Detecting Traps

While some errors are represented at the WIT type level and expected, some internal errors may cause a component instance to trap. Jco-generated bindings report the component-model traps they detect with the built-in WebAssembly.RuntimeError class.

After an instance has trapped, Jco marks the component instance as unusable. Any subsequent call throws the original trap without re-entering the component.

To detect these traps, use the built-in WebAssembly.RuntimeError class:

import { instantiate } from "./dist/transpiled/component.js";
import { WASIShim } from "@bytecodealliance/preview2-shim/instantiation";

const shim = new WASIShim().getImportObject();
const instance = await instantiate(undefined, shim);

try {
    instance['ns:pkg/iface'].someFunction();
} catch (err) {
    if (err instanceof WebAssembly.RuntimeError) {
        console.error(`TRAP: ${err}`);
        // The instance is now disabled and cannot be called again.
    } else {
        // Other exceptions are unexpected and should be handled separately.
        throw err;
    }
}

WIT result<T, E> values remain part of the component’s normal return contract and are not thrown as WebAssembly.RuntimeError instances.