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Handling errors in promises with then()'s onReject parameter versus using catch

Is it true that then()'s rejection handler only handles errors that occur as a result of the promise failing to resolve, e.g. a fetch request failing to return a result? Whereas, using catch instead of then()'s rejection handler will catch a failure of the promise to resolve, and also catch any failure that might occur somewhere in the callback function that is called in the case that the promise did resolve.