An API test consists of a series of steps (these are sometimes HTTP requests) that can be executed collectively or individually.
An assertion is a specification that indicates the expected outcome (response) to a request executed in a test.
A test is unsuccesful if an assertion fails i.e. the actual outcome is not equal to the expected outcome
You can create assertions for status codes, response time, reponse content, header values, etc.
When you execute an assertion, you can determine the type of operation you want to perform with your expected outcomes.
Comparison Logic Available in Scenarios
equals
greater than
greater than or equals
less than
less than or equals
no equal
exists
length equals
contains
validate pass
validate fail
Why
Assertions are checked any time a test is executed.
Assertions are used to detemine the state of a test (pass or fail).
Assertions are ideal for discovering if an API satisfies stipulated objectives.
Assertions in Scenarios
Scenarios in Stoplight are grouped into collections. To create an assertion for a step in Scenarios, you need to create a collection and add your Scenarios to it.