MyMatcher
being the name of your custom matcher function:expectation
object and a second argument which is a structure of all the arguments with which the matcher function was called with. It must then return a true or a false depending if it passes your criteria. It will most likely use the expectation
object to retrieve the actual and isNot values. It can also set a custom failure message on the expectation object itself by using the message
property of the expectation
object.beforeAll()
or beforeEach()
life-cycle method blocks for performance considerations and global availability.