functions

Returns an array of function property names from own (and optionally inherited) enumerable properties of an object.

Use Object.keys(obj) to iterate over the object's own properties. If inherited is true, use Object.get.PrototypeOf(obj) to also get the object's inherited properties. Use Array.prototype.filter() to keep only those properties that are functions. Omit the second argument, inherited, to not include inherited properties by default.

const functions = (obj, inherited = false) =>
  (inherited
    ? [...Object.keys(obj), ...Object.keys(Object.getPrototypeOf(obj))]
    : Object.keys(obj)
  ).filter((key) => typeof obj[key] === "function");

function Foo() {
  this.a = () => 1;
  this.b = () => 2;
}
Foo.prototype.c = () => 3;
functions(new Foo()); // ['a', 'b']
functions(new Foo(), true); // ['a', 'b', 'c']