to have arity
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