Async function breaks assigning to new objects in for loops

[tobysmith568]

Verify canary release

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

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Home
Binaries:
  Node: 16.16.0
  npm: N/A
  Yarn: N/A
  pnpm: N/A
Relevant packages:
  next: 12.3.1-canary.1
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0

What browser are you using? (if relevant)

Not relevant

How are you deploying your application? (if relevant)

Not relevant

Describe the Bug

Assigning to an object in a for loop inside an async function seems to not work properly. It seems to always assign the final value to be iterated over.

The works array only has string literals pushed to it, so it works as expect and ends up with ["one", "two", "three"].

The broken array has objects assigned to it, so it is broken and gets [{ input: "three }, { input: "three }, { input: "three }] even though it's assigned to with the exact same variable in the exact same for loop.

Everything works as expected when it's not an async function.

This cannot be reproed on next@12.2.6-canary.8
But can be reproed on next@12.2.6-canary.9 and it remains present in next@canary as of 14/09/22.

See the README in the attached repro for more details.

Expected Behavior

Both the works and broken arrays should get "one", "two", and "three".

Link to reproduction

https://github.com/tobysmith568/nextjs-repro

To Reproduce

Push objects to an array using a for loop in an async function.

[discretegames]

I think I have the same issue from pushing object literals in an async function, though my code isn't in a for loop. I had a line like

myarray.push({ a: await myfunc(), b: 123 });

in an async function and it works in next@12.2.6-canary.8 but fails in next@12.2.6-canary.9 saying TypeError: Cannot convert undefined or null to object. However, these (should-be) equivalent snippets did both work in next@12.2.6-canary.9:

const a = await myfunc();
myarray.push({ a, b: 123 });

const obj = { a: await myfunc(), b: 123 };
myarray.push(obj);

[magic-akari]

This is a bug in swc.

https://play.swc.rs/?version=1.3.0&code=H4sIAAAAAAAAA11QQQqDMBC8%2B4ohpwTEB1TSQ99Q6KF4aCVWUbJiFCni3xs3RqSXJMzMzsymJOtGdPRpSmi83NeWkAr6iiUBSmYb20%2Bj8%2FRTkDUihRhn4qsejBFFcihnGloWFvmBvQdqjT1AwB8VDZAnc1C1pyjODbPUmcw3k8ykWHbtyk9338IvsQS01pFXecIe3CbrJ1cHCxXyQ6GAL6cZYI2bxGTx2Cz8qmyl8n%2F6xlaeD55esPps%2Fk2p8h9C6eqlXAEAAA%3D%3D&config=H4sIAAAAAAAAAy2LQQ6AIAwE%2F9IzVz3wBx%2FRYDUYUNKWRGL4u9R4252dfeCQAP6BgizElqSdijd4oJBRAsei4IY20IZJqDtQ5J3UFJlg9HytNZGdtRX6%2BGw8yvIvypX6C9UPjkhtAAAA

[tobysmith568]

@magic-akari thanks for the quick fix in swc-project/swc#5861.
However, I wonder if this should this have been auto-closed by the completion of swc-project/swc#5861 ?

I would have assumed this should stay open until there's a new patch release of @swc/helpers and next.js is updated to use it. In the meantime other people might come and open more issues here while the bug is still reproducible in next@canary.

[kdy1]

Oh I missed it.
It will be fixed by #40520

[github-actions]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.