Why is binding with child components rendering two times on the first load?

[frederikhors]

Describe the bug

Why do I get two times the console.log()?

Reproduction

https://svelte.dev/repl/e27091c87e9f429b9eac1e3fdc6d20d3?version=3.41.0

• App.svelte:

<script>
  import Child from './Child.svelte'  
	
  let filter = { 
    value: "hello world"
  }
	
  $: console.log(filter)
</script>

<h2>Parent:</h2> {filter.value}

<input bind:value={filter.value}>

<br><br>

<h2>Child:</h2> <Child bind:filter />

• Child.svelte:

<script>
  export let filter = undefined
	
</script>

<input bind:value={filter.value}>

Severity

performances penalty

[Prinzhorn]

This looks like the issue I'm having (with stores though) #6298

So maybe it is a more widespread issue with bind: triggering unnecessary updates. But I just noticed it because my stores are expensive.

[motzel]

Additionally, this does not happen if the filter variable is a regular string. console.log() is called only once in this case

[techniq]

Related issue that was resolved - #5555

[Prinzhorn]

Another one for the collection #5689

[sourcecaster]

Yep, this is an old bug and was described in details here: #4447.
Simplest way to reproduce:
https://svelte.dev/repl/9b44ad8378f340cb87c1960a4b9b8caa?version=3.42.1
(see console output)

If you change from object value to scalar value then bug no longer occurs.

[frederikhors]

What a crazy bug!

[Florian-Schoenherr]

Simplest way to reproduce:
https://svelte.dev/repl/9b44ad8378f340cb87c1960a4b9b8caa?version=3.42.1

This even happens if you put
$: console.log(param);
into Component.svelte instead of App.svelte

[dummdidumm]

Closing as duplicate of #5689