Extending non-writeable by writing prototype properties fails

[conartist6]

Version

v25.9.0

Platform

Darwin EOL 25.2.0 Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:40 PST 2025; root:xnu-12377.61.12~1/RELEASE_ARM64_T6000 arm64

Subsystem

No response

What steps will reproduce the bug?

class Base {
  method() {}
}

// this usually happens during Object.freeze(Base.prototype)
Object.defineProperty(Base.prototype, 'method', {
  value: Base.prototype.method,
  writable: false,
});

class SafeExtended extends Base {
  // this works fine
  method() {}
}

class Extended extends Base {}

// TypeError: Cannot assign to read only property 'method' of object '#<Extended>'
Extended.prototype.method = function method() {};

How often does it reproduce? Is there a required condition?

100% reproduction.

What is the expected behavior? Why is that the expected behavior?

No error. I can't be sure what the spec says, but there's no error on this snippet in either firefox or chrome.

What do you see instead?

TypeError: Cannot assign to read only property 'method' of object '#<Extended>'

Additional information

No response

[Renegade334]

This is entirely working as intended. You are probably evaluating in non-strict mode in your browser tests, where assigning to a non-writable property fails silently instead of throwing.

[conartist6]

Aha, you're right. So in the browser the way to get the error is:

(function () {
  "use strict"
  class Base {
    method() {}
  }

  Object.defineProperty(Base.prototype, "method", {
    value: Base.prototype.method,
    writable: false,
  });

  class SafeExtended extends Base {
    method() {}
  }

  class Extended extends Base {
    static {
      this.prototype.method = function method() {};
    }
  }
})();

Firefox calls it TypeError: "method" is read-only