Why does SWC treat a field called "mock" specially?

[fatso83]

I was writing a how-to on using a new Sinon feature for auto-cleanup of stubs injected using accessors when I discovered it did not work when I exposed accessors on an exported object called 'mock'. You can see the full diff here: fatso83/sinon-swc-bug@eace1a7

Basically, if I write something like this:

export const mock = {
    get toBeMocked(){ return toBeMocked; },
    set toBeMocked(mock){ toBeMocked = mock; }
}

it will be transpiled to this by SWC (very different from tsc!):

export const _$mock = {
    get toBeMocked(){ return toBeMocked; },
    set toBeMocked(mock){ toBeMocked = mock; }
}

Somehow 'mock' gets turned into _$mock? Why does this happen? This breaks compatibility with Typescript as well, as TSC does not do this.

[kdy1]

It's because jest mode is enabled and a variable named mock is special in jest

[fatso83]

The tests run inside of Mocha

[kdy1]

Then please file an issue

[fatso83]

OK, I will! Was hoping it was not an issue and just me being stupid 😄

[fatso83]

Reported as #8148. It is actually ES5 as the target that will trigger the issue. Easy to see on the playground