Guard check on variable doesn't work as expected

[MSDev201]

Describe the bug

I'm experiencing a very strange bug where a variable check for undefined is seemingly ignored. This causes lots of issues down the line.

(the first if doesn't need to be this long but wanted to make sure)

This issue seems to be introduced in version 5.38.2 since it works without issues in version 5.38.1. Its still happening in the most recent version.

Reproduction

Unfortunately after lots of trying I'm not able to reproduce this issue in a simpler form yet.

Logs

Chrome:

error-handling.js:68 Uncaught TypeError: Cannot read properties of undefined (reading 'depth')

Firefox:

Uncaught TypeError: can't access property "depth", $.get(...) is undefined

System Info

System:
    OS: Windows 11 10.0.22631
    CPU: (28) x64 13th Gen Intel(R) Core(TM) i7-13850HX
    Memory: 33.43 GB / 63.61 GB
  Binaries:
    Node: 22.13.1 - C:\nvm4w\nodejs\node.EXE
    npm: 10.9.2 - C:\nvm4w\nodejs\npm.CMD
  Browsers:
    Edge: Chromium (140.0.3485.54)
    Internet Explorer: 11.0.22621.3527

Tested on:
* Google Chrome Version 140.0.7339.186 (Official Build) (64-bit)
* Firefox 140.3.0esr (64-Bit)

Severity

blocking an upgrade

[MSDev201]

Could be similar to #16795

[MSDev201]

Reproduction

https://svelte.dev/playground/hello-world?version=5.39.6#H4sIAAAAAAAACnVSTY-bMBD9K9PZ1QoklOz2SAFt1Ut7rSr1EHIAMyTuOjayh2UrxH-vjJMGtql8Gb1573m-RtTViTDFr6SUgcFY1UBEjWRqYkywlYocprsR-XfneR7A5KL63HUb90qKPVZXjm7hwmgmzQ5TzJywsuOi1CUrYniVlvtKfWM6Ocjh3nHFFO1GqRt6S-FxSuASPy3ij9M-vlgI0kz2uxlWBg11fPTE5BI-LcNZfjZw5kRfjpU-0NWgrZSj-JOn3FPbkuAoiiEvYPRQybKF6KqL10Xs9rNw8vpse-1Yj3dUieO66cqBNQNE1gybubt48uLxWRjtFt1Bvvhk95e9D-w72S6oH3LodUOt1NTAw8M1s5kH8E_er9YsDfI8hxJN_YsEl-gpJc7SEkHqBfGG-PxHsND9qSZb4hTGtq7zP9W8zxfweJaXHK70p7_SrLbF2XUr2zCGEIxbP2aP6KzumY0Go4WS4iUfwxaj1c7Z9hRPxQ8rDwey2TZoinAhtx8myPTGmHrttE-QK6kGqRtM59OZ_gAUWQbrWAMAAA

Working with version 5.38.1:
https://svelte.dev/playground/hello-world?version=5.38.1#H4sIAAAAAAAACnVSTY-bMBD9K9PZ1QoklOz2SAFt1Ut7rSr1EHIAMyTuOjayh2UrxH-vjJMGtql8Gb1573m-RtTViTDFr6SUgcFY1UBEjWRqYkywlYocprsR-XfneR7A5KL63HUb90qKPVZXjm7hwmgmzQ5TzJywsuOi1CUrYniVlvtKfWM6Ocjh3nHFFO1GqRt6S-FxSuASPy3ij9M-vlgI0kz2uxlWBg11fPTE5BI-LcNZfjZw5kRfjpU-0NWgrZSj-JOn3FPbkuAoiiEvYPRQybKF6KqL10Xs9rNw8vpse-1Yj3dUieO66cqBNQNE1gybubt48uLxWRjtFt1Bvvhk95e9D-w72S6oH3LodUOt1NTAw8M1s5kH8E_er9YsDfI8hxJN_YsEl-gpJc7SEkHqBfGG-PxHsND9qSZb4hTGtq7zP9W8zxfweJaXHK70p7_SrLbF2XUr2zCGEIxbP2aP6KzumY0Go4WS4iUfwxaj1c7Z9hRPxQ8rDwey2TZoinAhtx8myPTGmHrttE-QK6kGqRtM59OZ_gAUWQbrWAMAAA

[ivenuss]

Yeah, that's identical issue to #16795, just another side-effect.

You can useing a hack setTimeout or tick() before changing the state to temporary fix this issue, but hopefully someone will look into it soon 🙏