TS doesn't understand `mount()` return type

[Bilge]

Describe the bug

If I assign the result of mount(Foo) to a variable typed Foo, TS gets confused and emits:

TS2322: Type '{ $on?(type: string, callback: (e: any) => void): () => void; $set?(props: Partial<Record<string, any>>): void; } & Record<string, any>' is not assignable to type 'Comp | undefined'.

Nevertheless, svelte-check finds no errors and my editor understands the return type of mount(Foo) is Foo. It is only TS compilation via webpack that is problematic.

Reproduction

new class {
    private foo?: Foo;

    constructor() {
        this.foo = mount(Foo, {target: document.body}); // TS2322
    }
}

Logs

Here are my tsconfig.json files.

{
  "extends": "..",
  "compilerOptions": {
    "verbatimModuleSyntax": true,
  },
}

(This is ..)

{
  "extends": "@tsconfig/recommended/tsconfig.json",
  "compilerOptions": {
    "target": "ES2022",
    "module": "ES2020",
    "moduleResolution": "node",
    "moduleSuffixes": ["", ".svelte"],
    "allowJs": true,
  }
}

Relevant portion of webpack.config.js.

module.exports = (env, options) => {
    return {
        module: {
            rules: [
                {
                    test: /\.tsx?$/,
                    loader: 'ts-loader',
                    options: {
                        compilerOptions: {
                            sourceMap: !prod,
                        },
                    },
                    resolve: {
                        extensions: ['.ts', '.svelte.ts', '.js'],
                    },
                },
                {
                    test: /\.svelte$/,
                    loader: 'svelte-loader',
                    options: {
                        emitCss: true,
                        preprocess: sveltePreprocess({
                            sourceMap: !prod,
                        }),
                        compilerOptions: {
                            dev: !prod,
                        },
                    },
                    resolve: {
                        extensions: ['.ts'],
                    },
                },
            ],
        },
    };
}; 

System Info

System:
    OS: Linux 5.15 Ubuntu 20.04.3 LTS (Focal Fossa)
    CPU: (8) x64 Intel(R) Core(TM) i7-9700K CPU @ 3.60GHz
    Memory: 8.36 GB / 11.69 GB
    Container: Yes
    Shell: 5.0.17 - /bin/bash
  Binaries:
    Node: 22.12.0 - /run/user/1000/fnm_multishells/1717_1736443941005/bin/node
    Yarn: 1.22.22 - /run/user/1000/fnm_multishells/1717_1736443941005/bin/yarn
    npm: 10.9.0 - /run/user/1000/fnm_multishells/1717_1736443941005/bin/npm
  npmPackages:
    svelte: ^5.0.0 => 5.16.1
    webpack: ^5.11.0 => 5.97.1

Severity

blocking all usage of svelte

[7nik]

The TS's type ({ $on?(...): () => void; $set?(...): void; } & Record<string, any>) is the correct one.
The component constructor (Foo) and component instance (returned by mount) are entirely different things.

[webJose]

@Bilge so as to try to clear things up: Svelte v5 components are no longer classes and instead they are functions. So your private foo not be storing an instance of Foo since it is no longer a class. Furthermore, what mount() returns is an object with the component's exports. If the component exports nothing, then you'll get an empty object.

[Bilge]

OK? But I need some way to type my local variable such that I know what it exports and what methods I can call on it.

[webJose]

Don't type it explicitly. TypeScript will infer the type from the construction assignment.

[Bilge]

lol, no.

TS7008: Member foo implicitly has an any type.

[webJose]

Works fine for me:

[Bilge]

Yeah, that's because you don't have strict mode on. It's part of @tsconfig/recommended.

[webJose]

Eh, apparently not the case. I checked the configuration, and by default, strict mode seems to be set:

I also explicitly added strict: true to tsconfig.json and restarted VS Code's TypeScript server and it still worked.

[7nik]

In Svelte 5, the instance type is ReturnType<Foo>.

my editor understands the return type of mount(Foo) is Foo

You also need to fix this

[dummdidumm]

You need to disable webpack's type checking, it does not know about the exact value of Svelte component exports: https://stackoverflow.com/questions/60278796/how-to-ignore-typescript-errors-when-compile-by-webpack-production-mode

[Jojoshua]

You need to disable webpack's type checking, it does not know about the exact value of Svelte component exports: https://stackoverflow.com/questions/60278796/how-to-ignore-typescript-errors-when-compile-by-webpack-production-mode

I'm now looking at migrating to svelte 5 and I'm experiencing this same issue. Although I also use webpack + fork-ts-checker-webpack-plugin so TS errors are reported back I also see the same errors are reported just doing npx tsc --noEmit which I would think takes webpack out of the equation right?