Supported Method for Detecting Proxies

[SBrazzCode]

Describe the problem

A common concern with svelte 5's introduction of rune proxies is the inability to detect proxies i.e issue-13562. A common issue when migrating code is sudden errors erupting from libraries/apis that don't accept proxies. Adding to that issue, there is no supported way to method to detect if an object is a reactive proxy, so user's have to use try/catch or other homegrown methods to detect if an object is a proxy.

Describe the proposed solution

Can we have a supported method for detecting proxies

Ideally, the proxy check is something integrated in Svelte's current proxy logic (linked above). Alternatively, a more generic javascript proxy detection method could work.

Importance

would make my life easier

[Ocean-OS]

I don't think this is really that necessary; if you're concerned about whether an object is a $state proxy, you can just use $state.snapshot to unproxy the object.

[webJose]

Interesting thing: The Svelte framework will always refer to the value the proxy is for instead of the proxy itself:

<script>
	let name = $state('John')
</script>

<pre>Is Proxy?  {name instanceof Proxy}</pre>
<pre>Is string?  {typeof name === 'string'}</pre>

And:

<script>
	let name = $state('John')

	const isProxy = $derived(name instanceof Proxy)
	const isString = $derived(typeof name === 'string')
</script>

<pre>Is Proxy?  {isProxy}</pre>
<pre>Is string?  {isString}</pre>

Tells you that name is not a proxy, and instead is a string. So how does one go about passing the actual proxy? Honestly curious.

[7nik]

name instanceof Proxy won't work because Proxy has no prototype to compare with.

If some code cannot accept proxies, as said @Ocean-OS, just do $state.snapshot - it will unproxify the object or return the value it isn't Svelte's proxy.

[brunnerh]

The Svelte framework will always refer to the value the proxy is for instead of the proxy itself

That seems like a bit of a misunderstanding. Svelte will not create proxies for primitive values (strings, numbers, booleans) at all. You need to set the state to a plain object or array for a proxy to be created. This proxy then can be passed around.

[webJose]

I see. So I guess we learn something new everyday.

• So Proxy has no prototype.
• Primitive values don't create proxies.

Thanks for the lessons, @7nik and @brunnerh .

[Ocean-OS]

• So Proxy has no prototype.
• Primitive values don't create proxies.

Yeah, the new Proxy syntax is kind of a misnomer, since proxies aren't class instances. I think of Proxy as a sort of intrinsic that tells the JS engine to override certain operations on the provided object.
Additionally some object-related functions don't work with Proxies, such as structuredClone.