For instance, look for places where the input/output isn't consistent or changes based on a condition. Or when a function is doing too much (except for larger user-facing functions). Or when a function has too much code that could be split out more into smaller more composable functions.
The easier and simpler a function is that doesn't depend on too many other things, the easier it is to test. This follows the SOLID principles of software engineering: https://en.wikipedia.org/wiki/SOLID. It also follows functional programming.
For instance, look for places where the input/output isn't consistent or changes based on a condition. Or when a function is doing too much (except for larger user-facing functions). Or when a function has too much code that could be split out more into smaller more composable functions.
The easier and simpler a function is that doesn't depend on too many other things, the easier it is to test. This follows the SOLID principles of software engineering: https://en.wikipedia.org/wiki/SOLID. It also follows functional programming.