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roblox-ts's assert uses "JavaScript truthiness" for it's condition. This means that "" (empty string), 0, and NaN values will cause the assertion to fail in addition to undefined and false.

The reason for this is so that we can take advantage of TypeScript's asserts value predicate feature.

function foo(instance: Instance) {
print(instance.Size); // instance _must_ be a Part to reach this line

Because of this change, assert(0) or assert("") will cause an error in roblox-ts, but not in Luau.


Unfortunately, TypeScript already has an operator named "typeof" in the form of typeof x. Because of this, we cannot expose the Luau typeof() function directly. To get around this, we compile typeOf(value) into typeof(value).


Checking types with typeOf is usually not very useful with roblox-ts unless you need the string value that is returned. This is because TypeScript cannot infer that your if-statement confirmed the value was type checked:

function foo(value: unknown) {
if (typeOf(value) === "Vector3") {
print(value.X); // error: value is still unknown!

To get around this, typeIs(value, "type") compiles to typeof(value) == "type" and helps TypeScript infer the value was type checked:

function foo(value: unknown) {
if (typeIs(value, "Vector3")) {
print(value.X); // success!


Similar to typeIs, classIs(value, "ClassName") compiles to value.ClassName == "ClassName". This is useful for cases where you might want to avoid instance.IsA().

function foo(value: Instance) {
// value.IsA("Script") would return true for LocalScripts!
if (classIs(value, "Script")) {


While pcall is available, it returns the type LuaTuple<[false, string] | [true, T]> which is difficult to use!

const [success, valueOrError] = pcall(() => 123);
// valueOrError is string | number
// if it worked -> number
// if it errored -> string
if (success) {
print(valueOrError); // string | number
} else {
print(valueOrError); // string | number

The problem here is that success and valueOrError are not linked after the destructure happens. You can avoid destructuring the result, but that makes things very hard to read.

To get around this, you can use opcall which works the same as pcall, but returns an object instead of the LuaTuple<T>.

// result is { success: true; value: T; } | { success: false; error: string; }
const result = opcall(() => 123);
if (result.success) {
print(result.value); // number
} else {
print(result.error); // string


The identity macro compiles to just the inner value you pass into it, allowing for a zero-cost type constraint abstraction. This is useful for verifying that a given value is the type you expect:

interface MyInterface {
a: number;
b: string;
c: boolean;
const objects = {
abc: identity<MyInterface>({
a: 123,
b: "abc",
c: true,