"Count where" in a collection

Question

Using lodash, what would be a good way to count the number of objects in a collection conditionally? Say I wanted to count the number of objects where

a < 4

in the following collection

[{a : 1}, {a : 2}, {a : 3}, {a : 4}, {a : 5}, {a : 6}]

Answer 1

Below you can find an easy way to achieve that using the filter method:

var b = _.filter(a, function(o) { if (o.a < 4) return o }).length;