evaluation order with object rest #11352

fix(es/compat): Fix parameter default value evaluation order with object rest #11352

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