Hello, World

The Goal

Before learning any syntax, you’ll write a test. This is the pattern you’ll use for every chapter in this guide:

1. Write a failing test that describes what you want.
2. Write the minimal code to make it pass.
3. Refactor without breaking the test.

Your First File

Create a file called hello.lua:

local function hello()
  return "Hello, world!"
end

return { hello = hello }

Now create hello_test.lua alongside it:

local lu = require('luaunit')
local hello_module = require('hello')

TestHello = {}

function TestHello:test_returns_greeting()
  lu.assertEquals(hello_module.hello(), "Hello, world!")
end

os.exit(lu.LuaUnit.run())

Run the test:

lua hello_test.lua

You should see:

.
Ran 1 tests in 0.000 seconds, 1 success, 0 failures

Tip

If require('luaunit') fails, jump to Setting Up LuaUnit first.

The Red Step

The TDD cycle starts with a failing test. Let’s prove that by writing a test for something that doesn’t exist yet.

Add a new test case to hello_test.lua:

function TestHello:test_greets_by_name()
  lu.assertEquals(hello_module.greet("Ada"), "Hello, Ada!")
end

Run the tests again. You’ll see a failure:

F.
hello_test.lua:12: expected: "Hello, Ada!"
got: nil
Ran 2 tests in 0.000 seconds, 1 success, 1 failure

That red output is exactly what you want. You’ve described a behavior that doesn’t exist yet.

The Green Step

Now write the minimum code to make it pass. Update hello.lua:

local function hello()
  return "Hello, world!"
end

local function greet(name)
  return "Hello, " .. name .. "!"
end

return {
  hello = hello,
  greet = greet,
}

Run the tests:

..
Ran 2 tests in 0.000 seconds, 2 successes, 0 failures

The Refactor Step

Both functions share the same greeting pattern. Now that tests are green, it’s safe to clean up:

local function greet(name)
  return "Hello, " .. (name or "world") .. "!"
end

return { greet = greet }

Update hello_test.lua to use greet everywhere:

function TestHello:test_default_greeting()
  lu.assertEquals(hello_module.greet(), "Hello, world!")
end

function TestHello:test_greets_by_name()
  lu.assertEquals(hello_module.greet("Ada"), "Hello, Ada!")
end

Run tests. Still green. You’ve completed one full TDD cycle.

What You Learned

• Lua modules are just tables returned from a file.
• require loads a module by filename (without .lua).
• The .. operator concatenates strings.
• or provides default values since Lua has no default parameters.

Next Steps

Head to Setting Up LuaUnit for a deeper look at the test framework, or skip straight to Variables and Types if you’re already comfortable with the tooling.