Function isFunction

tests/js/quint.js:13–16

(x)

    if (fn.length !== length) throw new Error(`'${fn}' does not have arity of ${length}`);
  },
  isFunction(x) 
  {
    if (typeof x !== 'function') throw new Error(`'${x}' is not a function`);
  },
  name(x, name)
  {
    if (x.name !== name) throw new Error(`'${x}' does not have a name of ${name}`);