how to transform a sync function that returns a value or throws an exception, as async that fires a callback

asyncify.js

function asyncify(f) {
    return function _asyncify() {
        var args = Array.prototype.slice.call(arguments, 0);
        var argsWithoutLast = args.slice(0, -1);
        var callback = args[args.length - 1];
        var result, error;

        try {
            result = f.apply(this, argsWithoutLast);
        } catch (e) {
            error = e;
        }

        setTimeout(function() {
            callback(error, result);
        }, 0);
    }
}

// example

function head(xs) {
    if (_.isArray(xs) && !_.isEmpty(xs)) {
        return xs[0];
    } else {
        throw "Can't get head of empty array.";
    }
}

var cbHead = asyncify(head);

cbHead([1, 2, 3], function(err, result) {
    // result === 1
});

cbHead([], function(err, result) {
    // err === "Can't get head of empty array.";
});