Modern JavaScript Web Development Cookbook

Using spies

The functional tests we wrote are quite good, but just won't do for some situations, such as when you work with callbacks. Let's turn to another piece of code we wrote: the user validation routine we used for JWT. Basically, this function received a username, a password, and an error-first callback, which was used to signal whether the username really had that password or not. We wrote very basic validation code (a single user was accepted!), but that doesn't matter here; we want to look at how we can deal with the callback. The important parts we care about now are highlighted in the following code extract:

const validateUser = (    userName: string,    password: string,    callback: (?string, ?string) => void) => { if (!userName || !password) ...