Symbols in component module scripts are remade during HMR

[ottomated]

Describe the bug

With the following component:

<script module lang="ts">
  const KEY = Symbol('context-key');
  export function set_context() {
    setContext(KEY, /* ... */);
  }
  export function get_context() {
    return getContext<T>(KEY);
  }
</script>
<!-- ... -->

Every time HMR happens, KEY changes. This means that if set_context is called in a parent layout, all the children layouts will be unable to call get_context because the symbol is different.

This causes a hard-to-track-down error.

---

This might be impossible to solve outright, but a possible measure could be something like this:

export function getContext(key) {
  const context_map = get_or_init_context_map('getContext');
  if (DEV && typeof key === 'symbol') {
    for (const existing_key of context_map.keys()) {
      if (typeof existing_key !== 'symbol') continue;
      if (key === existing_key) continue;
      if (existing_key.description === key.description) {
        // Log some kind of warning that two symbols with the same name are being used
      }
    }
  }
  const result = /** @type {T} */ (context_map.get(key));
  return result;
}

Reproduction

REPL - will need to be ran locally because the playground doesn't support HMR.

Logs

N/A

System Info

System:
    OS: Linux 6.14 Fedora Linux 41 (Forty One)
    CPU: (32) x64 AMD Ryzen 9 5950X 16-Core Processor
    Memory: 42.24 GB / 62.69 GB
    Container: Yes
    Shell: 3.7.1 - /bin/fish
  Binaries:
    Node: 22.15.1 - ~/.volta/tools/image/node/22.15.1/bin/node
    Yarn: 4.3.1 - ~/.volta/tools/image/yarn/4.3.1/bin/yarn
    npm: 10.9.2 - ~/.volta/tools/image/node/22.15.1/bin/npm
    pnpm: 10.10.0 - ~/.volta/bin/pnpm
    bun: 1.2.4 - ~/.bun/bin/bun
  Browsers:
    Chrome: 136.0.7103.92
  npmPackages:
    svelte: ^5.33.4 => 5.33.4

Severity

annoyance

[Mr-Zero88]

That is intended behavior, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/for

[ottomated]

@Mr-Zero88 I know how symbols work. I'm saying it's unintuitive that changing an unrelated area of your code leads to symbols changing and breaking things, and suggesting a DX improvement.

[dm-de]

don't use script module
use global.js or something

#14398 (comment)