Function property names

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

• Use Object.keys() to iterate over the object's own properties.
• If inherited is true, use Object.getPrototypeOf() 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']