Ruby Academy · Lesson

Reading and Writing JSON

JSON.parse and generate.

Lesson 1 of 413 steps

Reading and Writing JSON is a free Ruby Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Ruby Academy learning path, and your progress syncs across the web and the CoddyKit app. The Ruby Academy course includes 4 lessons in total.

What Is JSON?

JSON (JavaScript Object Notation) is a lightweight text format for data exchange. It maps cleanly to Ruby hashes and arrays.

Require the json library to use it.

require 'json'
data = { name: 'Ruby', year: 1995 }
puts(JSON.generate(data))

Parsing JSON Text

JSON.parse turns a JSON string into Ruby objects: objects become Hashes, arrays become Arrays.

require 'json'
text = '{"name":"Ruby","year":1995}'
h = JSON.parse(text)
puts(h['name'])
puts(h['year'])

Generating JSON Text

JSON.generate (or to_json on objects) converts Ruby data into a compact JSON string.

require 'json'
arr = [1, 2, { ok: true }]
puts(arr.to_json)

Pretty Printing

JSON.pretty_generate produces human-readable, indented JSON, ideal for config files and debugging.

require 'json'
data = { name: 'Ruby', tags: ['fun', 'oop'] }
puts(JSON.pretty_generate(data))

Symbol Keys on Parse

By default parsed keys are Strings. Pass symbolize_names: true to get Symbol keys instead.

require 'json'
h = JSON.parse('{"name":"Ruby"}', symbolize_names: true)
puts(h[:name])

Nested Structures

JSON supports nesting. Parsing rebuilds the full tree of Hashes and Arrays.

require 'json'
text = '{"user":{"name":"Ada","roles":["admin","dev"]}}'
h = JSON.parse(text)
puts(h['user']['name'])
puts(h['user']['roles'][1])

Type Mapping

JSON types map to Ruby:

object → Hash, array → Array
string → String, number → Integer/Float
true/false → true/false, null → nil

require 'json'
h = JSON.parse('{"a":1,"b":1.5,"c":null,"d":true}')
puts(h['a'].class)
puts(h['c'].inspect)

Writing JSON to a File

Combine pretty_generate with File.write to save data to disk.

require 'json'
require 'tmpdir'
path = File.join(Dir.tmpdir, 'data.json')
File.write(path, JSON.pretty_generate({ ok: true }))
puts(File.read(path))

Reading JSON from a File

Read the file's contents and parse them in one step.

require 'json'
require 'tmpdir'
path = File.join(Dir.tmpdir, 'cfg.json')
File.write(path, '{"port":8081}')
cfg = JSON.parse(File.read(path))
puts(cfg['port'])

Handling Parse Errors

Malformed JSON raises JSON::ParserError. Rescue it to handle bad input gracefully.

require 'json'
begin
  JSON.parse('{not valid}')
rescue JSON::ParserError => e
  puts 'Invalid JSON caught'
end

Round Trip

Generating then parsing should return equivalent data, a useful sanity check.

require 'json'
original = { id: 7, tags: ['a', 'b'] }
back = JSON.parse(original.to_json, symbolize_names: true)
puts(back == original)

Quick Check

Which option makes JSON.parse return Symbol keys instead of String keys?

Answer it in the interactive lesson

Recap: Reading and Writing JSON

You can move data in and out of JSON:

JSON.parse string to Ruby objects
to_json / JSON.generate to compact JSON
pretty_generate for readable output
symbolize_names: true for Symbol keys
Rescue JSON::ParserError on bad input

require 'json'
puts(JSON.parse('[1,2,3]').sum)

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Frequently Asked Questions

Is the “Reading and Writing JSON” lesson free?

Yes — any 3 lessons of the Ruby Academy learning path, including “Reading and Writing JSON”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Ruby Academy course includes 4 lessons in total.

What will I learn in “Reading and Writing JSON”?

JSON.parse and generate. You practise Ruby Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Ruby Academy?

No prior experience is required. Ruby Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Reading and Writing JSON” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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