Feature Request: Support resolving promises when executing Javascript in Sandbox

[seanrcollings]

Description

When executing JavaScript code with async/await patterns using sandbox.runCode(), Promises are not awaited and result.results[0] contains an empty object instead of the resolved value.

Reproduction

See minimal reproduction repo: https://github.com/seanrcollings/cloudflare-worker-async-js

const code = `
(async () => {
  const response = await fetch("https://api.github.com");
  const data = await response.json();
  return data;
})()
`;

const result = await sandbox.runCode(code, {
  language: 'javascript'
});

// Expected: result.results[0].json contains API response
// Actual: result.results[0].json = {}
console.log(result.results[0]);

Issue: The returned expression from the isolate instance is a promise, but this promise is never awaited so we can't get the result.

Investigation

Through experimentation, I noticed the JavaScript execution environment appears to use Node.js vm.runInContext() internally.

Node.js docs: https://nodejs.org/api/vm.html#timeout-interactions-with-asynchronous-tasks-and-promises

Potential Solution

I've attempted a fix in my fork: seanrcollings#1

Change: Check if the result is promise-like and await it if that is the case. in brief this

• Configure the VM with microtaskMode: 'afterEvaluate' which gives each context it's own microtask queue
• Adds additional timeout handling for the async portion of the code flow
• Manually drains the context's microtask queue until the promise settles
• returns the resolve value of the promise.

In my local testing, this allows:

• Promises to be resolved before returning
• Timeouts to apply to async code
• The resolved value to be captured in results

However, I'm not deeply familiar with the JavaScript runtime implementation, so there may be other considerations or better approaches.

Impact

High - Currently, any modern JavaScript using async/await cannot be executed properly, limiting the Sandbox SDK to synchronous code only.

Questions

1. Is async/await support intended for runCode()?
2. If my approach looks reasonable, would you accept a PR with this fix?
3. Are there other considerations or alternative approaches I should be aware of?

[whoiskatrin]

@ghostwriternr can you review this please?