Default Parameter Values

Optional parameters leave you a nil to handle. Often what you really want is a fallback value: "if the caller didn't say, assume X". Luau has no default syntax in the signature (there is no greeting: string = "Hello"); instead you use the classic Lua idiom, now with types:

function greet(name: string, greeting: string?): string
    greeting = greeting or "Hello"
    return `{greeting}, {name}`
end

print(greet("Alice"))            -- Hello, Alice
print(greet("Alice", "Howdy"))   -- Howdy, Alice

The parameter is annotated optional (string?), and the first line of the body replaces nil with the default. It works because nil is falsy: nil or "Hello" evaluates to "Hello". After that line, the checker knows greeting can no longer be nil.