There is an issue with the conversion of TypeScript Classes, which does not meet expectations and results in unexpected code running outcomes.

[MyDataHouse]

Reproduction link or steps

The results of class conversion by tsdown differ from those converted by TypeScript, causing problems when I migrate my project to tsdown.

The main issue is that tsdown retains uninitialized properties during conversion, whereas TypeScript does not. This makes it impossible to read properties with the same name on the prototype via the prototype chain. I hope that the results of tsdown's TypeScript conversion can be consistent with those of TypeScript's conversion.

This is the result of conversion by tsdown.

This is the result of conversion by TypeScript.

What is expected?

Consistently align with TypeScript's transpilation results by automatically removing uninitialized properties in classes.

What is actually happening?

It is inconsistent with the TypeScript transpilation results.

Any additional comments?

No response

[Gugustinette]

Can you provide a reproduction ? This kind of behavior depends on your tsconfig.json configuration.

[Boshen]

@Dunqing mind taking a look?

[MyDataHouse]

Can you provide a reproduction ? This kind of behavior depends on your tsconfig.json configuration.

https://github.com/MyDataHouse/temple

Run pnpm build to check the conversion results of tsdown in the dist directory; run pnpm tsc ./src/index.ts to check the TypeScript conversion results in the same directory.

https://www.typescriptlang.org/zh/play/?#code/KYDwDg9gTgLgBAYwDYEMDOa4FkCeBhVDOAbwCg44A7FAW2AEIAuONGKAS0oHNyXQmWbTjwC+pUrgLo0AOjBQIMRTjDAZ1OnAC8cAOQAvABYpuaE7on5Cs+YuWqZaUNr00USYBaA

This link is a TypeScript online editor. With the same example, you can also check the conversion results.

[Dunqing]

I've found out that both esbuild and Babel keep properties with ! (definite assignment assertion). I don't know why, but consider that TypeScript is the source of truth, so keep it as well. It will be fixed in oxc-project/oxc#12713

[sxzz]

It will be fixed in the next release of Rolldown.

[Dunqing]

@MyDataHouse, can you show the tsconfig.json and tsdown configuration? I just found out that this is related to the useDefineForClassFields option. The fields would be preserved when this option is enabled. See https://www.typescriptlang.org/play/?useDefineForClassFields=true#code/KYDwDg9gTgLgBAYwDYEMDOa4FkCeBhVDOAbwCg44A7FAW2AEIAuONGKAS0oHNyXQmWbTjwC+pUrgLo0AOjBQIMRTjDAZ1OnAC8cAOQAvABYpuaE7on5Cs+YuWqZaUNr00USYBaA. So I guess this is the main problem, so my fix is wrong, and I am going to revert that PR.