An integration test checks whether two or more parts of a system work correctly when they are joined up. Each piece might pass its own checks, yet still fail the moment it has to hand data to the next piece. Integration tests look exactly at those handoffs.
Imagine assembling furniture from two boxes. Each shelf is fine on its own, but the test that matters is whether the screws from box A actually fit the holes in box B. An integration test is that fit check for software: does the checkout page send the right total to the payment service, and does the database store it the way the app expects? It sits one level above a unit test, which only inspects a single piece, and one level below an end-to-end test, which runs the whole journey a user takes.
A lot of real-world bugs live in these seams. Two parts each behave correctly, but they disagree about a date format or a missing field, and only a test that runs them together will reveal it.
Writing them takes a little setup. An integration test often needs a real database or a stand-in for an outside service, so it runs slower than a unit test and is more work to keep stable. The usual rule of thumb is to write many fast unit tests and a smaller, well-chosen set of integration tests around the connections that matter most, like payments or sign-in.
That balance is where they earn their keep. You catch the mismatches that unit tests cannot see, without paying the slow, brittle cost of testing everything end to end.
At TopDevs we write integration tests around the connections in your system, so the moment two components stop agreeing, we know about it before your users do.
