Description forthcoming.

Usage no npm install needed!

<script type="module">
  import agoricXsnap from 'https://cdn.skypack.dev/@agoric/xsnap';



Xsnap is a utility for taking resumable snapshots of a running JavaScript worker, using Moddable’s XS JavaScript engine.

Xsnap provides a Node.js API for controlling Xsnap workers.

const worker = xsnap();
await worker.evaluate(`
  // Incrementer, running on XS.
  function handleCommand(message) {
    const number = Number(String.fromArrayBuffer(message));
    return ArrayBuffer.fromString(String(number + 1));
await worker.snapshot('bootstrap.xss');
await worker.close();

Some time later, possibly on a different computer…

const decoder = new TextDecoder();
const worker = xsnap({ snapshot: 'bootstrap.xss' });
const response = await worker.issueCommand('1');
console.log(decoder.decode(response)); // 2
await worker.close();

The parent and child communicate using "commands".

  • The XS child uses the synchronous issueCommand function to send a request and receive as response from the Node.js parent.
  • The XS child can implement a synchronous handleCommand function to respond to commands from the Node.js parent.
    • The XS child handleCommand may be asynchronous after a fashion: it may return an object and, before the promise queue becomes empty, set the result property of this object to an ArrayBuffer. See the evaluate and report test for an example.
  • The Node.js parent uses an asynchronous issueCommand method to send a request and receive a response from the XS child.
  • The Node.js parent can implement an asynchronous handleCommand function to respond to commands from the XS child.

state diagram


With xsnap comes an xsrepl command line tool. Use yarn global add @agoric/xsnap to add xsrepl to your path. During development, run yarn repl.

The REPL supports special commands load and save for snapshots, and quit to quit. Load and save don't take arguments; just type the file name on the next prompt.

$ xsrepl
xs> globalThis.x = 42;
xs> x
xs> save
file> temp.xss
xs> quit
$ xsrepl
xs> load
file> temp.xss
xs> x
xs> quit