What meaning is return a(function () )?

Question

I studied simple asynchronized JavaScript code, but stuck with return value(function() ). I don't understand it.

How does operate this return value(function())?

function _async(func) {
    return function () {
        arguments[arguments.length++] = function (result) {
            _callback(result);
        };

        (function wait(args){
            for (var i = 0; i < args.length; i++) {
                if (args[i] && args[i].name == '_async_cb_receiver')
                    return args[i](function (arg) {
                        args[i] = arg; wait(args);
                    })}
            func.apply(null, args);

        })(arguments);

        var _callback;

        function _async_cb_receiver(callback) {
            _callback = callback;
        }

        return _async_cb_receiver;
    };
}

---

return args[i](function (arg) {
    args[i] = arg; wait(args);
})

I don't understand this part. wait(args) just returns args[i] , but parentheses appear and anonymous function is executed. How does this function handle args[i] and what arguments are in arg ?

Answer 1

args is an array of functions. So args[i] is one of these functions, and args[i](something) calls that function with something as the argument.

In this case, something is another function. This function replaces args[i] with its own argument, then calls wait(args) recursively.

This code is pretty convoluted, so I'm not sure what it's doing as a whole. I suspect it's doing something similar to the way promises work, but was written before they were added to the language.