mapMany (SelectMany) in JavaScript

gistfile1.js

function mapMany (arr, mapper) {
    return arr.reduce(function (prev, curr, i) {
        return prev.concat(mapper(curr));        
    },[]);
}

// usage
var res1 = mapMany(['abcd', '1234'], function (item) {
  return item.split('');
});
console.log(res1); 
// --> ["a", "b", "c", "d", "1", "2", "3", "4"]