Unused parameters in nested function compiles incorrectly

[fong]

Describe the bug

When there is a state parameter in a function declaration and the state is not being used inside the function, the signal gets compiled to get() instead of get(x). The function fails to execute and throws an undefined type error in the browser console.

This seems to only apply to nested functions and on npm run build (works correctly on local dev). The error can also surface if the state parameter is conditionally used inside the function.

Error thrown in the browser console:

Blc3Cr0p.js:95 Uncaught TypeError: Cannot read properties of undefined (reading 'f')
    at get (DJF-AMb5.js:1268:22)
    at func1 (2.DzV7yYn4.js:21:13)
    at HTMLButtonElement.on_click_1 (2.DzV7yYn4.js:8:34)
    at HTMLDivElement.handle_event_propagation (Blc3Cr0p.js:72:16)

Workarounds:

• if parameter is unused, remove the unused parameter from the function declaration
• if parameter is used inside a conditional, access it somewhere up the chain so that it is used earlier
  e.g.

  // this will compile incorrectly  
  const func = (a, d) => {
    if (d) {
      a++;
      console.log(d);
    }
  };

  // this will work  
  const func = (a, d) => {
    if (a && d) {
      a++;
      console.log(d);
    }
  };

Reproduction

repo: https://github.com/fong/svelte-get-signal-undefined

• npm run build and then npm run preview will throw error
• npm run dev will work correctly

example build: https://svelte-get-signal-undefined.pages.dev/

source code:

  let a = $state(0);

  const func1 = () => {
    let d = a * a;
    if (d <= 4) {
      func2(a, d);
    } else {
      func3(a, d);
    }
  };

  // compiles to "func2(get(a), d);" - correct
  const func2 = (a, d) => {
    if (a && d) {
      console.log("func2", d);
    }
  };

  // compiles to "func3(get(), d);" - incorrect
  const func3 = (a, d) => {
    console.log("func3", d);
  };

compiles to:

  let a = state(0);
  const func1 = () => {
    let d = get(a) * get(a);
    if (d <= 4) {
      func2(get(a), d);
    } else {
      func3(get(), d);      <--- this is throwing the undefined TypeError
    }
  };
  const func2 = (a2, d) => {
    if (a2 && d) {
      console.log("func2", d);
    }
  };
  const func3 = (a2, d) => {
    console.log("func3", d);
  };

Logs

System Info

System:
    OS: macOS 15.2
    CPU: (12) arm64 Apple M2 Max
    Memory: 79.14 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.11.0 - ~/.nvm/versions/node/v22.11.0/bin/node
    npm: 10.9.0 - ~/.nvm/versions/node/v22.11.0/bin/npm
    pnpm: 9.15.4 - ~/Library/pnpm/pnpm
    bun: 1.1.34 - ~/.bun/bin/bun
  Browsers:
    Chrome: 132.0.6834.160
    Safari: 18.2
  npmPackages:
    svelte: ^5.0.0 => 5.19.6

Severity

annoyance

[fong]

Issue is to do with Rollup 4.34.0. Resolved in 4.34.2 and bumping up in devDependencies

rollup/rollup#5826

[elliott-with-the-longest-name-on-github]

Closing as external, thanks for crossposting back the solution ❤