[🐞] Optimizer: captured variable in `$()` parameter default throws ReferenceError

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
52/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
javascript, rust, typescript
Domain
frontend, performance

Research direction

Start by reproducing the failure with the supplied createOptimizer example and inspect the Rust and TypeScript optimizer paths that transform captured variables in parameter defaults. The issue names no source files or tests, so trace the transform from the emitted function body and add a regression test for a default parameter using an outer capture. Done means the QRL resolves the omitted argument from the capture without a ReferenceError in both optimizers.

Written by the indexing model from the issue text.

Description

Which component is affected?

Qwik Optimizer (rust)

Describe the bug

When a $() function uses a captured outer variable inside a parameter default (in @qwik.dev/optimizer 2.1.0-rc.10, both Rust and TS optimizers), the optimizer unpacks the capture inside the extracted function body instead of making it available in the parameter scope. Calling the QRL without that argument throws ReferenceError at runtime.

import { createOptimizer } from "@qwik.dev/optimizer";

const optimizer = await createOptimizer();
const { modules } = await optimizer.transformModules({
  input: [{
    path: "test.js",
    code: `import { $ } from "@qwik.dev/core";
export const make = (x) => $((a = x) => a);`,
  }],
  entryStrategy: { type: "inline" },
  mode: "prod",
});

console.log(modules[0].code);

This emits:

q_s_wVGAgzOzrJg.s((a = x)=>{
    let x = _capturesObj._[0];
    return a;
});
export const make = (x)=>q_s_wVGAgzOzrJg.w([x]);

Calling await make(3)() throws ReferenceError: x is not defined instead of resolving to 3, because parameter default initializers evaluate in the parameter scope before the function body's let x = _capturesObj._[0] declaration is reached.

Reproduction

https://gist.github.com/trackoor/0313dd231eecdf4ed1c356336f1a1b71

Steps to reproduce

Run npm install @qwik.dev/[email protected] @qwik.dev/[email protected] and node repro.mjs.

System Info
  System:
    OS: macOS 26.3
    CPU: (14) arm64 Apple M3 Max
    Memory: 1.59 GB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.14.1 - /Users/hanzhi/.nvm/versions/node/v24.14.1/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 11.11.0 - /Users/hanzhi/.nvm/versions/node/v24.14.1/bin/npm
    pnpm: 11.24.0 - /opt/homebrew/bin/pnpm
  Browsers:
    Chrome: 156.0.8078.80
    Safari: 26.3
  npmPackages:
    @qwik.dev/core: 2.1.0-rc.10 => 2.1.0-rc.10
    @qwik.dev/optimizer: 2.1.0-rc.10 => 2.1.0-rc.10
Additional Information

Credit: https://ucsbplse.github.io/

Dominant language
TypeScript
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Forks
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Avg merge
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Merged PRs (30d)
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