@rbxts/jsnatives

Object.isCallable

Determines whether a value is a callable function.

Signature

function isCallable<T>(obj: unknown): obj is Callback
function isCallable<T extends object>(obj: T): obj is T & Callback

Description

The Object.isCallable() method determines whether a value is a function that can be called. This includes regular functions, methods, and objects with a __call metatable or apply trap for proxies.

The method returns a type predicate that narrows the type to Callback or T & Callback depending on the input.

Examples

Basic function check

const func = function() { return "hello"; };
const obj = {a: 1, b: 2};

print(Object.isCallable(func)); // true
print(Object.isCallable(obj)); // false

Working with proxies

const callableProxy = new Proxy({}, {
  apply: () => {
    return 1;
  }
});

const regularProxy = new Proxy({}, {});

print(Object.isCallable(callableProxy)); // true
print(Object.isCallable(regularProxy)); // false

Working with metatables

const callableMetatable = setmetatable({}, {
  __call: () => {
    return 1;
  }
});

print(Object.isCallable(callableMetatable)); // true

Type narrowing

function processValue(value: unknown) {
  if (Object.isCallable(value)) {
    // In this block, value is typed as Callback
    const result = value();
    return result;
  } else {
    // In this block, value is still unknown
    return "Not a function";
  }
}

function processObject<T extends object>(obj: T) {
  if (Object.isCallable(obj)) {
    // In this block, obj is typed as T & Callback
    const result = obj();
    return result;
  } else {
    // In this block, obj is still T
    return obj;
  }
}