recurse_callback.js

var array = [
    1,
    2,
    [10, 11, 12],
    [20, 21, [30, 31]],
    3
];

console.log('before');
console.log(array);

flatten(array, function(res) {
    console.log('after');
    console.log(res);
});

function flatten(array, callback) {
    var done    = [];
    var doneNum = 0;

    function check(res) {
        done = done.concat(res);
        ++doneNum;

        if (array.length <= doneNum) {
            callback(done);
        }
    }

    for (var i = 0; i < array.length; ++i) {
        if (array[i] instanceof Array) {
            flatten(array[i], function(res) {
                check(res);
            });
        }
        else {
            check([array[i]]);
        }
    }
}