Function arity

tests/js/quint.js:9–12

(fn, length)

const QUnitAssert = {
  arity(fn, length) 
  {
    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`);