How to convert a ruby hash object to JSON?

One of the numerous niceties of Ruby is the possibility to extend existing classes with your own methods. That's called "class reopening" or monkey-patching (the meaning of the latter can vary, though).

So, take a look here:

car = {:make => "bmw", :year => "2003"}
# => {:make=>"bmw", :year=>"2003"}
car.to_json
# NoMethodError: undefined method `to_json' for {:make=>"bmw", :year=>"2003"}:Hash
#   from (irb):11
#   from /usr/bin/irb:12:in `<main>'
require 'json'
# => true
car.to_json
# => "{"make":"bmw","year":"2003"}"

As you can see, requiring json has magically brought method to_json to our Hash.

require 'json/ext' # to use the C based extension instead of json/pure

puts {hash: 123}.to_json

You can also use JSON.generate:

require 'json'

JSON.generate({ foo: "bar" })
=> "{\"foo\":\"bar\"}"

Or its alias, JSON.unparse:

require 'json'

JSON.unparse({ foo: "bar" })
=> "{\"foo\":\"bar\"}"

Add the following line on the top of your file

require 'json'

Then you can use:

car = {:make => "bmw", :year => "2003"}
car.to_json

Alternatively, you can use:

JSON.generate({:make => "bmw", :year => "2003"})