sanitize.rb

require 'json'

string = { foo: 'bar', baz: { bill: 'bob' } }.to_json

def santize(string)
  # Turn string into Hash or Array
  result = JSON.parse(
    string,
    # This option lets us mutate strings
    # By default strings are frozen
    symbolize_names: true
  )

  # JSON implements our traverse function for us
  JSON.recurse_proc(
    result,
    # Sanitize method goes inside this proc
    &proc { |x| x.upcase!.reverse! if x.is_a? String }
  )

  result
end

puts santize(string)