I write tests when I know the answers before to some complicated code. A good example of this is writing some code to tell if a point is inside a triangle or n

I write tests when I know the answers before to some complicated code. A good example of this is writing some code to tell if a point is inside a triangle or not. Setting up the test case is easy with three points for the triangle, a point or two inside, points outside, etc... Writing the code is simple enough, but possibly prone to flipping a sign for a slope, or maybe translating the equation wrong. If I'm writing a web app, I just don't bother to write them before hand. I mostly code until I get what I want and then write the test. I think of this as "nailing it in place". I want to make sure through the test that this code continues to produce what I expect. I'll write as many tests as I feel are necessary but probably not explore every branch.