to call the callback

• <function> to call the callback

Asserts that a node.js-style asynchronous function taking a single callback will call it.

function mySuccessfulAsyncFunction(cb) {
  setTimeout(function () {
    cb();
  });
}
 
return expect(mySuccessfulAsyncFunction, 'to call the callback');

If the callback is never called, it will hang until your test framework marks it as timed out. So the assertion itself only ever fails if the function throws an exception synchronously:

function errorOut(cb) {
  throw new Error('ugh');
}
return expect(errorOut, 'to call the callback');

ugh

The assertion will pass even if the function passes an error to the callback. If you want that case to fail, look into the to call the callback without error assertion.

If you want the parameters passed to the callback to be the subject of further assertions, you might be able to use the when passed as parameters to assertion.

The parameters passed to the callback are also provided as the value of the returned promise, so you can do further assertions like this:

function asyncFn(cb) {
  setTimeout(function () {
    cb(null, 'foo');
  });
}

return expect(asyncFn, 'to call the callback').then(function (args) {
  // args will be [null, 'foo'];
});

Or using the Bluebird-specific .spread extension:

return expect(asyncFn, 'to call the callback').spread(function (err, result) {
  expect(err, 'to be null');
  expect(result, 'to equal', 'foo');
});