API stands for Application Programming Interface, which specifies how one component should interact with the other. It consists of a set of routines, protocols and tools for building the software applications.
The API Testing is performed for the system, which has a collection of API that ought to be tested. During Testing, a test of following things is looked at.
Exploring boundary conditions and ensuring that the test harness varies parameters of the API calls in ways that verify functionality and expose failures.
Generating more value added parameter combinations to verify the calls with two or more parameters.
Verifying the behaviour of the API which is considering the external environment conditions such as files, peripheral devices, and so forth.
Verifying the Sequence of API calls and check if the API's produce useful results from successive calls.
Return Value based on input condition - The return value from the API's are checked based on the input condition.
Verify if the API's does not return anything.
Verify if the API triggers some other event or calls another API. The Events output should be tracked and verified.
Verify if the API is updating any data structure.