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SES 1.9.0 introduces immutable ArrayBuffers

SES 1.9.0 introduces immutable ArrayBuffer, exposes its console shim, and permits a global ModuleSource in the shared intrinsics.

Immutable ArrayBuffer

JavaScript strings are immutable, but JavaScript lacks an immutable analog for ArrayBuffer. So, one can have a string of text and send it without making a copy to defend its invariance, but not a string of bytes.

Agoric’s Mark Miller presented a proposal at the October plenary meeting of the JavaScript standard committee, ECMA TC39. The committee adopted Immutable Array Buffers into the body of problems under consideration for a solution in a future standard, called “Stage 1”.

SES 1.9.0 is the first release to introduce an emulation of the proposed immutable ArrayBuffer.

On platforms without Array.prototype.transfer but with a global structuredClone, the ses shim’s lockdown will now install an emulation of Array.prototype.transfer. On platforms with neither, ses will currently not install such an emulation. However, once we verify that ses is not intended to support platforms without both, we may change lockdown to throw, failing to lock down.

  • XS and Node >= 22 already have Array.prototype.transfer.
  • Node 18, Node 20, and all browsers have structuredClone
  • Node <= 16 have neither, but are also no longer supported by ses.

Console Shim

The ses shim consists of mostly separable layers:

  • ses/assert-shim.js
  • ses/lockdown-shim.js
  • ses/compartment-shim.js
  • ses/console-shim.js

Version 1.9.0 is the first release to expose an entry point for ses/console-shim.js so the user can choose whether to wrap and replace the global console from the hardened JavaScript realm with a version that can unredact error messages.

ModuleSource

Agoric champions a number of JavaScript standard proposals under the umbrellas of Hardened JavaScript, Compartments, and a broad effort toward “Module Harmony”. These include a proposal for a global ModuleSource that takes the source code of a JavaScript module and prepares it for evaluation.

The package @endo/module-source provides an emulation of a ModuleSource constructor suitable for use in combination with ses and Moddable’s XS provides a native implementation. The module @endo/module-source/shim.js can install the emulated ModuleSource in global scope so it becomes available as a shared intrinsic.

Version 1.9.0 adds ModuleSource to the shared intrinsics that lockdown will repair if necessary and then harden, which consequently is available by default in every new Compartment.