Hash remove all except specific keys

What about slice?

hash.slice('firstName', 'lastName')
 # => { 'firstName' => 'John', 'lastName' => 'Smith' }

Available in Ruby since 2.5

Some other options:

h.select {|k,v| ["age", "address"].include?(k) }

Or you could do this:

class Hash
  def select_keys(*args)
    select {|k,v| args.include?(k) }
  end
end

So you can now just say:

h.select_keys("age", "address")

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