Unknown prop using export let and export function - error comeback

[lukaszpolowczyk]

Describe the bug

It's the same problem:
#4323

Reproduction

This is the REPL from that bug:
https://svelte.dev/repl/79aa242f7e6d4af684099868189d3cb4?version=3.18.0

If you click on button "toggle modal" in console you should have this warning:

"

was created with unknown prop 'saveForm'"

Logs

No response

System Info

System:
    OS: Linux 5.10 Arch Linux
    CPU: (4) x64 AMD Athlon(tm) X4 950 Quad Core Processor
    Memory: 1.73 GB / 15.07 GB
    Container: Yes
    Shell: 5.1.8 - /bin/bash
  Binaries:
    Node: 16.3.0 - /usr/bin/node
    Yarn: 1.22.10 - /usr/bin/yarn
    npm: 7.17.0 - /usr/bin/npm
  Browsers:
    Brave Browser: 100.1.37.109
    Firefox: 87.0

Severity

annoyance

[gtm-nayan]

I may have misunderstood but if the bug is the unknown prop warning then here's a simpler repro: https://svelte.dev/repl/3b8631768b874557b3987bdd10e29d76?version=3.47.0

Warning seems to stem from line 3 in App.svelte, where null is assigned to thing (also if you assign any value to thing other than undefined)

Arguably export function is a one way binding from the child to the parent and shouldn't get a value from the parent and the warning is expected behavior here.

[lukaszpolowczyk]

@gtm-nayan The expected behavior?

And how to do it differently so that the warning doesn't display?
The function exported from C.svelte, must work inside it, because it refers to inside it.

[gtm-nayan]

@gtm-nayan The expected behavior?

For me it is, but the consensus might be different.

And how to do it differently so that the warning doesn't display?

Just don't assign anything to the receiver of the binding and the warning goes away. (undefined is fine). In my repl, it'd be changing let thing = null; to let thing;, in your repl it'd be changing export let saveForm = null to export let saveForm = undefined; inside Wrapper.svelte.

[Rich-Harris]

We got rid of these warnings in Svelte 5, so I'll close this issue