Filtering truthy values with select(&:itself)

TL;DR

select keeps the elements for which the block returns a truthy value, and &:itself turns each element into its own block result. In Ruby only nil and false are falsy, so those two are dropped and everything else survives, including the empty string: [1, 2, 2, "", 3].

Step by step

&:itself converts the symbol :itself into a block via Symbol#to_proc, so the call is equivalent to:

a = [1, 2, nil, 2, "", false, 3]

a.select { |x| x.itself } # => [1, 2, 2, "", 3]

Kernel#itself returns its receiver unchanged (42.itself # => 42), which makes the block an identity function: each element is judged by its own truthiness. nil and false are the only falsy values in Ruby, so 0 and "" count as truthy.

Edge cases

This idiom is stricter than compact and looser than Active Support's compact_blank:

a = [1, 2, nil, 2, "", false, 3]

a.select(&:itself) # => [1, 2, 2, "", 3]
a.compact          # => [1, 2, 2, "", false, 3]

Array#compact removes only nil, so false and "" stay. Array#compact_blank (Active Support, Rails 6.1+) removes everything blank?: nil, false, "", empty arrays and hashes, and whitespace-only strings:

require "active_support/all"

a = [1, 2, nil, 2, "", false, 3]

a.compact_blank # => [1, 2, 2, 3]

Pick the one that matches the values you actually want to drop; they are not interchangeable.