fix: bindable props on class instances (spread + mount)#18143
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ashishkr96 wants to merge 1 commit intosveltejs:mainfrom
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fix: bindable props on class instances (spread + mount)#18143ashishkr96 wants to merge 1 commit intosveltejs:mainfrom
ashishkr96 wants to merge 1 commit intosveltejs:mainfrom
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`Object.getOwnPropertyDescriptor` skips inherited accessors, so spread/`mount` props that come from class instances (including class fields with `$state`) silently lost their setter and bindings became no-ops. Walks the prototype chain to find accessor descriptors, routes spread-proxy writes through `props[key] = v` so `this` resolves correctly, and applies the same fix to SSR `spread_props` / `bind_props`. Closes sveltejs#18140 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Closes #18140.
Object.getOwnPropertyDescriptoronly inspects own properties, so when a class instance was passed as props (<Comp {...instance} />ormount(Comp, instance)), Svelte couldn't find the inherited accessor and silently dropped the setter. Reads worked, writes didn't — bindings became no-ops, which made it awkward to use classes in component tests wheremount/renderis the natural entry point.This walks the prototype chain (stopping at
Object.prototype) when resolving property descriptors in:spread_props_handler.setandgetOwnPropertyDescriptor(client) — so<Comp {...instance} />finds the prototype accessor.prop()(client, themountpath) — same, plus the setter is now invoked viaprops[key] = vsothisresolves to the class instance and re-enters the spread proxy'ssettrap when relevant.spread_propsandbind_props(server) — SSR previously relied onObject.keys/getOwnPropertyDescriptor, which also missed inherited accessors. Inherited accessors are now copied withget/setbound to the source object sothissurvives.A shared
get_descriptor_in_chainhelper lives ininternal/shared/utils.js.Test plan
bind-and-spread-class-instance—<Comp {...accessor} />and<Comp {...field} />(class fields with$state) round-trip writes through to the parent's class instance, on dom/hydrate/ssr/async-ssr.mount-class-instance-props— same coverage viamount(Component, classInstance).runtime-runes,runtime-legacy,server-side-rendering,hydrationsuites still green.pnpm checkandpnpm lintpass.