Very few developers know how to use mocks effectively when writing tests, and I spent a decade figuring out why. Let's take a journey, starting with the most egregious ways people abuse mocks. Slowly, we'll work our way to all the seemingly-good-but-actually-harmful mocking advice that's popular today. Our last stop will be a demonstration of how mocks can be used to arrive at highly modular, usable code designs, using our own library testdouble.js. Attendees will leave this talk with precise and useful language for discussing mocking and some ideas for how to write better tests.