Why aren't Temporary Credentials being generated using aws-sdk?

I am using the aws-sdk package for node.js and wish to generate temporary credentials.

I have:

  AWS.Config.credentials = new AWS.Credentials 'myKey', 'mySecret'
  console.log AWS.Config.credentials

  AWS.config.credentials = new AWS.TemporaryCredentials()
    # DurationSeconds: 1200

  console.log '===='
  console.log AWS.config.credentials

However, when I log out the new credentials, it shows undefined for the accessKeyId. Am I doing something incorrectly?