Can't find variable _ after upgrading to Next 12.3

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System: Platform: darwin Arch: x64 Version: Darwin Kernel Version 21.6.0: Wed Aug 10 14:25:27 PDT 2022; root:xnu-8020.141.5~2/RELEASE_X86_64 Binaries: Node: 16.13.1 npm: 6.14.8 Yarn: 1.22.19 pnpm: N/A Relevant packages: next: 12.3.0 eslint-config-next: 11.1.2 react: 17.0.2 react-dom: 17.0.2

What browser are you using? (if relevant)

Reproduced with Firefox 104.0.1 and Safari 15.6.1

How are you deploying your application? (if relevant)

No response

Describe the Bug

After upgrading to Next 12.3 this morning, I’ve immediately had a console error saying “Can’t find variable _”.

I’ve debugged my code, because it made no sense. I’ve upgraded Typescript, no change.

I’ve finally fixed it by finding the _ variable that was created after compilation, create a new named variable and use it.

My code used to be:

switch (component.type) {

it’s now:

let componentType: string = component.type;
switch (componentType) {

It is a silly thing, but I’ve lost plenty of time because of this, and I’m still unclear what’s the root cause.

Expected Behavior

No error after compilation.

Link to reproduction

It’s a private repo

To Reproduce

Hard to say how to reproduce outside of the code that is provided above. Let me know if I can provide anything more helpful.

[SukkaW]

Just disscussed the issue with some SWC contributors. It seems that issue might be related with SWC’s recent generator transpiling changes.

[balazsorban44]

Using the reproduction from #40405 indeed, it seems to be an SWC issue.