Create assertions.md

articles/testing/assertions.md

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+# Assertions 
+
+## What is an Assertion? 
+
+- 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.