Names, Values, and the Only Two Things That Are False

Write health = 100 in Luau and it works. Write local health = 100 and it also works, and the second one is right. Without local, the name becomes global — visible to every function anywhere in that script, kept alive for as long as the script is, and slower to read every single time.

local laser = script.Parent      -- lives in this script, and only herelocal damage = 100 count = 0                        -- global: works, and you did not mean it

The practical damage is collisions inside your own script. A helper function that forgets local on its own count is now writing to the same count as the rest of the script, and the number changes for reasons that are nowhere near the line you are staring at.

count = 0                -- meant as the script's touch counter local function tallyCoins(coins)    count = 0                -- meant as a fresh counter, and it is not    for _ in coins do        count += 1    end    return countend tallyCoins({ "a", "b", "c" })print(count)                 --> 3, and the touch counter is gone

Each script does get its own set of globals, so two separate scripts will not trample each other this way. That is little comfort: a long script with a few helper functions is exactly where this bites. Studio's Script Analysis window flags some suspicious globals, but not every one — the habit is the real protection.

Luau has a short list of types and you will use six of them constantly.

Use typeof() rather than Lua's older type(). The old one reports every Roblox object as "userdata", which tells you nothing; typeof() reports "Instance", "Vector3", "CFrame" and the rest by name.

Luau also lets you annotate a type, and Studio will check it as you write rather than when you run.

local damage: number = 100local partName: string = "Laser"local target: BasePart? = nil   -- the ? means "or nil", and is honest

nil is not an error and not an absence of a variable. It is a value meaning "there is nothing here", and it is what you get back from FindFirstChild and every other lookup that is allowed to come up empty.

The trouble is that nil travels. A part that was not found is nil; storing it in a variable stores nil; passing that variable to a function passes nil; and the crash finally happens wherever somebody tries to use it, which can be a long way from where the lookup went wrong.

local platform = workspace.Obby:FindFirstChild("Platfrom")  -- typoprint(platform)              --> nil, and no error yet platform.CanCollide = false  --> attempt to index nil with 'CanCollide'

Line one is the bug and line four is the error. This is a common shape of failure in Roblox scripting, and the chapter on debugging is largely about reading your way back from the second line to the first.

Here is the one that will catch you. In Python, an empty list is false. In JavaScript, zero is false. In Luau, nil and false are false, and every other value in the language is true.

Zero is true. An empty string is true. An empty table is true. The string "false" is true.

Luau joins strings with .., not +. A plus between two strings is an arithmetic error, and the message it gives you names the operation rather than the mistake.

local name = "Amara" print("Welcome, " .. name)     --> Welcome, Amaraprint("Score: " .. 42)         --> Score: 42   (numbers convert on their own) -- print takes several arguments and adds the spaces itself:print("Welcome,", name)        --> Welcome, Amara

That last form is the one to prefer while debugging. It needs no concatenation, so it cannot fail on a nil, and it prints something useful even when a value is not what you expected — print("part:", part) on a nil prints part: nil, where the .. version would throw before telling you anything.