Do you use anything like Redux? When writing Redux state reducers and actions, it all seems so... simple. There's a lot of code like `return { ...currentState, newProp }` which seems too silly to test.
I often leave off tests for strictly "synchronous" actions like `return { ...currentState, newProp }`. Most of my tests are for confirming that the right paths are taken for actions that trigger one or more async operations to ensure correct failure case handling.
JS tests like this are so stupid cheap that we still write them for even the most basic test. Not sure I'm convinced it's worth it yet and I might regret it the first time we refactor a larger reducer.