to have arity

• <function> to have arity <number>

Asserts that the function takes the given number of arguments.

expect(Math.max, 'to have arity', 2);
expect('wat'.substring, 'to have arity', 2);

In case of a failing expectation you get the following output:

expect(
  function wat(foo, bar) {
    return foo + bar;
  },
  'to have arity',
  3
);

expected function wat(foo, bar) { return foo + bar; } to have arity 3