version: "1.0.1" name: roblox-luau-core description: "Use when writing, editing, or debugging any Luau: tables, control flow, string patterns, scope, closures, and errors ported from JS/Python." when_to_use: "Also for Luau syntax errors, nil indexing, wrong loop or table behavior, string formatting, and reviewing Luau code for correctness." last_reviewed: 2026-10-14 sources:

• https://luau.org/syntax
• https://create.roblox.com/docs/luau/control-structures
• https://luau.org/library
• https://raw.githubusercontent.com/Kampfkarren/kampfkarren-luau-guidelines/main/README.md
• https://raw.githubusercontent.com/Roblox/creator-docs/main/content/en-us/reference/engine/libraries/vector.yaml

Luau Core Language

When to Load

Load for Luau semantics and JS/Python translation. Types: roblox-luau-types; lifecycle: roblox-luau-patterns; engine APIs: domain skills.

Quick Reference

• Only false and nil are falsy. 0, "", and {} are truthy.
• Equality does not coerce types. 0 == "0" is false. Inequality is ~=.
• Array conventions are 1-based. #array is meaningful only for a sequence without nil gaps.
• Assigning nil removes a table key; assigning a table copies the reference; table.clone is shallow.
• Dictionary iteration order is unspecified. Do not make behavior depend on it.
• Dynamic keys require brackets: record[field]. Dot syntax uses a literal identifier.
• Missing arguments become nil; extra results and arguments can be discarded.
• Prefer an if expression over condition and a or b when a may be falsy.
• Locals start at their declaration; repeat locals reach until. Forward-declare mutual recursion.
• Luau string patterns are not regular expressions. Their syntax and capabilities differ.
• Backtick interpolation and .. are valid. Join many fragments once in hot loops.
• NaN does not equal itself and defeats </>; test with x ~= x.
• Binary data uses the buffer library: fixed size, 0-based offsets, explicit-width reads/writes. Avoid buffer.readinteger/writeinteger (stubs, not released runtime).
• For Base64, hashing, or compression use EncodingService (buffers, not strings; JSON via HttpService). Details: see full reference.

local label = if enabled then "On" else "Off"
local message = `{name}: {score}`
 
local byId: {[number]: string} = {}
byId[userId] = name
 
for index, value in values do
    print(index, value)
end

Translation traps

• JavaScript ===, !==, null, optional chaining, spread, arrow functions, and array methods are not Luau syntax.
• Python None, zero-based list assumptions, list comprehensions, exceptions, and implicit tuple behavior do not translate directly.
• x or fallback tests truthiness, not only absence. It replaces a valid false value.
• typeof(value) recognizes Roblox datatypes; type(value) reports the underlying Luau type category.

Review

Valid Luau, no nil gaps, no assumed dictionary order, no accidental aliases, no cross-language syntax, no fallbacks hiding false or nil.

Detailed language traps and translation examples: references/full.md