Bottom-up testing is a specific type of integration testing that tests the lowest components of a code base first. More generally, it refers to a middle phase in software testing that involves taking integrated code units and testing them together, before testing an entire system or code base.
IT professionals refer to design assemblies, or groups of code modules that are evaluated for overall functioning, in integrated testing environments.
Bottom-up testing is the opposite of top-down testing, where the top integrated elements are tested first. Experts explain that while bottom-up testing can be useful in producing a smaller, independent test basis, it doesn’t provide the same kind of skeletal program demonstration design that top-down testing does. With bottom-up testing, developers build up to that scale where the program mainly coincides with the user interface. Some people see this as a kind of backward testing, but each individual developer team must decide for reasons of purpose and practicality which type of software testing suits the project better.