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Tests individual units or pieces of code for a system. The aim of component testing is to determine whether an individual function works properly in isolation before integrate with the system then combine it with other components.
Component testing is different from all later test stages as it is not done by testers but by the developers of the code, in the developer’s environment using IDE, debugging tools and test frameworks incorporating stubs and drivers.
As an example, in the V-model method Component Specification and Component Testing level unity is where is located at.