async-wasm.zig

extern fn slowAddJS(a: u32, b: u32, frame: *@Frame(slowAdd)) void;

var addResult: u32 = 0;

export fn onSlowAdd(frame: *@Frame(slowAdd), result: u32) void {
    addResult = result;
    resume frame;
}

fn slowAdd(a: u32, b: u32) u32 {
    slowAddJS(a, b, @frame());
    suspend;
    return addResult;
}

extern fn print(val: u32) void;

fn realMain(a: u32, b: u32) void {
    print(a);
    print(b);
    print(0);
    const result = slowAdd(a, b);
    print(a);
    print(b);
    print(result);
}

var mainFrame1: @Frame(realMain) = undefined;
var mainFrame2: @Frame(realMain) = undefined;

export fn main() void {
    mainFrame1 = async realMain(1, 2);
    mainFrame2 = async realMain(10, 20);
}

main.js

let exports;
var importObject = {
    env: {
        slowAddJS(a, b, frame) {
            console.log("slowAddJS", { a, b, frame });
            setTimeout(() => {
                console.log("Calling zig callback...")
                exports.onSlowAdd(frame, a + b);
            }, 100);
        },
        print(a) {
            console.log("print", { a });
        }
    }
};

WebAssembly.instantiateStreaming(fetch('main.wasm'), importObject)
    .then(obj => {
        exports = obj.instance.exports;
        console.log("Calling zig main via wasm...")
        exports.main()
    });

log:

Calling zig main via wasm...
main.js:13 print {a: 1}
main.js:13 print {a: 2}
main.js:13 print {a: 0}
main.js:6 slowAddJS {a: 1, b: 2, frame: 5360}
main.js:13 print {a: 10}
main.js:13 print {a: 20}
main.js:13 print {a: 0}
main.js:6 slowAddJS {a: 10, b: 20, frame: 5440}
main.js:8 Calling zig callback...
main.js:13 print {a: 1}
main.js:13 print {a: 2}
main.js:13 print {a: 3}
main.js:8 Calling zig callback...
main.js:13 print {a: 10}
main.js:13 print {a: 20}
main.js:13 print {a: 30}