Future proofing of static blocks transform

[nicolo-ribaudo]

Describe the bug

There is a stage 2.7 TC39 proposal that would break the way transpilers transform static blocks to private fields, if the static block freezes the class: tc39/proposal-nonextensible-applies-to-private#1.

It'd be great to get all the transpilers updated to be compatible, just in case.

Input code

class A {
    static #foo = 1;
    static { Object.freeze(this); }
}

Config

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "jsx": false
    },
    "loose": false,
    "minify": {
      "compress": false,
      "mangle": false
    }
  },
  "module": {
    "type": "es6"
  },
  "minify": false,
  "isModule": true,
  "env": {
    "targets": "chrome 90"
  }
}

Link to the code that reproduces this issue

https://play.swc.rs/?version=1.13.2&code=H4sIAAAAAAAAA0vOSSwuVnBUqOZSAILiksSSzGQF5bT8fAVbBUNrZMFqBf%2BkrNTkEr20otTUqlSNkozMYk1rhVquWgDElPrbRAAAAA%3D%3D&config=H4sIAAAAAAAAA1WOSw7CMAxE9zlF5DULVkhwBw4RBbekaj6yU0RU9e4kaVPanT0zfp5ZSAkDa3jIOY95CYoYad%2BzwslF9c0KoLaKNZkQ4dLcgYvVqZGxSsvqwOg9Y3M2zRpnunRka28DIfM5WKLK9SOeyWKjg%2FWvqZpb55gC1n58g3%2BoPdvBYPjZLiNNq4bucwAp6jGWNqDf5C3K%2B7USxfIDz55nsCoBAAA%3D

SWC Info output

No response

Expected behavior

See Babel PR for example: babel/babel#17443

Given this input:

class C {
  static #FOO = "#FOO";
  static {
    C.#FOO = #BAR in C;
  }
  static #BAR = C.#FOO;
  static {
    Object.freeze(C);
  }
}

Something like

var lastBlocks;
class C {
  static #FOO = "#FOO";
  static #BAR = (() => {
    C.#FOO = #BAR in C;
  })(), lastBlocks = (() => {
    Object.freeze(C);
  }), C.#FOO;
}
lastBlocks();

Actual behavior

For this code:

class A {
    static #foo = 1;
    static { Object.freeze(this); }
}

SWC generates

class A {
    static #foo = 1;
    static #_ = Object.freeze(this);
}

which with that proposal will throw because it's trying to add #_ to a frozen object.

Version

1.13.2

Additional context

No response

[kdy1]

@claude fix it

[claude]

Claude finished @kdy1's task in 17m 16s —— View job • claude/issue-10956-20260122-0322 • Create PR ➔

---

Fixing Static Blocks Transform for Future Compatibility

Task: Fix the static blocks transform to be compatible with the TC39 proposal that would make frozen classes throw when adding private fields.

• Gather context and understand the current implementation
• Understand the required transformation approach
• Implement the fix
  • Add trailing static block store to TraverseCtx
  • Modify ClassStaticBlock to detect and handle trailing static blocks
  • Add hooks to inject call statements after class declarations
• Add/update tests
• Run tests to verify (15/15 static blocks tests passing)
• Commit and push changes

Summary:

This fix future-proofs the static blocks transform against the TC39 proposal that would make frozen classes throw when adding private fields.

Changes made:

1. Added TrailingStaticBlocksStore to track trailing static blocks that need deferred execution
2. Modified ClassStaticBlock transformer to detect and handle trailing static blocks specially
3. Added TrailingStaticBlocks hook to inject call statements after class declarations
4. Updated existing test expectations to reflect the new behavior
5. Added new test cases for the frozen class scenario

Example transformation:

// Input
class C {
  static #foo = 1;
  static { Object.freeze(this); }
}

// Output
var _initStaticBlock0;
class C {
  static #foo = 1;
  static #_ = _initStaticBlock0 = () => {
    Object.freeze(this);
  };
}
_initStaticBlock0.call(C);

The trailing static blocks (those after all static fields) are now wrapped in arrow functions and executed after the class definition, ensuring no new private fields are added after a potential Object.freeze().

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