Proper error handling in Promises
The standard way of catching errors in promises is to use .catch(onError). The onError handler runs if the previous promise is rejected.
But you can handle rejections using the second parameter of the then call too. A common source of confusion is that these two are not equivalent:
.then(onSuccess).catch(onError)
vs
.then(onSuccess, onError)
In the second case, rejections in onSuccess are not handled by onError.
Why is that?
The source of confusion is that rejection handling is somewhat similar to the try-catch structure. But unlike the try-catch where everything written before the catch is protected, promises work in steps. And rejections are handled in the next step.
Under the hood, catch(onError) is just a shorthand for .then(undefined, onError).
In the example above, the catch-ed version translates to:
.then(onSuccess).then(undefined, onError)
Which defines a separate step to handle rejections, while the .then(onSuccess, onError) defines both handlers in a single step.