Testing rejected promise in Mocha/Chai

I have a class that rejects a promise:

Sync.prototype.doCall = function(verb, method, data) {
  var self = this;

  self.client = P.promisifyAll(new Client());

  var res = this.queue.then(function() {
    return self.client.callAsync(verb, method, data)
      .then(function(res) {
        return;
      })
      .catch(function(err) {    
        // This is what gets called in my test    
        return P.reject('Boo');
      });
  });

  this.queue = res.delay(this.options.throttle * 1000);
  return res;
};

Sync.prototype.sendNote = function(data) {
  var self = this;
  return self.doCall('POST', '/Invoice', {
    Invoice: data
  }).then(function(res) {
    return data;
  });
};

In my test:

return expect(s.sendNote(data)).to.eventually.be.rejectedWith('Boo');

However while the test passes it throws the error to the console.

Unhandled rejection Error: Boo ...

With non promise errors I have used bind to test to prevent the error from being thrown until Chai could wrap and test:

return expect(s.sendNote.bind(s, data)).to.eventually.be.rejectedWith('Boo');

However this does not work with this and returns:

TypeError: [Function] is not a thenable.

What is the correct way to test for this?

You're getting the error because sendNote is being rejected and you're not catching it.

Try:

var callPromise = self.doCall('POST', '/Invoice', {
  Invoice: data
}).then(function(res) {
  return data;
});

callPromise.catch(function(reason) {
  console.info('sendNote failed with reason:', reason);
});

return callPromise;

Looks like you'll also have to move your existing catch one block out:

var res = this.queue.then(function() {
  return self.client.callAsync(verb, method, data)
    .then(function(res) {
      return;
    });
  }).catch(function(err) {    
    // This is what gets called in my test    
    return P.reject('Boo');
  });