Draft: Initial implementation of ShadowRealm built-in with realm isolation

[ParthMozarkar]

Draft PR: Initial Implementation of ShadowRealm Built-in

This PR introduces an initial implementation of the ShadowRealm built-in object based on the TC39 ShadowRealm proposal.

The purpose of this draft PR is to establish the core realm isolation mechanism and provide a foundation for the remaining features of ShadowRealm.

---

Implemented Features

ShadowRealm Constructor

• Added the ShadowRealm built-in constructor.
• Verified that new ShadowRealm() correctly creates a ShadowRealm instance.

Realm Isolation

• Implemented basic support for ShadowRealm.prototype.evaluate.
• Code executed through evaluate runs in a separate and fully isolated realm.
• The global state between the host realm and the ShadowRealm remains isolated.

Example behavior:

const realm = new ShadowRealm();

realm.evaluate(`
  globalThis.testValue = 42;
`);

console.log(globalThis.testValue); // undefined

This confirms that state changes inside the ShadowRealm do not affect the host realm.

---

Tests

Unit tests were added to verify:

• Constructor behavior
• Realm isolation through evaluate

All added tests are currently passing.

Test file:

core/engine/src/builtins/shadow_realm/tests.rs

---

Code Changes

New files:

core/engine/src/builtins/shadow_realm/mod.rs
core/engine/src/builtins/shadow_realm/tests.rs

Updated files:

core/engine/src/builtins/mod.rs
core/engine/src/context/intrinsics.rs
core/string/src/common.rs

These updates register the ShadowRealm built-in and integrate it into the engine intrinsics and static strings.

---

Remaining Work

This PR is opened as a Draft because some parts of the ShadowRealm specification are not yet implemented.

Callable Masking

Functions returned across realms are not yet supported. Proper wrapped callable objects still need to be implemented.

importValue

Support for dynamic module imports inside the ShadowRealm (ShadowRealm.prototype.importValue) has not yet been implemented.

---

Next Steps

Planned follow-up work includes:

• Implement callable masking for cross-realm functions
• Add support for ShadowRealm.prototype.importValue
• Expand test coverage
• Ensure closer alignment with the TC39 proposal

---

Request for Feedback

Opening this as a Draft PR to get early feedback on the approach and integration before continuing with the remaining parts of the implementation.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,070	49,932	-138
Ignored	2,072	2,210	+138
Failed	821	821	0
Panics	0	0	0
Conformance	94.54%	94.28%	-0.26%

Tested main commit: 3016f492d02a06033e11269fdcef603d293222c4
Tested PR commit: b929c1828a59744835761154287f8ce6ef1274aa
Compare commits: 3016f49...b929c18

[jedel1043]

Don't forget to gate everything behind the experimental feature

[ParthMozarkar]

Don't forget to gate everything behind the experimental feature

yaa sure you could check my next commit and review once if anything is else needed?