.css("--custom") throws TypeError: ret is undefined (3.6.0 → 3.6.1 regression)

[andersk]

Description

In jQuery 3.6.0, $element.css("--custom") correctly returned undefined for a custom property that isn’t set. But in jQuery 3.6.1, it crashes with TypeError: ret is undefined.

This breaks the Zulip settings dialog.

This was presumably introduced by #4930 (cc @fecore1 @mgol).

Link to test case

https://codepen.io/anderskaseorg/pen/rNvNZWW

[mgol]

Thanks for the report. That said, I'm not sure the undefined return value was correct either. We are using .getPropertyValue() for custom property values and it returns an empty string for unknown CSS custom properties. We just then fallback to computed[ value ] which will be undefined. The logic is at:
https://github.com/jquery/jquery/blob/3.6.1/src/css/curCSS.js#L31

That said, for regular unknown properties that is the logic we are using. I guess we should discuss what's our goal here.

[andersk]

Yeah, jQuery has given undefined here for standard properties since 1.8.0, and for custom properties since they were first supported in 3.2.0. “Correct” or not, it seems to me that a patch release isn’t a good time to break this compatibility.

[mgol]

I agree with that but there’s also a case with a custom property set to an empty string or a whitespace-only string. IMO then we should return an empty string, not undefined. I’ll need to check if this is possible with the Web APIs we have.