Svelte 5: Assignment to a private non-reactive property of a class in a .svelte.ts file will throw an error

[AhmadMayo]

Describe the bug

1- create a .svelte.ts or .svelte.js file
2- create a class with a private non-reactive property (doesn't matter if it's initialized or not)
3- assign a value to the private (in the constructor or in a method)
4- import the class into a svelte file and initialize it

A compilation error is thrown

[vite] Error when evaluating SSR module /src/routes/+page.svelte: failed to import "/src/routes/test.svelte.ts"
|- TypeError: Cannot read properties of undefined (reading 'includes')
    at validate_assignment (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/2-analyze/validation.js:605:35)
    at AssignmentExpression (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/2-analyze/validation.js:550:3)
    at go (file:///$test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:49:5)
    at visitor (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:64:4)
    at Object.next (file:///test-svelte-5/node_modules/zimmerframe/src/walk.js:106:21)
    at go (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:45:29)
    at next (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:55:7)
    at _ (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/scope.js:666:4)
    at go (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:49:5)
    at visitor (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:64:4)

Copying the same code to a .svelte file doesn't reproduce the error.

Note that I created the application using npm create svelte@next test-svelte-5, and then I updated svelte to next. I don't know if I need to update any dependencies other than svelte or not.

Reproduction

reproduction

Logs

[vite] Error when evaluating SSR module /src/routes/+page.svelte: failed to import "/src/routes/test.svelte.ts"
|- TypeError: Cannot read properties of undefined (reading 'includes')
    at validate_assignment (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/2-analyze/validation.js:605:35)
    at AssignmentExpression (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/2-analyze/validation.js:550:3)
    at go (file:///$test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:49:5)
    at visitor (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:64:4)
    at Object.next (file:///test-svelte-5/node_modules/zimmerframe/src/walk.js:106:21)
    at go (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:45:29)
    at next (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:55:7)
    at _ (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/scope.js:666:4)
    at go (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:49:5)
    at visitor (file:///test-svelte-5/node_modules/svelte/src/compiler/phases/visitors.js:64:4)

System Info

System:
    OS: Linux 5.15 Ubuntu 22.04.3 LTS 22.04.3 LTS (Jammy Jellyfish)
    CPU: (12) x64 Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz
    Memory: 4.49 GB / 7.67 GB
    Container: Yes
    Shell: 5.8.1 - /usr/bin/zsh
  Binaries:
    Node: 20.9.0 - ~/.asdf/installs/nodejs/20.9.0/bin/node
    npm: 10.1.0 - ~/.asdf/plugins/nodejs/shims/npm
  npmPackages:
    svelte: ^5.0.0-next.1 => 5.0.0-next.1

Severity

blocking an upgrade

[trueadm]

Could this be because you've used the private keyword in a TS module? What happens if you change it to another name?

[AhmadMayo]

Same error. In my implementation it’s more complex than this, with more members and other names. I kept removing different pieces of the code to get to the simplest reproduction code. Things that didn’t cause the error:

• Public non-reactive members
• Mixing reactive and non-reactive members
• Mixing private and public reactive members
• Mixing public and private reactive members with public non-reactive members
• Any combination of public, private, reactive, and non-reactive members in a script tag in a svelte file

[Rich-Harris]

Not sure what I'm looking at here — where am I supposed to see the error? Not seeing it in the CodeSandbox. If you can create a repro with https://svelte-5-preview.vercel.app (note: no TypeScript) it'll be much easier to get to the bottom of

[trueadm]

@AhmadMayo You can create js files in the repl though, just add a module and rename it to be something.svelte.js.

[AhmadMayo]

Oh. thanks @trueadm . @Rich-Harris here's a reproduction link

[trueadm]

Ah looks like #9394 fixes this same issue. However that was pending @Rich-Harris's work on #9435 which is marked as WIP still.

[Rich-Harris]

Looks like it is indeed fixed — closing