In general, don't worry (too much) about tests in the beginning of a project until it's likely you'll have more than one user (aka yourself) and/or paying custo

In general, don't worry (too much) about tests in the beginning of a project until it's likely you'll have more than one user (aka yourself) and/or paying customers. If you have neither users nor customers, don't feel too guilty about your lack of tests. In many cases, it's more important to make something useful enough that makes an investment in more testing later worthwhile. Later, when you do have users and customers, you will want your tests to discover regressions before your users do. However, the longer you wait, the higher the risk that you'll discover your code is not easily testable... which will require lots of refactoring to make testable. And that refactoring is made harder by the lack of tests... In the meantime, if you're truly stuck, write down a list of how you're currently manually testing your code and write down edge and corner cases you're worried about. (disclosure: I make testing tools for a living.)