Question

Can someone explain meaning and difference of keep function from simple map(array, fun)

var array = [1, 2, 3, 4];
var even = function (x) { return x % 2 === 0; };

array.map(even);

Description of keep function in Closure docs

Answer 1

Look at this example from the docs you linked:

user> (keep #(if (odd? %) %) (range 10))
(1 3 5 7 9)
user> (map #(if (odd? %) %) (range 10))
(nil 1 nil 3 nil 5 nil 7 nil 9)

As you can see keep does not include nil values in the resulting list – map does.