Async iterator gives TypeError: undefined is not a function

I am trying to learn how to use the async module in node.js and I fall into to the following problem:

I first wrote this script which works fine (to my understanding)

var async = require('async');
var seconds_values = [];

process();

function process() {
    var names = ['One', 'Two', 'ten', 'hello'];

    modifications(names);
}

function modifications(somenames) {
    async.each(somenames, function (one_name, callback) {
        var random_num = Math.floor((Math.random() * 100000000) + 1);

        seconds_values.push(one_name + '_' + random_num);
        callback();
    }, function(err){
        async.each(seconds_values, function (one_value, callback) {
            console.log(one_value);
            callback();
        },
        function(err){
            console.log('==== The END ===');
        });
    });
}

The I tried to assign the iterator to a variable, to make the code more readable.

var async = require('async');
var seconds_values = [];

process();

function process() {
    var names = ['One', 'Two', 'ten', 'hello'];

    modifications(names);
}

var first_function = function (one_name, callback) {
    var random_num = Math.floor((Math.random() * 100000000) + 1);

    seconds_values.push(one_name + '_' + random_num);
    callback();
}

function modifications(somenames) {
    async.each(somenames, first_function, function(err){
        async.each(seconds_values, function (one_value, callback) {
            console.log(one_value);
            callback();
        },
        function(err){
            console.log('==== The END ===');
        });
    });
}

The second script gives me the following error:

iterator(x, only_once(done) );
        ^
TypeError: undefined is not a function

I think I am missing something fundamental here. Could you please point me out what, if possible?