Bug 247427 – Setting [[Prototype]] of global object in JSC shell should not throw `TypeError`

• Status: NEW
• Component: JavaScriptCore

[Kanguk Lee]

// input.js
Object . setPrototypeOf ( this , { } ) ;
__________________________________________

Hello,

Executing the input.js using JSC throws TypeError, but it is expected to terminate normally.

Running the input.js with JSC gives

---
$ jsc input.js
Exception: TypeError: Cannot set prototype of immutable prototype object
setPrototypeOf@[native code]
global [e-mail]:1:24
---

while other engines behave like

---
# V8
$ node input.js
// no error

# GraalJS
$ js input.js
// no error
---


WebKit version: 615.1.10

[Radar WebKit Bug Importer]

<rdar://problem/101940462>

[Alexey Shvayka]

Great catch, thank you!

Indeed the ECMA-262 doesn't require global object to have immutable prototype; it's the WebIDL spec which makes global objects' [[Prototype]] slots immutable: https://webidl.spec.whatwg.org/#platform-object-setprototypeof. That shouldn't affect the JSC shell of course.

When fixing that we need to ensure that scope resolution is sane when `globalThis.__proto__` is modified, which might get a little tricky.

[Yusuke Suzuki]

I wonder if we should fix this. Since DOM window is super more common in various places, should we just keep our behavior aligned to DOM window?

[Alexey Shvayka]

(In reply to Yusuke Suzuki from comment #3)
> I wonder if we should fix this. Since DOM window is super more common in
> various places, should we just keep our behavior aligned to DOM window?

I would say we should fix this (not prioritize of course) just for the spec purity and non-browser environments like Bun, which claims to be functionally-equivalent to Node.js.