How to pass JavaScript functions from V8 to Rust?

[adhamsalama]

Hello everyone

I am following this tutorial about how to write a custom JavaScript runtime using Deno and Rust
https://deno.com/blog/roll-your-own-javascript-runtime
and I'm stuck on implementing setTimeout, because I don't know how JavaScript functions are passed from V8 to Rust, so I don't know what op_set_timeout function signature would look like.

Can anyone help?

Thanks in advance!

[bartlomieju]

Example:

deno/core/ops_builtin_v8.rs

Lines 53 to 66 in 26f42a2

	#[op(v8)]
	fn op_set_promise_reject_callback<'a>(
	scope: &mut v8::HandleScope<'a>,
	cb: serde_v8::Value,
	) -> Result<Option<serde_v8::Value<'a>>, Error> {
	let cb = to_v8_fn(scope, cb)?;
	let context_state_rc = JsRealm::state_from_scope(scope);
	let old = context_state_rc
	.borrow_mut()
	.js_promise_reject_cb
	.replace(Rc::new(cb));
	let old = old.map(|v| v8::Local::new(scope, &*v));
	Ok(old.map(|v| from_v8(scope, v.into()).unwrap()))
	}

.

That said - this is limited to only synchronous ops. You won't be able to pass and call a JS function in an asynchronous op.

If you want to see how to implement setTimeout API watch this talk where I walked through it: https://www.youtube.com/watch?v=xJXQwuTB_7Y

[adhamsalama]

Sorry for the late reply, thanks for the answer and the video!

[adhamsalama]

@bartlomieju This is great, but can you tell how much more complex implementing setInterval is?
The thing with setInterval is that it returns and intervalId which can be used to cancel that interval using clearInterval.
So, how would that be implemented in a single Rust function?