Testing with Minitest

Minitest is Ruby's built-in testing framework that lets you verify your code automatically with concise assertions, so you catch bugs before your users do.

Learn Testing with Minitest in our free Ruby course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Ruby course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

You'll write test classes and assertions, use setup , check for raised errors, and try the spec style.

What You'll Learn in This Lesson

1️⃣ Your First Test

A Minitest test is a class that inherits from Minitest::Test . Every method whose name starts with test_ is a test. Inside, call assertions like assert_equal(expected, actual) or assert(condition) . The line require 'minitest/autorun' makes the file run its tests automatically.

2️⃣ More Assertions, setup & assert_raises

setup runs before every test_ method, giving each test fresh, isolated state. Beyond assert_equal there's assert_nil , assert_includes , and assert_raises , which passes only if the block raises the expected exception.

3️⃣ Spec Style & Minitest vs RSpec

Minitest also offers a spec style that reads like a sentence: describe groups tests, it defines one, and expectations use _(value).must_equal expected . It's a lighter alternative to RSpec's expect(...).to DSL, with no extra gem to install.

Your turn. Complete two assertions about strings so the suite passes. Fill in each ___ .

Mini-Challenge: Test a Stack

Build a tiny Stack and write a Minitest suite for it with a setup , an equality assertion, and an assert_raises . This mirrors how you'd test a real class. Run with ruby test_stack.rb .

Common Errors (and the fix)

• No tests run — ❌ Method names don't start with test_ . ✅ Minitest only runs test_ -prefixed methods.
• "uninitialized constant Minitest" — ❌ Missing the require. ✅ Add require 'minitest/autorun' at the top.
• Args backwards — ❌ assert_equal actual, expected . ✅ Order is assert_equal(expected, actual) .
• assert_raises without a block — ❌ Passing a value instead of code. ✅ Give it a block: assert_raises(E) { ... } .

Pro Tips

• 💡 One behaviour per test: small focused tests give clearer failure messages.
• 💡 teardown too: pair setup with teardown for cleanup after each test.
• 💡 Floats: use assert_in_delta instead of assert_equal for floating-point results.

📋 Quick Reference — Minitest

Concept

Ruby

Load + run

require 'minitest/autorun'

Test class

< Minitest::Test

A test

def test_thing

Equality

assert_equal exp, act

Raises

assert_raises(E) { }

Before each

def setup

Frequently Asked Questions

🎉 Lesson Complete!

• ✅ A test is a test_ method in a Minitest::Test class
• ✅ require 'minitest/autorun' runs everything
• ✅ assert , assert_equal , assert_raises
• ✅ setup gives each test fresh state
• ✅ Spec style ( describe/it ) is a lighter alternative to RSpec
• ✅ Next lesson: Checkpoint — OOP & Stdlib

Practice quiz

Continue this course

• Previous: JSON & YAML
• Next: Checkpoint: OOP & Stdlib