A function A is dependent on B, B can be another function, interface or anything.function B is inside function A, so function A should take care of the things that are also related to B, so in dependency injection we pass the dependency of A as a parameter to the function A, so here we pass B as a parameter to A
DI makes testing easy, if the function is hard to test, it is possible that it is hard wired with a dependency
try to separate from how the data is generated from how the data is used both are different things
A function A uses function B in it’s implementation in real world, function A could be something related money transfer or a function with a timer which takes time to run and function B is the dependency of function A, now we can’t test function A using real money or waste time waiting for it to run, so what do we do, we will try to create something really similar to the function B, which is known as mocking and pass it as a parameter, so now instread of calling function related to the original function B we can call those function from the mock that we made, which solves the problem and makes testing easy
When testing Function A, you don't want to perform actual money transfers or wait for real-time delays, as this can be impractical, time-consuming, and might involve real-world consequences. Instead, you create a mock or a simulated version of Function B. This mock behaves similarly to the real Function B but allows you to control its responses, simulate various scenarios, and speed up the testing process.
By substituting the real Function B with its mock during testing, you can ensure that Function A behaves as expected without relying on the actual implementation of Function B. This not only makes testing more efficient but also allows you to test different scenarios, error conditions, and edge cases more easily.