Functions — reusable instructions

Here's a script that gives a player coins twice:

local coins = 0
coins = coins + 5
coins = coins + 5

It works, but imagine doing this for fifty features. Copy-pasting the same lines everywhere is how bugs are born. Functions are the cure: you write the job once, give it a name, and call it whenever you need it.

#Writing your first function

function addCoins(amount)
    coins = coins + amount
end

Reading it left to right:

• function — I'm about to define a job.
• addCoins — its name. You call the job by this name.
• (amount) — the parameter: a box the function gets filled when you call it.
• coins = coins + amount — the actual job.
• end — the job stops here.

Defining a function does nothing by itself. You have to call it:

addCoins(5)  -- gives 5 coins
addCoins(10) -- gives 10 coins

One function, two calls, two different results. The magic is the parameter — every call fills the box with a different value.

#Parameters and arguments

These two words get used interchangeably, but here's the precise version: parameters are the names in the definition (amount), arguments are the values you pass at the call site (5, 10).

You can have as many parameters as you need:

function giveReward(playerName, amount, reason)
    print(playerName .. " earned " .. amount .. " coins for " .. reason)
end

giveReward("Sam", 100, "winning")

#The pattern you'll see everywhere

Nearly every useful function is: take inputs, do work. Even the built-in tools are functions:

print("hi", 5)       -- takes whatever, shows it
tonumber("10")       -- takes a string, returns a number

You've been calling functions since the second lesson. print(...) is a function with a variable number of inputs.

#Common mistakes

• Forgetting end. Every function needs its end. Luau errors — the Output tells you the line, and usually it's the missing end.
• Calling without passing the arguments. addCoins() when the function needs (amount) gives you nil inside, and nil + 5 errors.
• Forgetting to call at all. Defining addCoins and never calling it means nothing happens.

Next: getting answers back from a function with return.