Check if variable is callable

Question

I want to create a helper function isCallback that returns whether a function is callable.

I have a type that can either be true or a callback with a specific argument. In my current code, I have lots of checks like typeof foo === 'function' and I want to refactor those with the isCallback function.

I created this helper function isCallback:

export const isCallback = (maybeFunction: unknown): boolean =>
  typeof maybeFunction === 'function'

My problem is that when I use it TypeScript is confused:

if(isCallback(foo)) {
  foo(myArgs)  // Error here
}

and complains with:

This expression is not callable.
  Not all constituents of type 'true | ((result: Result) => void)' are callable.
    Type 'true' has no call signatures.

How can I create a function that returns true if the variable is callable and TypeScript knows about it, too?

Answer 1

As @jonrsharpe pointed out, using type predicates works.

export const isCallback = (
  maybeFunction: true | ((...args: any[]) => void),
): maybeFunction is (...args: any[]) => void =>
  typeof maybeFunction === 'function'