Writing intuitive assertions

The core of any test is its assertions. An assertion prescribes exactly what we expect to occur, and so it is vital not only that we craft it accurately but that we craft it in a way that our expectation is made utterly clear.

A single test will usually involve several assertions. And a test will typically follow the form of: given an input of X, do I receive an output of Y? Sometimes, establishing Y is complex and may not be constrained to a singular assertion. We may want to introspect Y to confirm that it is truly the desired output.

Consider a function named getActiveUsers(users), which will return only the active users from a set of all users. We may wish to make several assertions about its output:

const activeUsers = getActiveUsers([
  { name: 'Bob', active: false },
  { name: 'Sue', active: true },
  { name: 'Yin&apos...