Difference between Javascript code generated from Typescript between esbuild and tsc

[sloonz]

Typescript :

class Test {
    id?: number;
}

Code generated by tsc :

class Test {
}

Code generated by esbuild :

class Test {
    id;
}

This results in an observable difference for console.log({...new Test()}) : tsc version results in {}, esbuild version in {id: undefined}. This difference breaks, for example, the .patch() method of Objection, because patch() does something like this : initialObject.$set(partialUpdate), which overrides non-patched fields to undefined (instead of preserving previous values).

[hyrious]

This behavior is controlled by tsconfig: useDefineForClassFields, which is enabled by default in esbuild if:

• bundle
• there is target: esnext in tsconfig

In other words, it is disabled if

• transform --target=esnext

If you need this behavior in transform, you have to append tsconfig like this:
--tsconfig-raw={"compilerOptions":{"useDefineForClassFields":true}}.

I have suggested enable it in transform if target is esnext in another issue: #1480 (comment)

[sloonz]

Awesome, thanks for the answer !