Ask for Feedback: Make `JSObject` / `JSFunction` calls be `throws`

[kateinoigakukun]

Every dynamic JS function calls can throw an exception, but the current JSObject and JSFunction are non-throws by default. They can be throws by .throws or .throwing but it's still opt-in.

For example, if an exception is thrown during a JS function call but the call site in Swift side is not throws, the control unwinding wasm call frames without executing function epilogues even if there is do { } catch {} clause in Swift. And also in that case, defer blocks won't be called.

This behavior is quite tricky and unwinding wasm call frames without executing function epilogues can lead to memory and resource leak.

I'm going to introduce an API breaking change to make JSObject / JSFunction calls be throws in the next next release to improve the situation.

This change has a performance tradeoff as throws variants are typically a little bit slower than non-throwing variants. We should measure the amount of the penalty but it shouldn't be a big deal. We can make non throwing calls as an opt-in feature so that those who want best performance can satisfy their needs.

// Before
let foo = JSObject.global.document.getElementById!("foo")
let foo = try JSObject.global.document.throwing.getElementById!("foo")

// After
let foo = try JSObject.global.document.getElementById!("foo")
let foo = JSObject.global.document.nonthrowing.getElementById!("foo")

Let me know here if you have a strong objection 🙏 Thanks.

[austintatiousness]

I have a project with precisely thousands of calls to JavaScriptKit I would be absolutely devastated if I had to rewrite all of it just to add .nonthrowing everywhere or wrap everything in a do.

[kateinoigakukun]

After experimenting this on a large project, it seems not a realistic idea to change the API. To avoid unwinding without Swift epilogues, we should make "method calls withour try but throwing exceptions" crash at that point.