Nil Safety In Strict Mode

The most common Lua crash is "attempt to index nil": code that assumed a value existed when it didn't. Luau's answer is nil discipline: under --!strict, a plain string can never be nil. If nil is a real possibility, the type must say so with ?:

--!strict
local nickname: string? = nil   -- ok: string OR nil
local username: string = nil    -- ✗ type error in strict mode

The discipline cuts both ways: given a string?, strict mode won't let you use it as a string until you've ruled out nil. The standard tool is a plain comparison, which narrows just like a typeof guard:

local function shout(name: string?): string
    if name ~= nil then
        return string.upper(name) -- name is string here
    end
    return "NOBODY"
end

When nil would be a bug rather than a valid case, use assert: after assert(x ~= nil) the checker treats x as non-nil for the rest of the scope, and at runtime the assert crashes early, at the point of the broken assumption, instead of somewhere far away.

The payoff: the checker forces every "might be missing" value to be handled while you edit, and the runtime error simply never happens.