[2020-03-31] Promise Usage Tips

Just some personal observations while working extensively with asynchronous code and promises in particular.

1. In a promise then() function, returned values are automatically resolved, and throw's are automatically rejected.

2. One extremely common anti-pattern I see from JavaScript developers is the misuse of Promise.reject(...). When using async/await promise rejection is analogous to throwing an exception, which should not be used for business logic. A good rule of thumb is to throw or reject only when assumptions made by your business logic have not been met, eg. null parameters and such.

3. async/await is just a layer on top of promises. Adding async to a function that doesn’t need it (e.g. it doesn’t call await inside) will add a wasteful promise wrapper and a next-tick operation. This is because a promise always resolves on a next-tick and async methods always return a promise.