Difference in map between underscore/lodash and jQuery/Zepto.

map-with-filter-differences.js

// Difference in map between underscore/lodash and jQuery/Zepto.
// 
// $ allows filtering at the same time; _ does not.
// 
// In your client-side JavaScript apps that use _ and $, this can come as a
// surprise since the assumption is that all your map, each, filter, etc. needs
// are fulfilled by _, and you'd be right to use them for speed since they
// alias native methods where available. However, sometimes $ can be useful,
// and this is one such case where that's true.
// 
// The battle is between speed and verbosity, where $ potentially wins the
// former (half the loop iterations) but the last line using _ wins the latter.

var numbers = [1,2,3,4];

var doubleEven = function (n) {
    if (n % 2) return;
    return n * 2;
};

_.map(numbers, doubleEven);
// -> [undefined, 4, undefined, 8]

$.map(numbers, doubleEven);
// -> [4, 8]

// Desired outcome using _
var oddFilter = function (n) {
    return ! (n % 2);
};
_.chain(numbers).filter(oddFilter).map(doubleEven).value();
// -> [4, 8]