JavaScript Scheduler should use window.postMessage, when setImmediate not available

[alexandru]

The Scheduler implementation for JavaScript tries to use setImmediate, when available (and right now it's only available on Node.js). Otherwise, it does a fallback to setTimeout, but this operation is throttled by browsers on repeated calls to 4ms, which can be terrible for performance.

An alternative, in browsers, is to fallback on window.postMessage.

See this setImmediate polyfill for inspiration: https://github.com/YuzuJS/setImmediate

[armanbilge]

@alexandru why not use the MacrotaskExecutor as done in #1522? It is based on that setImmediate polyfill you linked.

https://github.com/scala-js/scala-js-macrotask-executor

[alexandru]

@armanbilge I haven't checked the implementation. If it is, then it might be a good idea.

Thanks for the suggestion.

[armanbilge]

I can confirm it is, having worked on that project 😁

[armanbilge]

This can be closed as of #1522.