Supporting NodeJS style callbacks using async/await

async_await_callbacks.js

// Usage is like -
//
//   let x = await task(obj, obj.method, arg1, arg2);
//   do_something(x);
//
// where obj.method is a method that takes a NodeJS style callback
// as the last argument, which is left out when making the above call.
// The task function turns the callback backed function into a 
// pseudo promise which the async/await mechanism can work with.
function task(target, fn, ...argv) {
    let _succeed = null, _fail = null;
    let callback = function cb(err, result) {
        // Account for when the callback happens
        // too quickly - perhaps synchronously -
        // so that the _succeed and _fail variables
        // have not yet been populated. Just force an
        // async call in that case.
        if (!_succeed) {
            setTimeout(cb, 100, err, result);
            return;
        }

        if (err) {
            return _fail(err);
        }

        _succeed(result);
    };

    // Do the async task right away.
    argv.push(callback);
    fn.apply(target, argv);
    
    return {
        then: function (succeed, fail) {
            // Store away the one shot continuations so that
            // when the callback arrives, they can be called.
            _succeed = succeed;
            _fail = fail;
        }
    };
};