Abstract methods don't correctly compile in .svelte files

[dooshiifox]

Describe the bug

Compiling an abstract class with an abstract method does not remove the abstract method from the JS output, instead just removing the abstract keyword and all types. This produces a runtime SyntaxError. This only occurs within .svelte components and not in normal TypeScript.

A temporary workaround is to simply remove the abstracts and define a method body that errors, or by extracting the class into a .svelte.ts file where this issue does not occur.

Reproduction

https://svelte.dev/playground/hello-world?version=5.19.9#H4sIAAAAAAAAA22PwW4CMQxEf8XyCSTUvW9ppaqXXvgCQkVI3BLIJlHsQFer_XcS6KGHniy9GY_HEwY9EPb4Qd5HuMbsLSzIOiG7xBV-OU-M_XZCGVPzNVD579ZbSk98IS-NHTTTf9zEIBSkxuCaTXZJwOvw_aJQWCEM0RZPryoo6Tr4BMdcCLTnCNGYkhmuTo6xCOwf1v0KnMCpsMCgz8QgR4ITQ7Wk6jKedKDc8vSBJWvTmGaGzfh-n1PT_qjDuKF6wS6WPVyis89Nn1VYd4-6tVt9Q-hHsJdcaN7NNwmrwrI4AQAA

Logs

System Info

System:
    OS: Linux 6.6 cpe:/o:nixos:nixos:25.05 25.05 (Warbler)
    CPU: (32) x64 13th Gen Intel(R) Core(TM) i9-13900HX
    Memory: 16.25 GB / 31.19 GB
    Container: Yes
    Shell: 5.2.37 - /nix/store/0irlcqx2n3qm6b1pc9rsd2i8qpvcccaj-bash-5.2p37/bin/bash
  Binaries:
    Node: 20.18.0 - /nix/store/z0ivg3dbal4sk5h0c7p1pfkxc7ff98ri-nodejs-20.18.0/bin/node
    npm: 10.8.2 - /nix/store/z0ivg3dbal4sk5h0c7p1pfkxc7ff98ri-nodejs-20.18.0/bin/npm
    pnpm: 9.12.3 - /nix/store/3xd8lpj86wbjkvxd2qjd28fd98gqsypc-pnpm-9.12.3/bin/pnpm
  Browsers:
    Chromium: 131.0.6778.204
  npmPackages:
    svelte: ^5.19.0 => 5.19.8

Severity

annoyance

[thes01]

I believe the abstract keyword is TypeScript-specific, so according to the documentation, you should modify the preprocessor setup to use such features.

[brunnerh]

interface would also be very TypeScript-specific. I think abstract methods fall within the category of erasable type annotations and could maybe be supported directly.