The catch attribute allows catching a JavaScript exception. This can be attached to any imported function or method, and the function must return a Result where the Err payload is a JsValue:

# #![allow(unused_variables)]
#fn main() {
extern "C" {
    // `catch` on a standalone function.
    fn foo() -> Result<(), JsValue>;

    // `catch` on a method.
    type Zoidberg;
    #[wasm_bindgen(catch, method)]
    fn woop_woop_woop(this: &Zoidberg) -> Result<u32, JsValue>;

If calling the imported function throws an exception, then Err will be returned with the exception that was raised. Otherwise, Ok is returned with the result of the function.

By default wasm-bindgen will take no action when wasm calls a JS function which ends up throwing an exception. The wasm spec right now doesn't support stack unwinding and as a result Rust code will not execute destructors. This can unfortunately cause memory leaks in Rust right now, but as soon as wasm implements catching exceptions we'll be sure to add support as well!