to throw a

• <function> to throw (a|an) <function>

Asserts that the function throws an instance of a specific constructor.

function willThrow() {
  throw new SyntaxError('The error message');
}

expect(willThrow, 'to throw a', SyntaxError);

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

expect(willThrow, 'to throw a', RangeError);

expected
function willThrow() {
  throw new SyntaxError('The error message');
}
to throw a RangeError
  
expected SyntaxError('The error message') to be a RangeError

The assertion also fails if the function doesn't throw at all:

function willNotThrow() {}
 
expect(willNotThrow, 'to throw a', RangeError);

expected function willNotThrow() {} to throw a RangeError
  
expected function willNotThrow() {} to throw
  
did not throw

The thrown error is provided as the fulfillment value of the returned promise, so you can do further assertions like this:

return expect(willThrow, 'to throw a', SyntaxError).then(function (err) {
  expect(err, 'to have message', /\bmessage/);
});

To test functions that require input wrap the function invocation in an anonymous function:

function willThrow(input) {
  if (input) throw new SyntaxError('The error message');
  return input;
}
expect(
  function () {
    willThrow('input.here');
  },
  'to throw a',
  SyntaxError
);